COMPUTER GATE

Hard Disk Drive (HDD)

2011-04-30T16:28:00.000-07:00

A hard disk drive^[2] (HDD) is a non-volatile, random access device for digital data. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the platters.

Introduced by IBM in 1956, hard disk drives have fallen in cost and physical size over the years while dramatically increasing in capacity. Hard disk drives have been the dominant device for secondary storage of data in general purpose computers since the early 1960s.^[3] They have maintained this position because advances in their areal recording density have kept pace with the requirements for secondary storage.^[3] Today's HDDs operate on high-speed serial interfaces; i.e., serial ATA (SATA) or serial attached SCSI (SAS).

[edit] History

Main article: History of hard disk drives

Hard disk drives were introduced in 1956 as data storage for an IBM accounting computer^[4] and were developed for use with general purpose mainframe and mini computers.

Driven by areal density doubling every two to four years since their invention, HDDs have changed in many ways, a few highlights include:

Capacity per HDD increasing from 3.75 megabytes to greater than 1 terabyte, a greater than 270-thousand-to-1 improvement.
Size of HDD decreasing from 87.9 cubic feet (a double wide refrigerator) to 0.002 cubic feet (2½-inch form factor, a pack of cards), a greater than 44-thousand-to-1 improvement.
Price decreasing from about $15,000 per megabyte to less than $0.0001 per megabyte ($100/1 terabyte), a greater than 150-million-to-1 improvement.^[5]
Average access time decreasing from greater than 0.1 second to a few thousandths of a second, a greater than 40-to-1 improvement.
Market application expanding from general purpose computers to most computing applications including consumer applications.

[edit] Technology

[edit] Magnetic recording

[edit] Components

A hard disk drive with the disks and motor hub removed showing the copper colored stator coils surrounding a bearing at the center of the spindle motor. The orange stripe along the side of the arm is a thin printed-circuit cable. The spindle bearing is in the center. The actuator is in the upper left.

A typical hard disk drive has two electric motors; a disk motor to spin the disks and an actuator (motor) to position the read/write head assembly across the spinning disks.

The disk motor has an external rotor attached to the disks; the stator windings are fixed in place.

Opposite the actuator at the end of the head support arm is the read-write head (near center in photo); thin printed-circuit cables connect the read-write heads to amplifier electronics mounted at the pivot of the actuator. A flexible, somewhat U-shaped, ribbon cable, seen edge-on below and to the left of the actuator arm continues the connection to the controller board on the opposite side.

The head support arm is very light, but also stiff; in modern drives, acceleration at the head reaches 550 Gs.

The silver-colored structure at the upper left of the first image is the top plate of the actuator, a permanent-magnet and moving coil motor that swings the heads to the desired position (it is shown removed in the second image). The plate supports a squat neodymium-iron-boron (NIB) high-flux magnet. Beneath this plate is the moving coil, often referred to as the voice coil by analogy to the coil in loudspeakers, which is attached to the actuator hub, and beneath that is a second NIB magnet, mounted on the bottom plate of the motor (some drives only have one magnet).

A disassembled and labeled 1997 hard drive. All major components were placed on a mirror, which created the symmetrical reflections.

The voice coil itself is shaped rather like an arrowhead, and made of doubly coated copper magnet wire. The inner layer is insulation, and the outer is thermoplastic, which bonds the coil together after it is wound on a form, making it self-supporting. The portions of the coil along the two sides of the arrowhead (which point to the actuator bearing center) interact with the magnetic field, developing a tangential force that rotates the actuator. Current flowing radially outward along one side of the arrowhead and radially inward on the other produces the tangential force. If the magnetic field were uniform, each side would generate opposing forces that would cancel each other out. Therefore the surface of the magnet is half N pole, half S pole, with the radial dividing line in the middle, causing the two sides of the coil to see opposite magnetic fields and produce forces that add instead of canceling. Currents along the top and bottom of the coil produce radial forces that do not rotate the head.

[edit] Error handling

Modern drives also make extensive use of Error Correcting Codes (ECCs), particularly Reed–Solomon error correction. These techniques store extra bits for each block of data that are determined by mathematical formulas. The extra bits allow many errors to be fixed. While these extra bits take up space on the hard drive, they allow higher recording densities to be employed, resulting in much larger storage capacity for user data.^[14] In 2009, in the newest drives, low-density parity-check codes (LDPC) are supplanting Reed-Solomon. LDPC codes enable performance close to the Shannon Limit and thus allow for the highest storage density available.^[15]

Typical hard drives attempt to "remap" the data in a physical sector that is going bad to a spare physical sector—hopefully while the errors in that bad sector are still few enough that the ECC can recover the data without loss. The S.M.A.R.T. system counts the total number of errors in the entire hard drive fixed by ECC, and the total number of remappings, in an attempt to predict hard drive failure.

[edit] Future development

Because of bit-flipping errors and other issues, perpendicular recording densities may be supplanted by other magnetic recording technologies. Toshiba is promoting bit-patterned recording (BPR),^[16] whiles Xyratex are developing heat-assisted magnetic recording (HAMR).^[17]

[edit] Capacity

[edit] Capacity measurements

Advertised capacity by manufacturer (using decimal multiples)		Expected capacity by consumers in class action (using binary multiples)		Reported capacity
				Windows (using binary multiples)	Mac OS X 10.6+ (using decimal multiples)
With prefix	Bytes	Bytes	Diff.	Windows (using binary multiples)	Mac OS X 10.6+ (using decimal multiples)
100 MB	100,000,000	104,857,600	4.86%	95.4 MB	100.0 MB
100 GB	100,000,000,000	107,374,182,400	7.37%	93.1 GB, 95,367 MB	100.00 GB
1 TB	1,000,000,000,000	1,099,511,627,776	9.95%	931 GB, 953,674 MB	1,000.00 GB

The capacity of hard disk drives is given by manufacturers in megabytes (1 MB = 1,000,000 bytes), gigabytes (1 GB = 1,000,000,000 bytes) or terabytes (1 TB = 1,000,000,000,000 bytes).^[18]^[19] This numbering convention, where prefixes like kilo- and mega- denote powers of 1000, is also used for data transmission rates and DVD capacities. However, the convention is different from that used for RAM, ROM and CD capacities, where prefixes like kilo- and mega- mean powers of 1024.

When the standard unit prefixes like kilo- denote powers of two math in the measure of computer capacities, the 2¹⁰ⁿ progression (for n = 1, 2, …) is as follows:^[18]

kilo = 2¹⁰ = 1024¹ = 1024,
mega = 2²⁰ = 1024² = 1,048,576,
giga = 2³⁰ = 1024³ = 1,073,741,824,
tera = 2⁴⁰ = 1024⁴ = 1,099,511,627,776,

and so forth.

The practice of using suffixes assigned to every three powers of 10 within the hard drive industry (storage) dates back to the early days of computing. Some computers like the IBM 702, which in 1953 used magnetic-core memory (non-volatile memory comprising small magnetic rings), had precisely 10,000 memory locations. Other computers however, such as the IBM 704 (also with magnetic-core memory) in 1954 had up to 32,768 words of memory, which was referred to as “32k words.” Thus, the prefix “k” was being used to represent 1024 in the 1950s.^[20] The practice of using the prefix “K” to denote 1024 was further reinforced after magnetic-core memory was obsoleted by Intel Corporation in 1969 with the introduction of the Intel 1102 dynamic random-access memory (DRAM) chip featuring 1024 bits of memory, Intel marketed it as a “1K” or 1 kilobit chip.^[21]^[22] When supercomputers with large amounts of IC-based memory were developed, such as the Cray‑1 in 1976, which featured up to 8 × 1024 × 1024 bytes of DRAM in 64-bit words, terminology such as “4M words” helped to popularize the use of the standard prefix “M” to denote the multiple 1,048,576 (1024²). Today, computer memory in DIMM form is commercially available in multiples of gigabytes.

In the case of “mega-,” there is a nearly 5% difference between its decimal definition used by the HDD industry and its powers-of-two definition used for other computing purposes like memory. Furthermore, the difference is compounded by 2.4% with each incrementally larger prefix (gigabyte, terabyte, et cetera). The discrepancy between the two conventions for measuring capacity was the subject of two class action suits against HDD manufacturers. The plaintiffs argued that the use of decimal measurements effectively misled consumers (see Orin Safier v. Western Digital Corporation^[23] and Cho v. Seagate Technology (US) Holdings, Inc.).^[24]

Confusing the matter is different operating systems address this disparity in different ways. Microsoft Windows operating systems adhere to the binary convention when reporting files sizes; a file that the operating system reports as measuring precisely 4 megabytes comprises 4,194,304 bytes (1024² × 4) and therefore occupies nearly 4.2 megabytes of hard disk capacity as the unit of measure is defined by the drive manufacturers. However, starting with the 2009 introduction of OS X 10.6 (“Snow Leopard”), Apple Inc. began using decimal math when reporting file size using the prefixes.

In December 1998, an international standards organization attempted to address these dual definitions of the conventional prefixes by proposing unique binary prefixes and prefix symbols to denote multiples of 1024, such as “mebibyte (MiB)”, which exclusively denotes 2²⁰ or 1,048,576 bytes.^[25] In the over‑12 years that have since elapsed, the proposal has seen little adoption by the computer industry and the conventionally prefixed forms of “byte” continue to denote slightly different values depending on context.^[26]^[27]

[edit] Form factors

5¼″ full height 110 MB HDD,
2½″ (8.5 mm) 6495 MB HDD,
US/UK pennies for comparison.

A 2.5 inch SATA hard drive from a Sony Vaio E series laptop.

Six hard drives with 8″, 5.25″, 3.5″, 2.5″, 1.8″, and 1″ disks, partially disassembled to show platters and read-write heads, with a ruler showing inches.

Mainframe and minicomputer hard disks were of widely varying dimensions, typically in free standing cabinets the size of washing machines (e.g. HP 7935 and DEC RP06 Disk Drives) or designed so that dimensions enabled placement in a 19" rack (e.g. Diablo Model 31). In 1962, IBM introduced its model 1311 disk, which used 14 inch (nominal size) platters. This became a standard size for mainframe and minicomputer drives for many years,^[28] but such large platters were never used with microprocessor-based systems.

With increasing sales of microcomputers having built in floppy-disk drives (FDDs), HDDs that would fit to the FDD mountings became desirable, and this led to the evolution of the market towards drives with certain Form factors, initially derived from the sizes of 8-inch, 5.25-inch, and 3.5-inch floppy disk drives. Smaller sizes than 3.5 inches have emerged as popular in the marketplace and/or been decided by various industry groups.

8 inch: 9.5 in × 4.624 in × 14.25 in (241.3 mm × 117.5 mm × 362 mm)
In 1979, Shugart Associates' SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8″ FDD.
5.25 inch: 5.75 in × 3.25 in × 8 in (146.1 mm × 82.55 mm × 203 mm)
This smaller form factor, first used in an HDD by Seagate in 1980,^[29] was the same size as full-height 5¹⁄₄-inch-diameter (130 mm) FDD, 3.25-inches high. This is twice as high as "half height"; i.e., 1.63 in (41.4 mm). Most desktop models of drives for optical 120 mm disks (DVD, CD) use the half height 5¼″ dimension, but it fell out of fashion for HDDs. The Quantum Bigfoot HDD was the last to use it in the late 1990s, with "low-profile" (≈25 mm) and "ultra-low-profile" (≈20 mm) high versions.
3.5 inch: 4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm) = 376.77344 cm³
This smaller form factor, first used in an HDD by Rodime in 1983,^[30] was the same size as the "half height" 3½″ FDD, i.e., 1.63 inches high. Today it has been largely superseded by 1-inch high "slimline" or "low-profile" versions of this form factor which is used by most desktop HDDs.
2.5 inch: 2.75 in × 0.275–0.59 in × 3.945 in (69.85 mm × 7–15 mm × 100 mm) = 48.895–104.775 cm³
This smaller form factor was introduced by PrairieTek in 1988;^[31] there is no corresponding FDD. It is widely used today for hard-disk drives in mobile devices (laptops, music players, etc.) and as of 2008 replacing 3.5 inch enterprise-class drives.^[32] It is also used in the Playstation 3^[33] and Xbox 360^{[citation needed]} video game consoles. Today, the dominant height of this form factor is 9.5 mm for laptop drives (usually having two platters inside), but higher capacity drives have a height of 12.5 mm (usually having three platters). Enterprise-class drives can have a height up to 15 mm.^[34] Seagate has released a wafer-thin 7mm drive aimed at entry level laptops and high end netbooks in December 2009.^[35]
1.8 inch: 54 mm × 8 mm × 71 mm = 30.672 cm³
This form factor, originally introduced by Integral Peripherals in 1993, has evolved into the ATA-7 LIF with dimensions as stated. It was increasingly used in digital audio players and subnotebooks, but is rarely used today. An original variant exists for 2–5GB sized HDDs that fit directly into a PC card expansion slot. These became popular for their use in iPods and other HDD based MP3 players.
1 inch: 42.8 mm × 5 mm × 36.4 mm
This form factor was introduced in 1999 as IBM's Microdrive to fit inside a CF Type II slot. Samsung calls the same form factor "1.3 inch" drive in its product literature.^[36]
0.85 inch: 24 mm × 5 mm × 32 mm
Toshiba announced this form factor in January 2004^[37] for use in mobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba currently sells a 4 GB (MK4001MTD) and 8 GB (MK8003MTD) version [1]^{[dead link]} and holds the Guinness World Record for the smallest hard disk drive.^[38]

3.5-inch and 2.5-inch hard disks currently dominate the market.

By 2009 all manufacturers had discontinued the development of new products for the 1.3-inch, 1-inch and 0.85-inch form factors due to falling prices of flash memory,^[39]^[40] which is slightly more stable and resistant to damage from impact and/or dropping.

The inch-based nickname of all these form factors usually do not indicate any actual product dimension (which are specified in millimeters for more recent form factors), but just roughly indicate a size relative to disk diameters, in the interest of historic continuity.

[edit] Current hard disk form factors

Form factor	Width	Height	Largest capacity	Platters (Max)
3.5″	102 mm	25.4 mm	3 TB^[41] (2010)	5
2.5″	69.9 mm	7–15 mm	1.5 TB^[42] (2010)	4^[43]
1.8″	54 mm	8 mm	320 GB^[44] (2009)	3

[edit] Obsolete hard disk form factors

Form factor	Width	Largest capacity	Platters (Max)
5.25″ FH	146 mm	47 GB^[45] (1998)	14
5.25″ HH	146 mm	19.3 GB^[46] (1998)	4^[47]
1.3″	43 mm	40 GB^[48] (2007)	1
1″ (CFII/ZIF/IDE-Flex)	42 mm	20 GB (2006)	1
0.85″	24 mm	8 GB^[49] (2004)	1

[edit] Performance characteristics

[edit] Access time

The factors that limit the time to access the data on a hard disk drive (Access time) are mostly related to the mechanical nature of the rotating disks and moving heads. Seek time is a measure of how long it takes the head assembly to travel to the track of the disk that contains data. Latency is rotational delay incurred because the desired disk sector may not be directly under the head when data transfer is requested. These two delays are on the order of milliseconds each. The bit rate or data transfer rate once the head is in the right position creates delay which is a function of the number of blocks transferred; typically relatively small, but can be quite long with the transfer of large contiguous files. Delay may also occur if the drive disks are stopped to save energy, see Power management.

An HDD's Average Access Time is its average Seek time which technically is the time to do all possible seeks divided by the number of all possible seeks, but in practice is determined by statistical methods or simply approximated as the time of a seek over one-third of the number of tracks^[50]

Defragmentation is a procedure used to minimize delay in retrieving data by moving related items to physically proximate areas on the disk.^[51] Some computer operating systems perform defragmentation automatically. Although automatic defragmentation is intended to reduce access delays, the procedure can slow response when performed while the computer is in use.^[52]

Access time can be improved by increasing rotational speed, thus reducing latency and/or by decreasing seek time. Increasing areal density increases throughput by increasing data rate and by increasing the amount of data under a set of heads, thereby potentially reducing seek activity for a given amount of data. Based on historic trends, analysts predict a future growth in HDD areal density (and therefore capacity) of about 40% per year.^[53] Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.

[edit] Seek time

Average seek time ranges from 3 ms^[54] for high-end server drives, to 15 ms for mobile drives, with the most common mobile drives at about 12 ms^[55] and the most common desktop type typically being around 9 ms. The first HDD had an average seek time of about 600 ms and by the middle 1970s HDDs were available with seek times of about 25 ms. Some early PC drives used a stepper motor to move the heads, and as a result had seek times as slow as 80–120 ms, but this was quickly improved by voice coil type actuation in the 1980s, reducing seek times to around 20 ms. Seek time has continued to improve slowly over time.

[edit] Latency

Latency is the delay for the rotation of the disk to bring the required disk sector under the read-write mechanism. It depends on rotational speed of a disk, measured in revolutions per minute (RPM). Average rotational delay is shown in the table below, based on the empirical relation that the average latency in milliseconds for such a drive is one-half the rotational period:

Spindle [rpm]	Average latency [ms]
4200	7.14
5400	5.56
7200	4.17
10000	3
15000	2

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[edit] Data transfer rate

As of 2010, a typical 7200 rpm desktop hard drive has a sustained "disk-to-buffer" data transfer rate up to 1030 Mbits/sec.^[56] This rate depends on the track location, so it will be higher for data on the outer tracks (where there are more data sectors) and lower toward the inner tracks (where there are fewer data sectors); and is generally somewhat higher for 10,000 rpm drives. A current widely used standard for the "buffer-to-computer" interface is 3.0 Gbit/s SATA, which can send about 300 megabyte/s from the buffer to the computer, and thus is still comfortably ahead of today's disk-to-buffer transfer rates. Data transfer rate (read/write) can be measured by writing a large file to disk using special file generator tools, then reading back the file. Transfer rate can be influenced by file system fragmentation and the layout of the files.^[51]

HDD data transfer rate depends upon the rotational speed of the platters and the data recording density. Because heat and vibration limit rotational speed, advancing density becomes the main method to improve sequential transfer rates.^{[citation needed]} Areal density advances by increasing both the number of tracks across the disk and the number of sectors per track, the latter will increase the data transfer rate (for a given RPM). Since data transfer rate performance only tracks one of the two components of areal density, its performance improves at lower rate,

[edit] Power consumption

Power consumption has become increasingly important, not only in mobile devices such as laptops but also in server and desktop markets. Increasing data center machine density has led to problems delivering sufficient power to devices (especially for spin up), and getting rid of the waste heat subsequently produced, as well as environmental and electrical cost concerns (see green computing). Heat dissipation directly tied to power consumption, and as drive age, disk failure rates increase at higher drive temperatures.^[57] Similar issues exist for large companies with thousands of desktop PCs. Smaller form factor drives often use less power than larger drives. One interesting development in this area is actively controlling the seek speed so that the head arrives at its destination only just in time to read the sector, rather than arriving as quickly as possible and then having to wait for the sector to come around (i.e. the rotational latency).^{[citation needed]} Many of the hard drive companies are now producing Green Drives that require much less power and cooling. Many of these Green Drives spin slower (<5,400 rpm compared to 7,200, 10,000 or 15,000 rpm) and also generate less waste heat.^{[citation needed]} Power consumption can also be reduced by parking the drive heads when the disk is not in use reducing friction, adjusting spin speeds according to transfer rates, and disabling internal components when not in use.^[58]

Also in systems where there might be multiple hard disk drives, there are various ways of controlling when the hard drives spin up since the highest current is drawn at that time.

On SCSI hard disk drives, the SCSI controller can directly control spin up and spin down of the drives.

On Parallel ATA (aka PATA) and Serial ATA (SATA) hard disk drives, some support power-up in standby or PUIS. The hard disk drive will not spin up until the controller or system BIOS issues a specific command to do so. This limits the power draw or consumption upon power on.

Some SATA II hard disk drives support staggered spin-up, allowing the computer to spin up the drives in sequence to reduce load on the power supply when booting.^[59]

[edit] Power management

Most hard disk drives today support some form of power management which uses a number of specific power modes that save energy by reducing performance. When implemented an HDD will change between a full power mode to one or more power saving modes as a function of drive usage. Recovery from the deepest mode, typically called Sleep, may take as long as several seconds.^[60]

[edit] Audible noise

Measured in dBA, audible noise is significant for certain applications, such as DVRs, digital audio recording and quiet computers. Low noise disks typically use fluid bearings, slower rotational speeds (usually 5,400 rpm) and reduce the seek speed under load (AAM) to reduce audible clicks and crunching sounds. Drives in smaller form factors (e.g. 2.5 inch) are often quieter than larger drives.

[edit] Shock resistance

Shock resistance is especially important for mobile devices. Some laptops now include active hard drive protection that parks the disk heads if the machine is dropped, hopefully before impact, to offer the greatest possible chance of survival in such an event. Maximum shock tolerance to date is 350 g for operating and 1000 g for non-operating.^[61]

[edit] Access and interfaces

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Hard disk drives are accessed over one of a number of bus types, including parallel ATA (P-ATA, also called IDE or EIDE), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses that they cannot communicate with natively, such as IEEE 1394, USB and SCSI.

For the ST-506 interface, the data encoding scheme as written to the disk surface was also important. The first ST-506 disks used Modified Frequency Modulation (MFM) encoding, and transferred data at a rate of 5 megabits per second. Later controllers using 2,7 RLL (or just "RLL") encoding caused 50% more data to appear under the heads compared to one rotation of an MFM drive, increasing data storage and data transfer rate by 50%, to 7.5 megabits per second.

Many ST-506 interface disk drives were only specified by the manufacturer to run at the 1/3 lower MFM data transfer rate compared to RLL, while other drive models (usually more expensive versions of the same drive) were specified to run at the higher RLL data transfer rate. In some cases, a drive had sufficient margin to allow the MFM specified model to run at the denser/faster RLL data transfer rate (not recommended nor guaranteed by manufacturers). Also, any RLL-certified drive could run on any MFM controller, but with 1/3 less data capacity and as much as 1/3 less data transfer rate compared to its RLL specifications.

Enhanced Small Disk Interface (ESDI) also supported multiple data rates (ESDI disks always used 2,7 RLL, but at 10, 15 or 20 megabits per second), but this was usually negotiated automatically by the disk drive and controller; most of the time, however, 15 or 20 megabit ESDI disk drives were not downward compatible (i.e. a 15 or 20 megabit disk drive would not run on a 10 megabit controller). ESDI disk drives typically also had jumpers to set the number of sectors per track and (in some cases) sector size.

Modern hard drives present a consistent interface to the rest of the computer, no matter what data encoding scheme is used internally. Typically a DSP in the electronics inside the hard drive takes the raw analog voltages from the read head and uses PRML and Reed–Solomon error correction^[62] to decode the sector boundaries and sector data, then sends that data out the standard interface. That DSP also watches the error rate detected by error detection and correction, and performs bad sector remapping, data collection for Self-Monitoring, Analysis, and Reporting Technology, and other internal tasks.

SCSI originally had just one signaling frequency of 5 MHz for a maximum data rate of 5 megabytes/second over 8 parallel conductors, but later this was increased dramatically. The SCSI bus speed had no bearing on the disk's internal speed because of buffering between the SCSI bus and the disk drive's internal data bus; however, many early disk drives had very small buffers, and thus had to be reformatted to a different interleave (just like ST-506 disks) when used on slow computers, such as early Commodore Amiga, IBM PC compatibles and Apple Macintoshes.

ATA disks have typically had no problems with interleave or data rate, due to their controller design, but many early models were incompatible with each other and could not run with two devices on the same physical cable in a master/slave setup. This was mostly remedied by the mid-1990s, when ATA's specification was standardized and the details began to be cleaned up, but still causes problems occasionally (especially with CD-ROM and DVD-ROM disks, and when mixing Ultra DMA and non-UDMA devices).

Serial ATA does away with master/slave setups entirely, placing each disk on its own channel (with its own set of I/O ports) instead.

FireWire/IEEE 1394 and USB(1.0/2.0) HDDs are external units containing generally ATA or SCSI disks with ports on the back allowing very simple and effective expansion and mobility. Most FireWire/IEEE 1394 models are able to daisy-chain in order to continue adding peripherals without requiring additional ports on the computer itself. USB however, is a point to point network and does not allow for daisy-chaining. USB hubs are used to increase the number of available ports and are used for devices that do not require charging since the current supplied by hubs is typically lower than what's available from the built-in USB ports.

[edit] Disk interface families used in personal computers

Notable families of disk interfaces include:

Several Parallel ATA hard disk drives

Historical bit serial interfaces connect a hard disk drive (HDD) to a hard disk controller (HDC) with two cables, one for control and one for data. (Each drive also has an additional cable for power, usually connecting it directly to the power supply unit). The HDC provided significant functions such as serial/parallel conversion, data separation, and track formatting, and required matching to the drive (after formatting) in order to assure reliability. Each control cable could serve two or more drives, while a dedicated (and smaller) data cable served each drive.
- ST506 used MFM (Modified Frequency Modulation) for the data encoding method.
- ST412 was available in either MFM or RLL (Run Length Limited) encoding variants.
- Enhanced Small Disk Interface (ESDI) was an industry standard interface similar to ST412 supporting higher data rates between the processor and the disk drive.

Modern bit serial interfaces connect a hard disk drive to a host bus interface adapter (today typically integrated into the "south bridge") with one data/control cable. (As for historical bit serial interfaces above, each drive also has an additional power cable, usually direct to the power supply unit.)
- Fibre Channel (FC), is a successor to parallel SCSI interface on enterprise market. It is a serial protocol. In disk drives usually the Fibre Channel Arbitrated Loop (FC-AL) connection topology is used. FC has much broader usage than mere disk interfaces, and it is the cornerstone of storage area networks (SANs). Recently other protocols for this field, like iSCSI and ATA over Ethernet have been developed as well. Confusingly, drives usually use copper twisted-pair cables for Fibre Channel, not fibre optics. The latter are traditionally reserved for larger devices, such as servers or disk array controllers.
- Serial ATA (SATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. Similar differential signaling system is used in RS485, LocalTalk, USB, Firewire, and differential SCSI.
- Serial Attached SCSI (SAS). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses a mechanically identical data and power connector to standard 3.5-inch SATA1/SATA2 HDDs, and many server-oriented SAS RAID controllers are also capable of addressing SATA hard drives. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.

Inner view of a 1998 Seagate hard disk drive which used Parallel ATA interface

Word serial interfaces connect a hard disk drive to a host bus adapter (today typically integrated into the "south bridge") with one cable for combined data/control. (As for all bit serial interfaces above, each drive also has an additional power cable, usually direct to the power supply unit.) The earliest versions of these interfaces typically had a 8 bit parallel data transfer to/from the drive, but 16-bit versions became much more common, and there are 32 bit versions. Modern variants have serial data transfer. The word nature of data transfer makes the design of a host bus adapter significantly simpler than that of the precursor HDD controller.
- Integrated Drive Electronics (IDE), later renamed to ATA, with the alias P-ATA ("parallel ATA") retroactively added upon introduction of the new variant Serial ATA. The original name reflected the innovative integration of HDD controller with HDD itself, which was not found in earlier disks. Moving the HDD controller from the interface card to the disk drive helped to standardize interfaces, and to reduce the cost and complexity. The 40-pin IDE/ATA connection transfers 16 bits of data at a time on the data cable. The data cable was originally 40-conductor, but later higher speed requirements for data transfer to and from the hard drive led to an "ultra DMA" mode, known as UDMA. Progressively swifter versions of this standard ultimately added the requirement for a 80-conductor variant of the same cable, where half of the conductors provides grounding necessary for enhanced high-speed signal quality by reducing cross talk. The interface for 80-conductor only has 39 pins, the missing pin acting as a key to prevent incorrect insertion of the connector to an incompatible socket, a common cause of disk and controller damage.
- EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of direct memory access (DMA) to transfer data between the disk and the computer without the involvement of the CPU, an improvement later adopted by the official ATA standards. By directly transferring data between memory and disk, DMA eliminates the need for the CPU to copy byte per byte, therefore allowing it to process other tasks while the data transfer occurs.
- Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface, was an early competitor of ESDI. SCSI disks were standard on servers, workstations, Commodore Amiga, and Apple Macintosh computers through the mid-1990s, by which time most models had been transitioned to IDE (and later, SATA) family disks. Only in 2005 did the capacity of SCSI disks fall behind IDE disk technology, though the highest-performance disks are still available in SCSI and Fibre Channel only. The range limitations of the data cable allows for external SCSI devices. Originally SCSI data cables used single ended (common mode) data transmission, but server class SCSI could use differential transmission, either low voltage differential (LVD) or high voltage differential (HVD). ("Low" and "High" voltages for differential SCSI are relative to SCSI standards and do not meet the meaning of low voltage and high voltage as used in general electrical engineering contexts, as apply e.g. to statutory electrical codes; both LVD and HVD use low voltage signals (3.3 V and 5 V respectively) in general terminology.)

Acronym or abbreviation	Meaning	Description
SASI	Shugart Associates System Interface	Historical predecessor to SCSI.
SCSI	Small Computer System Interface	Bus oriented that handles concurrent operations.
SAS	Serial Attached SCSI	Improvement of SCSI, uses serial communication instead of parallel.
ST-506	Seagate Technology	Historical Seagate interface.
ST-412	Seagate Technology	Historical Seagate interface (minor improvement over ST-506).
ESDI	Enhanced Small Disk Interface	Historical; backwards compatible with ST-412/506, but faster and more integrated.
ATA (PATA)	Advanced Technology Attachment	Successor to ST-412/506/ESDI by integrating the disk controller completely onto the device. Incapable of concurrent operations.
SATA	Serial ATA	Modification of ATA, uses serial communication instead of parallel.

[edit] Integrity

An IBM HDD head resting on a disk platter. Since the drive is not in operation, the head is simply pressed against the disk by the suspension.

Close-up of a hard disk head resting on a disk platter. A reflection of the head and its suspension is visible on the mirror-like disk.

Due to the extremely close spacing between the heads and the disk surface, hard disk drives are vulnerable to being damaged by a head crash—a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, contamination of the drive's internal enclosure, wear and tear, corrosion, or poorly manufactured platters and heads.

The HDD's spindle system relies on air pressure inside the disk enclosure to support the heads at their proper flying height while the disk rotates. Hard disk drives require a certain range of air pressures in order to operate properly. The connection to the external environment and pressure occurs through a small hole in the enclosure (about 0.5 mm in breadth), usually with a filter on the inside (the breather filter).^[63] If the air pressure is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about 3,000 m (10,000 feet).^[64] Modern disks include temperature sensors and adjust their operation to the operating environment. Breather holes can be seen on all disk drives—they usually have a sticker next to them, warning the user not to cover the holes. The air inside the operating drive is constantly moving too, being swept in motion by friction with the spinning platters. This air passes through an internal recirculation (or "recirc") filter to remove any leftover contaminants from manufacture, any particles or chemicals that may have somehow entered the enclosure, and any particles or outgassing generated internally in normal operation. Very high humidity for extended periods can corrode the heads and platters.

For giant magnetoresistive (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) still results in the head temporarily overheating, due to friction with the disk surface, and can render the data unreadable for a short period until the head temperature stabilizes (so called "thermal asperity", a problem which can partially be dealt with by proper electronic filtering of the read signal).

[edit] Actuation of moving arm

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Head stack with actuator coil on the left side (partly hidden by the controller interface) and read/write heads on the right side

The hard drive's electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to servo feedback. These are either complete concentric circles (in the case of dedicated servo technology), or segments interspersed with real data (in the case of embedded servo technology). The servo feedback optimizes the signal to noise ratio of the GMR sensors by adjusting the voice-coil of the actuated arm. The spinning of the disk also uses a servo motor. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed.

[edit] Landing zones and load/unload technology

A read/write head from a circa-1998 Fujitsu 3.5-inch hard disk. The area pictured is approximately 2.0 mm x 3.0mm.

Microphotograph of an older generation hard disk head and slider (1990s). The size of the front face (which is the "trailing face" of the slider) is about 0.3 mm × 1.0 mm. It is the location of the actual head (magnetic sensors). The non-visible bottom face of the slider is about 1.0 mm × 1.25 mm (so-called "nano" size) and faces the platter. It contains the lithographically micro-machined air bearing surface (ABS) that allows the slider to fly in a highly controlled fashion. One functional part of the head is the round, orange structure visible in the middle—the lithographically defined copper coil of the write transducer. Also note the electric connections by wires bonded to gold-plated pads.

Modern HDDs prevent power interruptions or other malfunctions from landing its heads in the data zone by parking the heads either in a landing zone or by unloading (i.e., load/unload) the heads. Some early PC HDDs did not park the heads automatically and they would land on data. In some other early units the user manually parked the heads by running a program to park the HDD's heads.

A landing zone is an area of the platter usually near its inner diameter (ID), where no data are stored. This area is called the Contact Start/Stop (CSS) zone. Disks are designed such that either a spring or, more recently, rotational inertia in the platters is used to park the heads in the case of unexpected power loss. In this case, the spindle motor temporarily acts as a generator, providing power to the actuator.

Spring tension from the head mounting constantly pushes the heads towards the platter. While the disk is spinning, the heads are supported by an air bearing and experience no physical contact or wear. In CSS drives the sliders carrying the head sensors (often also just called heads) are designed to survive a number of landings and takeoffs from the media surface, though wear and tear on these microscopic components eventually takes its toll. Most manufacturers design the sliders to survive 50,000 contact cycles before the chance of damage on startup rises above 50%. However, the decay rate is not linear: when a disk is younger and has had fewer start-stop cycles, it has a better chance of surviving the next startup than an older, higher-mileage disk (as the head literally drags along the disk's surface until the air bearing is established). For example, the Seagate Barracuda 7200.10 series of desktop hard disks are rated to 50,000 start-stop cycles, in other words no failures attributed to the head-platter interface were seen before at least 50,000 start-stop cycles during testing.^[65]

Around 1995 IBM pioneered a technology where a landing zone on the disk is made by a precision laser process (Laser Zone Texture = LZT) producing an array of smooth nanometer-scale "bumps" in a landing zone,^[66] thus vastly improving stiction and wear performance. This technology is still largely in use today (2008), predominantly in desktop and enterprise (3.5 inch) drives. In general, CSS technology can be prone to increased stiction (the tendency for the heads to stick to the platter surface), e.g. as a consequence of increased humidity. Excessive stiction can cause physical damage to the platter and slider or spindle motor.

Load/Unload technology relies on the heads being lifted off the platters into a safe location, thus eliminating the risks of wear and stiction altogether. The first HDD RAMAC and most early disk drives used complex mechanisms to load and unload the heads. Modern HDDs use ramp loading, first introduced by Memorex in 1967,^[67] to load/unload onto plastic "ramps" near the outer disk edge.

All HDDs today still use one of these two technologies listed above. Each has a list of advantages and drawbacks in terms of loss of storage area on the disk, relative difficulty of mechanical tolerance control, non-operating shock robustness, cost of implementation, etc.

Addressing shock robustness, IBM also created a technology for their ThinkPad line of laptop computers called the Active Protection System. When a sudden, sharp movement is detected by the built-in accelerometer in the Thinkpad, internal hard disk heads automatically unload themselves to reduce the risk of any potential data loss or scratch defects. Apple later also utilized this technology in their PowerBook, iBook, MacBook Pro, and MacBook line, known as the Sudden Motion Sensor. Sony,^[68] HP with their HP 3D DriveGuard^[69] and Toshiba^[70] have released similar technology in their notebook computers.

This accelerometer-based shock sensor has also been used for building cheap earthquake sensor networks.^[71]

[edit] Disk failures and their metrics

Most major hard disk and motherboard vendors now support S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology), which measures drive characteristics such as operating temperature, spin-up time, data error rates, etc. Certain trends and sudden changes in these parameters are thought to be associated with increased likelihood of drive failure and data loss.

However, not all failures are predictable. Normal use eventually can lead to a breakdown in the inherently fragile device, which makes it essential for the user to periodically back up the data onto a separate storage device. Failure to do so can lead to the loss of data. While it may sometimes be possible to recover lost information, it is normally an extremely costly procedure, and it is not possible to guarantee success. A 2007 study published by Google suggested very little correlation between failure rates and either high temperature or activity level; however, the correlation between manufacturer/model and failure rate was relatively strong. Statistics in this matter is kept highly secret by most entities. Google did not publish the manufacturer's names along with their respective failure rates,^[72] though they have since revealed that they use Hitachi Deskstar drives in some of their servers.^[73] While several S.M.A.R.T. parameters have an impact on failure probability, a large fraction of failed drives do not produce predictive S.M.A.R.T. parameters.^[72] S.M.A.R.T. parameters alone may not be useful for predicting individual drive failures.^[72]

A common misconception is that a colder hard drive will last longer than a hotter hard drive. The Google study seems to imply the reverse—"lower temperatures are associated with higher failure rates". Hard drives with S.M.A.R.T.-reported average temperatures below 27 °C (80.6 °F) had higher failure rates than hard drives with the highest reported average temperature of 50 °C (122 °F), failure rates at least twice as high as the optimum S.M.A.R.T.-reported temperature range of 36 °C (96.8 °F) to 47 °C (116.6 °F).^[72]

SCSI, SAS, and FC drives are typically more expensive and are traditionally used in servers and disk arrays, whereas inexpensive ATA and SATA drives evolved in the home computer market and were perceived to be less reliable. This distinction is now becoming blurred.

The mean time between failures (MTBF) of SATA drives is usually about 600,000 hours (some drives such as Western Digital Raptor have rated 1.4 million hours MTBF),^[74] while SCSI drives are rated for upwards of 1.5 million hours.^{[citation needed]} However, independent research indicates that MTBF is not a reliable estimate of a drive's longevity.^[75] MTBF is conducted in laboratory environments in test chambers and is an important metric to determine the quality of a disk drive before it enters high volume production. Once the drive product is in production, the more valid metric is annualized failure rate (AFR).^{[citation needed]} AFR is the percentage of real-world drive failures after shipping.

SAS drives are comparable to SCSI drives, with high MTBF and high reliability.^{[citation needed]}

Enterprise S-ATA drives designed and produced for enterprise markets, unlike standard S-ATA drives, have reliability comparable to other enterprise class drives.^[76]^[77]

Typically enterprise drives (all enterprise drives, including SCSI, SAS, enterprise SATA, and FC) experience between 0.70%–0.78% annual failure rates from the total installed drives.^{[citation needed]}

Eventually all mechanical hard disk drives fail, so to mitigate loss of data, some form of redundancy is needed, such as RAID^[78] or a regular backup^[78] system.

[edit] External removable drives

External removable hard disk drives connect to the computer using a USB cable or other means. External drives are used for:

Backup of files and information
Data recovery
Disk cloning
Running virtual machines
Scratch disk for video editing applications and video recording.

A 6 GB Seagate Pocket hard drive with USB cable extended next to a 2 GB CompactFlash card.

Larger models often include full-sized 3.5" PATA or SATA desktop hard drives. Features such as biometric security or multiple interfaces generally increase cost.

[edit] Market segments

As of July 2010, the highest capacity consumer HDDs are 3 TB.^[79]
"Desktop HDDs" typically store between 120 GB and 2 TB and rotate at 5,400 to 10,000 rpm, and have a media transfer rate of 0.5 Gbit/s or higher. (1 GB = 10⁹ bytes; 1 Gbit/s = 10⁹ bit/s)
Enterprise HDDs are typically used with multiple-user computers running enterprise software. Examples are
- transaction processing databases;
- internet infrastructure (email, webserver, e-commerce);
- scientific computing software;
- nearline storage management software.

The fastest enterprise HDDs spin at 10,000 or 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/s.^[80] and a sustained transfer rate up to 1 Gbit/s.^[80] Drives running at 10,000 or 15,000 rpm use smaller platters to mitigate increased power requirements (as they have less air drag) and therefore generally have lower capacity than the highest capacity desktop drives.

Enterprise drives commonly operate continuously ("24/7") in demanding environments while delivering the highest possible performance without sacrificing reliability. Maximum capacity is not the primary goal, and as a result the drives are often offered in capacities that are relatively low in relation to their cost.^[81]

Mobile HDDs or laptop HDDs, smaller than their desktop and enterprise counterparts, tend to be slower and have lower capacity. A typical mobile HDD spins at either 4200 rpm, 5200 rpm, 5400 rpm, or 7200 rpm, with 5400 rpm being the most prominent. 7200 rpm drives tend to be more expensive and have smaller capacities, while 4200 rpm models usually have very high storage capacities. Because of smaller platter(s), mobile HDDs generally have lower capacity than their greater desktop counterparts.

The exponential increases in disk space and data access speeds of HDDs have enabled the commercial viability of consumer products that require large storage capacities, such as digital video recorders and digital audio players.^[82] In addition, the availability of vast amounts of cheap storage has made viable a variety of web-based services with extraordinary capacity requirements, such as free-of-charge web search, web archiving, and video sharing (Google, Internet Archive, YouTube, etc.).

[edit] Sales

Worldwide revenue from shipments of HDDs is expected to reach $27.7 billion in 2010, up 18.4% from $23.4 billion in 2009^[83] corresponding to a 2010 unit shipment forecast of 674.6 million compared to 549.5 million units in 2009.^[84]

[edit] Icons

Hard drives are traditionally symbolized as either a stylized stack of platters (in orthographic projection) or, more abstractly, as a cylinder. This is particularly found in schematic diagrams or on indicator lights, as on laptops, to indicate hard drive access. In most modern operating systems, hard drives are instead represented by an illustration or photograph of a hard drive enclosure. These are illustrated below.

Today, hard drives are symbolized by a picture of the enclosure.
Schematically, hard drives may be represented by cylinders or stacks of platters, as in this RAID diagram.
The cylinder schematic derives from hard drives internally being a stack of platters, as in these 1970s vintage disk pack (cover removed).

[edit] Manufacturers

LCD (Liquid Crystal Display)

2009-06-08T17:00:00.000-07:00

A liquid crystal display (LCD) is an electronically-modulated optical device shaped into a thin, flat panel made up of any number of color or monochrome pixels filled with liquid crystals and arrayed in front of a light source backlight) or reflector. It is often utilized in battery-powered electronic devices because it uses very small amounts of electric power. (

A comprehensive classification of the various types and electro-optical modes of LCDs is provided in the article LCD classification.

Reflective twisted nematic liquid crystal display.

Polarizing filter film with a vertical axis to polarize light as it enters.
Glass substrate with ITO electrodes. The shapes of these electrodes will determine the shapes that will appear when the LCD is turned ON. Vertical ridges etched on the surface are smooth.
Twisted nematic liquid crystal.
Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter.
Polarizing filter film with a horizontal axis to block/pass light.
Reflective surface to send light back to viewer. (In a backlit LCD, this layer is replaced with a light source.)

[edit] Overview

LCD alarm clock

Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases) perpendicular to each other. With no actual liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second (crossed) polarizer.

The surface of the electrodes that are in contact with the liquid crystal material are treated so as to align the liquid crystal molecules in a particular direction. This treatment typically consists of a thin polymer layer that is unidirectionally rubbed using, for example, a cloth. The direction of the liquid crystal alignment is then defined by the direction of rubbing. Electrodes are made of a transparent conductor called Indium Tin Oxide (ITO).

Before applying an electric field, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces. In a twisted nematic device (still the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arrange themselves in a helical structure, or twist. This reduces the rotation of the polarization of the incident light, and the device appears grey. If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray. will appear

LCD with top polarizer removed from device and placed on top, such that the top and bottom polarizers are parallel.

The optical effect of a twisted nematic device in the voltage-on state is far less dependent on variations in the device thickness than that in the voltage-off state. Because of this, these devices are usually operated between crossed polarizers such that they appear bright with no voltage (the eye is much more sensitive to variations in the dark state than the bright state). These devices can also be operated between parallel polarizers, in which case the bright and dark states are reversed. The voltage-off dark state in this configuration appears blotchy, however, because of small variations of thickness across the device.

Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of one particular polarity is applied for a long period of time, this ionic material is attracted to the surfaces and degrades the device performance. This is avoided either by applying an alternating current or by reversing the polarity of the electric field as the device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity of the applied field).

When a large number of pixels are needed in a display, it is not technically possible to drive each directly since then each pixel would require independent electrodes. Instead, the display is multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (typically in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (typically in rows), with each group getting a voltage sink. The groups are designed so each pixel has a unique, unshared combination of source and sink. The electronics, or the software driving the electronics then turns on sinks in sequence, and drives sources for the pixels of each sink.

[edit] Specifications

Important factors to consider when evaluating an LCD monitor:

Resolution: The horizontal and vertical size expressed in pixels (e.g., 1024x768). Unlike CRT monitors, LCD monitors have a native-supported resolution for best display effect.
Dot pitch: The distance between the centers of two adjacent pixels. The smaller the dot pitch size, the less granularity is present, resulting in a sharper image. Dot pitch may be the same both vertically and horizontally, or different (less common).
Viewable size: The size of an LCD panel measured on the diagonal (more specifically known as active display area).
Response time: The minimum time necessary to change a pixel's color or brightness. Response time is also divided into rise and fall time. For LCD Monitors, this is measured in btb (black to black) or gtg (gray to gray). These different types of measurements make comparison difficult. A response time of <16ms id="cite_ref-0" class="reference">[1] and the difference between response times below 10ms becomes imperceptible due to limitations of the human eye.^[2]^[3]

Refresh rate: The number of times per second in which the monitor draws the data it is being given. Since activated LCD pixels do not flash on/off between frames, LCD monitors exhibit no refresh-induced flicker, no matter how low the refresh rate.^[4] Many high-end LCD televisions now have a 120 or 240 Hz (current and former NTSC countries) or 100 or 200 Hz (PAL/SECAM countries) refresh rate. The rate of 120 was chosen as the least common multiple of 24 frame/s (cinema) and 30 frame/s (NTSC TV), and allows for less distortion when movies are viewed due to the elimination of telecine (3:2 pulldown). For PAL at 25 frame/s, 100 or 200 Hz is used as a fractional compromise of the least common multiple of 600 (24 x 25). Until a 600 Hz refresh rate becomes available, PAL video will speed up cinema by a small percentage (currently 1 to 4 percent). These higher refresh rates are most effective from a 24p-source video output (e.g. Blu-ray Disc), and/or scenes of fast motion.
Matrix type: Active TFT or Passive.
Viewing angle: (coll., more specifically known as viewing direction).
Color support: How many types of colors are supported (coll., more specifically known as color gamut).
Brightness: The amount of light emitted from the display (coll., more specifically known as luminance).
Contrast ratio: The ratio of the intensity of the brightest bright to the darkest dark.
Aspect ratio: The ratio of the width to the height (for example, 4:3, 5:4, 16:9 or 16:10).
Input ports (e.g., DVI, VGA, LVDS, DisplayPort, or even S-Video and HDMI).
Gamma correction

[edit] Brief history

Apple Studio LCD display

1888: Friedrich Reinitzer (1858-1927) discovers the liquid crystalline nature of cholesterol extracted from carrots (that is, two melting points and generation of colors) and published his findings at a meeting of the Vienna Chemical Society on May 3, 1888 (F. Reinitzer: Beiträge zur Kenntniss des Cholesterins, Monatshefte für Chemie (Wien) 9, 421-441 (1888)).^[5]

1904: Otto Lehmann publishes his work "Flüssige Kristalle" (Liquid Crystals).

1911: Charles Mauguin first experiments of liquids crystals confined between plates in thin layers.

1922: George Friedel describes the structure and properties of liquid crystals and classified them in 3 types (nematics, smectics and cholesterics).

1936: The Marconi Wireless Telegraph company patents the first practical application of the technology, "The Liquid Crystal Light Valve".

1962: The first major English language publication on the subject "Molecular Structure and Properties of Liquid Crystals", by Dr. George W. Gray.^[6]

1962: Richard Williams of RCA found that liquid crystals had some interesting electro-optic characteristics and he realized an electro-optical effect by generating stripe-patterns in a thin layer of liquid crystal material by the application of a voltage. This effect is based on an electro-hydrodynamic instability forming what is now called “Williams domains” inside the liquid crystal.^[7]

1964: In the fall of 1964 George H. Heilmeier, then working in the RCA laboratories on the effect discovered by Williams realized the switching of colors by field-induced realignment of dichroic dyes in a homeotropically oriented liquid crystal. Practical problems with this new electro-optical effect made Heilmeier to continue work on scattering effects in liquid crystals and finally the realization of the first operational liquid crystal display based on what he called the dynamic scattering mode (DSM). Application of a voltage to a DSM display switches the initially clear transparent liquid crystal layer into a milky turbid state. DSM displays could be operated in transmissive and in reflective mode but they required a considerable current to flow for their operation.^[8]^[9]^[10] George H. Heilmeier was inducted in the National Inventors Hall of Fame and credited with the invention of LCD.^[11]

1960s: Pioneering work on liquid crystals was undertaken in the late 1960s by the UK's Royal Radar Establishment at Malvern. The team at RRE supported ongoing work by George Gray and his team at the University of Hull who ultimately discovered the cyanobiphenyl liquid crystals (which had correct stability and temperature properties for application in LCDs).

1970: On December 4, 1970, the twisted nematic field effect in liquid crystals was filed for patent by Hoffmann-LaRoche in Switzerland, (Swiss patent No. 532 261) with Wolfgang Helfrich and Martin Schadt (then working for the Central Research Laboratories) listed as inventors.^[8] Hoffmann-La Roche then licensed the invention to the Swiss manufacturer Brown, Boveri & Cie who produced displays for wrist watches during the 1970s and also to Japanese electronics industry which soon produced the first digital quartz wrist watches with TN-LCDs and numerous other products. James Fergason at the Westinghouse Research Laboratories in Pittsburgh while working with Sardari Arora and Alfred Saupe at Kent State University Liquid Crystal Institute filed an identical patent in the USA on April 22, 1971.^[12] In 1971 the company of Fergason ILIXCO (now LXD Incorporated) produced the first LCDs based on the TN-effect, which soon superseded the poor-quality DSM types due to improvements of lower operating voltages and lower power consumption.
1972: The first active-matrix liquid crystal display panel was produced in the United States by T. Peter Brody.^[13]
2007: In the 4Q of 2007 for the first time LCD televisions surpassed CRT units in worldwide sales.^[14]
2008: LCD TVs become the majority with a 50% market share of the 200 million TVs forecast to ship globally in 2008 according to Display Bank.^[15]

A detailed description of the origins and the complex history of liquid crystal displays from the perspective of an insider during the early days has been published by Joseph A. Castellano in "Liquid Gold, The Story of Liquid Crystal Displays and the Creation of an Industry".^[16] Another report on the origins and history of LCD from a different perspective has been published by Hiroshi Kawamoto, available at the IEEE History Center.^[17]

[edit] Color displays

A subpixel of a color LCD

Simulation of an LCD monitor up close

Comparison of the OLPC XO-1 display (left) with a typical color LCD. The images show 1×1 mm of each screen. A typical LCD addresses groups of 3 locations as pixels. The XO-1 display addresses each location as a separate pixel.

Example of how the colors are generated (R-red, G-green and B-blue)

In color LCDs each individual pixel is divided into three cells, or subpixels, which are colored red, green, and blue, respectively, by additional filters (pigment filters, dye filters and metal oxide filters). Each subpixel can be controlled independently to yield thousands or millions of possible colors for each pixel. CRT monitors employ a similar 'subpixel' structures via phosphors, although the electron beam employed in CRTs do not hit exact 'subpixels'.

Color components may be arrayed in various pixel geometries, depending on the monitor's usage. If the software knows which type of geometry is being used in a given LCD, this can be used to increase the apparent resolution of the monitor through subpixel rendering. This technique is especially useful for text anti-aliasing.

To reduce smudging in a moving picture when pixels do not respond quickly enough to color changes, so-called pixel overdrive may be used.

[edit] Passive-matrix and active-matrix addressed LCDs

A general purpose alphanumeric LCD, with two lines of 16 characters.

LCDs with a small number of segments, such as those used in digital watches and pocket calculators, have individual electrical contacts for each segment. An external dedicated circuit supplies an electric charge to control each segment. This display structure is unwieldy for more than a few display elements.

Small monochrome displays such as those found in personal organizers, or older laptop screens have a passive-matrix structure employing super-twisted nematic (STN) or double-layer STN (DSTN) technology—the latter of which addresses a color-shifting problem with the former—and color-STN (CSTN)—wherein color is added by using an internal filter. Each row or column of the display has a single electrical circuit. The pixels are addressed one at a time by row and column addresses. This type of display is called passive-matrix addressed because the pixel must retain its state between refreshes without the benefit of a steady electrical charge. As the number of pixels (and, correspondingly, columns and rows) increases, this type of display becomes less feasible. Very slow response times and poor contrast are typical of passive-matrix addressed LCDs.

High-resolution color displays such as modern LCD computer monitors and televisions use an active matrix thin-film transistors (TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated transistor, allowing each column line to access one pixel. When a row line is activated, all of the column lines are connected to a row of pixels and the correct voltage is driven onto all of the column lines. The row line is then deactivated and the next row line is activated. All of the row lines are activated in sequence during a refresh operation. Active-matrix addressed displays look "brighter" and "sharper" than passive-matrix addressed displays of the same size, and generally have quicker response times, producing much better images. structure. A matrix of

[edit] Active matrix technologies

A Casio 1.8" color TFT liquid crystal display which equips the Sony Cyber-shot DSC-P93A digital compact cameras

Main articles: Thin film transistor liquid crystal display and Active-matrix liquid crystal display

[edit] Twisted nematic (TN)

Twisted nematic displays contain liquid crystal elements which twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, the light is polarized to pass through the cell. In proportion to the voltage applied, the LC cells twist up to 90 degrees changing the polarization and blocking the light's path. By properly adjusting the level of the voltage almost any grey level or transmission can be achieved.

For a more comprehensive description refer to the section on the twisted nematic field effect.

[edit] In-plane switching (IPS)

In-plane switching is an LCD technology which aligns the liquid crystal cells in a horizontal direction. In this method, the electrical field is applied through each end of the crystal, but this requires two transistors for each pixel instead of the single transistor needed for a standard thin-film transistor (TFT) display. This results in blocking more transmission area, thus requiring a brighter backlight, which will consume more power, making this type of display less desirable for notebook computers.

[edit] Advanced Fringe Field Switching (AFFS)

Advanced Fringe Field Switching is a similar technology to IPS or S-IPS offering superior performance and color gamut besides high luminosity. AFFS is developed by Boe Hydis Displays, Korea.

AFFS-applied notebook applications minimize color distortion while maintaining its superior wide viewing angle for a professional display. Color shift and deviation caused by light leakage is corrected by optimizing the white gamut which also enhances white/grey reproduction.

In premium IBM ThinkPad series notebooks, Boe Hydis AFFS displays are used to provide higher resolutions up to 1600x1200, UXGA (in some versions QXGA) in a relatively small 15 inch display setting. IBM also advertised these high end panels under their FlexView^TM label. AFFS panels are mostly classified under the VIEWIZ^TM name by Boe Hydis resembling premium performance.

As of 2008, Hitachi acquired AFFS license to manufacture high end panels in their product line. Boe Hydis suspended their production of high quality displays however the company still advertises the benefits of the superior technology.

[edit] Vertical alignment (VA)

Vertical alignment displays are a form of LC displays in which the liquid crystal material naturally exists in a horizontal state removing the need for extra transistors (as in IPS). When no voltage is applied the liquid crystal cell, it remains perpendicular to the substrate creating a black display. When voltage is applied, the liquid crystal cells shift to a horizontal position, parallel to the substrate, allowing light to pass through and create a white display. VA liquid crystal displays provide some of the same advantages as IPS panels, particularly an improved viewing angle and improved black level.

[edit] Blue Phase mode

Main article: Blue Phase Mode LCD

Blue phase LCDs do not require an LC top layer. Blue phase LCDs are relatively new to the market,and very expensive because of the low volume of production. They provide a higher refresh rate than normal LCDs, but normal LCDs are still cheaper to make and actually provide better colors and a sharper image.^{[citation needed]} .

[edit] Quality control

Some LCD panels have defective transistors, causing permanently lit or unlit pixels which are commonly referred to as stuck pixels or dead pixels respectively. Unlike integrated circuits (ICs), LCD panels with a few defective pixels are usually still usable. It is also economically prohibitive to discard a panel with just a few defective pixels because LCD panels are much larger than ICs. Manufacturers have different standards for determining a maximum acceptable number of defective pixels. The maximum acceptable number of defective pixels for LCD varies greatly. At one point, Samsung held a zero-tolerance policy for LCD monitors sold in Korea.^[18] Currently, though, Samsung adheres to the less restrictive ISO 13406-2 standard.^[19] Other companies have been known to tolerate as many as 11 dead pixels in their policies.^[20] Dead pixel policies are often hotly debated between manufacturers and customers. To regulate the acceptability of defects and to protect the end user, ISO released the ISO 13406-2 standard.^[21] However, not every LCD manufacturer conforms to the ISO standard and the ISO standard is quite often interpreted in different ways.

Examples of defects in LCDs

LCD panels are more likely to have defects than most ICs due to their larger size. In the example to the right, a 300 mm SVGA LCD has 8 defects and a 150 mm wafer has only 3 defects. However, 134 of the 137 dies on the wafer will be acceptable, whereas rejection of the LCD panel would be a 0% yield. The standard is much higher now due to fierce competition between manufacturers and improved quality control. An SVGA LCD panel with 4 defective pixels is usually considered defective and customers can request an exchange for a new one. Some manufacturers, notably in South Korea where some of the largest LCD panel manufacturers, such as LG, are located, now have "zero defective pixel guarantee", which is an extra screening process which can then determine "A" and "B" grade panels. Many manufacturers would replace a product even with one defective pixel. Even where such guarantees do not exist, the location of defective pixels is important. A display with only a few defective pixels may be unacceptable if the defective pixels are near each other. Manufacturers may also relax their replacement criteria when defective pixels are in the center of the viewing area.

LCD panels also have defects known as mura, which look like a small-scale crack with very small changes in luminance or color.^[22] It is most visible in dark or black areas of displayed scenes. Defects in various LCD panel components can cause mura effect.^{[clarification needed]}

[edit] Zero-power (bistable) displays

The zenithal bistable device (ZBD), developed by QinetiQ (formerly DERA), can retain an image without power. The crystals may exist in one of two stable orientations (Black and "White") and power is only required to change the image. ZBD Displays is a spin-off company from QinetiQ who manufacture both grayscale and color ZBD devices.

A French company, Nemoptic, has developed another zero-power, paper-like LCD technology which has been mass-produced since July 2003. This technology is intended for use in applications such as Electronic Shelf Labels, E-books, E-documents, E-newspapers, E-dictionaries, Industrial sensors, Ultra-Mobile PCs, etc. Zero-power LCDs are a category of electronic paper.

Kent Displays has also developed a "no power" display that uses Polymer Stabilized Cholesteric Liquid Crystals (ChLCD). The major drawback to the ChLCD is slow refresh rate, especially with low temperatures.

In 2004 researchers at the University of Oxford demonstrated two new types of zero-power bistable LCDs based on Zenithal bistable techniques.^[23]

Several bistable technologies, like the 360° BTN and the bistable cholesteric, depend mainly on the bulk properties of the liquid crystal (LC) and use standard strong anchoring, with alignment films and LC mixtures similar to the traditional monostable materials. Other bistable technologies (i.e. Binem Technology) are based mainly on the surface properties and need specific weak anchoring materials.

See Ferro Liquid Display for more information about ferro fluid based bistable displays.

[edit] Drawbacks

Two IBM ThinkPad laptop screens viewed at an extreme angle.

LCD technology still has a few drawbacks in comparison to some other display technologies:

While CRTs are capable of displaying multiple video resolutions without introducing artifacts, LCDs produce crisp images only in their native resolution and, sometimes, fractions of that native resolution. Attempting to run LCD panels at non-native resolutions usually results in the panel scaling the image, which introduces blurriness or "blockiness" and is susceptible in general to multiple kinds of HDTV blur. Many LCDs are incapable of displaying very low resolution screen modes (such as 320x200) due to these scaling limitations.

Some types of LCD displays have a more limited color resolution than advertised, and must use spatial and/or temporal dithering to increase the apparent color depth. This can cause a shimmering effect with some types of displays which can be distracting for some users.

Although LCDs typically have more vibrant images and better "real-world" contrast ratios (the ability to maintain contrast and variation of color in bright environments) than CRTs, they do have lower contrast ratios than CRTs in terms of how deep their blacks are. A contrast ratio is the difference between a completely on (white) and off (black) pixel, and LCDs can have "backlight bleed" where light (usually seen around corners of the screen) leaks out and turns black into gray or even a bluish / purple tint with TN-film based displays. However, as of 2009, the very best LCD TVs that do not use LED backlighting can achieve a dynamic contrast ratio of 150,000:1.

LCDs typically have longer response times than their plasma and CRT counterparts, especially older displays, creating visible ghosting when images rapidly change. For example, when moving the mouse quickly on an LCD, multiple cursors can sometimes be seen. **See also: CRT phosphor persistence

LCDs appear to exhibit motion blur as the human eye follows moving objects, where some CRT screens do not. This is because an individual LCD pixel is constantly visible for the entire duration of a frame (typically 16.7ms), whereas a CRT pixel is lit for only a fraction of a microsecond once per frame as the electron beam scans past it. The means that even on a hypothetical LCD panel with a response time of zero, a panning image will appear to have motion blur whereas a panning image on a CRT monitor will not. This is caused by the movement of our eyes during the time the frame is visible. Blur can be reduced by increasing the refresh rate to a multiple of the frame rate (e.g. 120 or 240 Hz) and employing various image processing techniques. Blur or ghosting can be partially "corrected" using software techniques that present a negative image of the blur to compensate by canceling-out the predicted blur. For example, if a ghost image is caused by a left-over spot that is 5% brighter than normal, the software will draw a negative of the ghost image that is minus-5 percent, and the result will add up to the expected value (n + 5 - 5 = n). However, this technique requires a processing delay, which can be problematic for fast-action video-game usage. Some monitors even come with a "gaming mode" to turn off anti-ghosting when needed.
- See also: CRT phosphor persistence

LCD panels using TN tend to have a limited viewing angle relative to CRT and plasma displays. This reduces the number of people able to conveniently view the same image – laptop screens are a prime example. Usually when looking below the screen, it gets much darker; looking from above makes it look lighter. This distorts the colors and makes cheap LCD monitors unsuitable for work where color is important (photography, fashion, etc) as the colors change when one moves one's eyes slightly up or down, or when looks at the top of the screen or at the bottom from a fixed position. Many displays based on thin film transistor variants such as IPS, MVA, or PVA, have much improved viewing angles; typically the color only gets a little brighter when viewing at extreme angles, though much of the improvements on viewing angles has been done on lateral angles, not on vertical ones.

Consumer LCD monitors tend to be more fragile than their CRT counterparts. The screen may be especially vulnerable due to the lack of a thick glass shield as in CRT monitors.

Dead pixels can occur when the screen is damaged or pressure is put upon the screen; few manufacturers replace screens with dead pixels under warranty.

Horizontal and/or vertical banding is a problem in some LCD screens. This flaw occurs as part of the manufacturing process, and cannot be repaired (short of total replacement of the screen). Banding can vary substantially even among LCD screens of the same make and model. The degree is determined by the manufacturer's quality control procedures.

The cold cathode fluorescent lamps typically used for back-lights in LCD screens contain mercury, a toxic substance, though LED-backlit LCD screens are mercury-free.

Pattern based flicker, caused by imperfect voltage balance. LCD Flicker tests - one or more of the tests will usually demonstrate objectionable flicker, which can also show up if the problem pattern occurs as a hatching pattern over a significant area.

[edit] Energy Efficiency

Among newer TV models, the LCD displays require less energy on average than their plasma counterparts. A 42-inch LCD display consumes 203 Watts on average compared to 271 Watts consumed by a 42-inch Plasma display.^[24]

Energy use per inch is a better way of comparing differing technologies, CRT technology is more efficient using 0.23 Watts/square inch, while LCDs require 0.27 Watts/square inch. Plasma displays are on the high end at 0.36 Watts/square inch and DLP/rear projection TVs represent the low end at 0.14 Watts/square inch.^[25]

According to the U.S. EPA, if all TVs sold in the United States met Energy Star requirements (operate 30% more efficiently than standard models in both stand-by and active modes), we would see the equivalent GHG emissions reduction of taking a million cars off the road.^[26]

[edit] See also

[edit] Manufacturers

[edit] References

^ CNET's Monitor Buying Guide
^ Contemporary LCD Monitor Parameters: Objective and Subjective Analysis
^ Temporal Resolution
^ Contemporary LCD Monitor Parameters: Objective and Subjective Analysis (page 3)
^ Tim Sluckin: Ueber die Natur der kristallinischen Flüssigkeiten und flüssigen Kristalle (The early history of liquid crystals), Bunsen-Magazin, 7.Jahrgang, 5/2005
^ George W. Gray, Stephen M. Kelly: "Liquid crystals for twisted nematic display devices", J. Mater. Chem., 1999, 9, 2037–2050
^ R. Williams, “Domains in liquid crystals,” J. Phys. Chem., vol. 39, pp. 382–388, July 1963
^ ^a ^b Castellano, Joseph A. (2006). "Modifying Light". American Scientist 94 (5): pp. 438–445.
^ G. H. Heilmeier and L. A. Zanoni, “Guest-host interactions in nematic liquid crystals. A new electro-optic effect,” Appl. Phys. Lett., vol. 13, no. 3, pp. 91–92, 1968
^ G. H. Heilmeier, L. A. Zanoni, and L. A. Barton, “Dynamic scattering: A new electrooptic effect in certain classes of nematic liquid crystals,” Proc. IEEE, vol. 56, pp. 1162–1171, July 1968
^ http://www.invent.org/2009induction/1_3_09_induction_heilmeier.asp
^ "Modifying Light". American Scientist Online. http://www.americanscientist.org/template/AssetDetail/assetid/53321/page/4;jsessionid=aaa6J-GFIciRx2%3Ci%3ELive.
^ Brody, T.P., "Birth of the Active Matrix", Information Display, Vol. 13, No. 10, 1997, pp. 28-32.
^ "Worldwide LCD TV shipments surpass CRTs for first time ever". engadgetHD. 2008-02-19. http://www.engadgethd.com/2008/02/19/worldwide-lcd-tv-shipments-surpass-crts-for-first-time-ever/. Retrieved on 2008-06-13.
^ "Displaybank's Global TV Market Forecasts for 2008 - Global TV market to surpass 200 million units". Displaybank. 2007-12-05. http://www.displaybank.com/eng/info/news/press_show.php?id=2996. Retrieved on 2008-06-13.
^ LIQUID GOLD, The Story of Liquid Crystal Displays and the Creation of an Industry, 2005 World Scientific Publishing Co. Pte. Ltd., ISBN 981-238-956-3
^ Hiroshi Kawamoto: The History of Liquid-Crystal Displays, Proc. IEEE, Vol. 90, No. 4, April 2002
^ "Samsung to Offer 'Zero-PIXEL-DEFECT' Warranty for LCD Monitors". Forbes.com. December 30, 2004. http://www.forbes.com/infoimaging/feeds/infoimaging/2004/12/30/infoimagingasiapulse_2004_12_30_ix_9333-0197-.html. Retrieved on 2007-09-03.
^ "What is Samsung's Policy on dead pixels?". Samsung. February 5, 2005. http://erms.samsungelectronics.com/customer/uk/jsp/faqs/faqs_view.jsp?SITE_ID=31&PG_ID=16&AT_ID=17628&PROD_SUB_ID=546. Retrieved on 2007-08-03.
^ "Display (LCD) replacement for defective pixels - ThinkPad". Lenovo. June 25, 2007. http://www-307.ibm.com/pc/support/site.wss/document.do?lndocid=MIGR-4U9P53. Retrieved on 2007-07-13.
^ "What is the ISO 13406-2 standard for LCD screen pixel faults?". Anders Jacobsen's blog. January 4, 2006. http://www.jacobsen.no/anders/blog/archives/2006/01/04/what_is_the_iso_134062_standard_for_lcd_screen_pixel_faults.html.
^ EBU – TECH 3320, "User requirements for Video Monitors in Television Production", EBU/UER, May 2007, p. 11.
^ Dr Chidi Uche. "Development of bistable displays". University of Oxford. http://www.eng.ox.ac.uk/lc/research/Gratingstructures.html. Retrieved on 2007-07-13.
^ [1]
^ "Draft Efficiency Standards for Television" (PDF). California Energy Commission. December 2008. http://www.energy.ca.gov/2008publications/CEC-400-2008-028/CEC-400-2008-028-SD.PDF. Retrieved on 2009-05-31.
^ [2]

[edit] External links - Tutorials

This article's external links may not follow Wikipedia's content policies or guidelines. Please improve this article by removing excessive or inappropriate external links.

Color LCD Interfacing,LCD Interfacing with microcontroller
LCD info forum
Animated tutorial of LCD technology by 3M

Wikimedia Commons has media related to: Liquid Crystal Displays

History and Physical Properties of Liquid Crystals by Nobelprize.org
Definitions of basic terms relating to low-molar-mass and polymer liquid crystals (IUPAC Recommendations 2001)
An intelligible introduction to liquid crystals from Case Western Reserve University
Liquid Crystal Physics tutorial from the Liquid Crystals Group, University of Colorado
Introduction to liquid crystals from the Liquid Crystal Technology Group, Oxford University
Liquid Crystals & Photonics Group - Ghent University (Belgium), good tutorial
Liquid crystals Liquid Crystals Interactive Online (not updated since 1999)
Liquid Crystal Institute Kent State University
Liquid Crystals a journal by Taylor&Francis
Molecular Crystals and Liquid Crystals a journal by Taylor&Francis
Hot-spot detection techniques for ic's
What are liquid crystals? from Chalmers University of Technology, Sweden
LCD display NEMA standards
NEMA information for LCD enclosures

[edit] General information

What is TFT and how it works, TFT LCD guide for dummies.
How LCDs are made, an interactive demonstration from AUO (LCD manufacturer).
Development of Liquid Crystal Displays: Interview with George Gray, Hull University, 2004 – Video by the Vega Science Trust.
History of Liquid Crystals – Presentation and extracts from the book Crystals that Flow: Classic papers from the history of liquid crystals by its co-author Timothy J. Sluckin
Oleg Artamonov (2007-01-23). "Contemporary LCD Monitor Parameters: Objective and Subjective Analysis". X-bit labs. http://www.xbitlabs.com/articles/other/display/lcd-parameters.html. Retrieved on 2008-05-17.
Overview of 3LCD technology, Presentation Technology
LCD Module technical resources and application notes, Diamond Electronics
LCD Phase and Clock Adjustment, Techmind offers a free test screen to get a better LCD picture quality than the LCDs "auto-tune" function.
How to clean your LCD screen About.com: PC Support
TFT CentralLCD Monitor Reviews, Specs, Articles and News
FlatpanelsHD - Guide to flat panel monitors and TVs - LCD Monitor and LCD-TV Reviews, Articles and News
Interfacing Alphanumeric LCD to Microcontroller
Animations explaining operation of LCD panels

[hide] v • d • e Display technology

Video	Vacuum fluorescent (VFD) · Cathode ray tube (CRT) · Plasma · Liquid crystal (LCD) ·Light emitting diode (LED) · Video projector display · Digital light processing (DLP) ·Liquid crystal on silicon (LCOS) Next generation of display technologies: Organic light emitting diode (OLED) · Surface-conduction electron-emitter (SED) ·Field emission (FED) · Laser TV · Ferro Liquid (FLD) · Interferometric modulator display (IMOD) ·Thick-film dielectric electroluminescent (TDEL) · Nanocrystal · Quantum dot LED (QDLED) ·Micro device (MDDP) · Time Multiplexed Optical Shutter (TMOS) ·Transparent electroluminescent · Carbon nanotube (CNT) · Telescopic pixel (TPD) ·Liquid crystal lasers (LCL) · High dynamic range (HDR)

Non-video	Electromechanical (Flip-dot · Split-flap · Vane) · Electronic paper · Rollable · Eggcrate ·Nixie tube · Transparency

3D display	Stereoscopic · Autostereoscopic · Computer Generated Holography · Volumetric · Laser beam

Static media	Hologram · Movie projector · Neon sign · Rollsign · Slide projector

Touchscreen

2009-01-16T16:50:00.001-08:00

A touchscreen is a display which can detect the presence and location of a touch within the display area. The term generally refers to touch or contact to the display of the device by a finger or hand. Touchscreens can also sense other passive objects, such as a stylus. However, if the object sensed is active, as with a light pen, the term touchscreen is generally not applicable. The ability to interact directly with a display typically indicates the presence of a touchscreen.

Until the early 1980s, most consumer touchscreens could only sense one point of contact at a time, and few have had the capability to sense how hard one is touching. This is starting to change with the commercialisation of multi-touch technology.

The touchscreen has two main attributes. First, it enables one to interact with what is displayed directly on the screen, where it is displayed, rather than indirectly with a mouse or touchpad. Secondly, it lets one do so without requiring any intermediate device, again, such as a stylus that needs to be held in the hand. Such displays can be attached to computers or, as terminals, to networks. They also play a prominent role in the design of digital appliances such as the personal digital assistant (PDA), satellite navigation devices and mobile phones.

History

One of the the Nintendo DS' main selling points is its second screen on the bottom, which is a touch screen.

Touchscreens emerged from academic and corporate research labs in the second half of the 1960s. One of the first places where they gained some visibility was in the terminal of a computer-assisted learning terminal that came out in 1972 as part of the PLATO project. They have subsequently become familiar in kiosk systems, such as in retail and tourist settings, on point of sale systems, on ATMs and on PDAs where a stylus is sometimes used to manipulate the GUI and to enter data. The popularity of smart phones, PDAs, portable game consoles and many types of information appliances is driving the demand for, and the acceptance of, touchscreens.

The HP-150 from 1983 was probably the world's earliest commercial touchscreen computer. It doesn't actually have a touchscreen in the strict sense, but a 9" Sony CRT surrounded by infrared transmitters and receivers which detect the position of any non-transparent object on the screen.

Touchscreens are popular in heavy industry and in other situations, such as museum displays or room automation, where keyboard and mouse systems do not allow a satisfactory, intuitive, rapid, or accurate interaction by the user with the display's content.

Historically, the touchscreen sensor and its accompanying controller-based firmware have been made available by a wide array of after-market system integrators and not by display, chip or motherboard manufacturers. With time, however, display manufacturers and System On Chip (SOC) manufacturers worldwide have acknowledged the trend toward acceptance of touchscreens as a highly desirable user interface component and have begun to integrate touchscreen functionality into the fundamental design of their products. In the portable consumer electronics space, the touchscreen has evolved from the Electronic Organizer to the Apple iphone, Samsung Eternity, LG Vu, and Blackberry Storm.^[1]

Technologies

There are a number of types of touchscreen technology.

Resistive

A resistive touchscreen panel is composed of several layers. The most important are two thin metallic electrically conductive and resistive layers separated by thin space. When some object touches this kind of touch panel, the layers are connected at a certain point; the panel then electrically acts similar to two voltage dividers with connected outputs. This causes a change in the electrical current which is registered as a touch event and sent to the controller for processing.

Resistive touchscreen panels are generally the most affordable technology but offer only 75% clarity^{[citation needed]} (premium films and glass finishes allow transmissivity to approach 85%^{[citation needed]}) and the layer can be damaged by sharp objects. Resistive touchscreen panels are not affected by outside elements such as dust or water and are the type most commonly used today. The Nintendo DS is an example of a product that uses resistive touchscreen technology.

Surface acoustic wave

Surface acoustic wave (SAW) technology uses ultrasonic waves that pass over the touchscreen panel. When the panel is touched, a portion of the wave is absorbed. This change in the ultrasonic waves registers the position of the touch event and sends this information to the controller for processing. Surface wave touchscreen panels can be damaged by outside elements. Contaminants on the surface can also interfere with the functionality of the touchscreen.^[2]

Capacitive

A capacitive touchscreen panel is coated with a material, typically indium tin oxide, that conducts a continuous electrical current across the sensor.^[3]^[4] The sensor therefore exhibits a precisely controlled field of stored electrons in both the horizontal and vertical axes - it achieves capacitance. The human body is also an electrical device which has stored electrons and therefore also exhibits capacitance. Capacitive sensors work based on proximity, and do not have to be directly touched to be triggered. It is a durable technology that is used in a wide range of applications including point-of-sale systems, industrial controls, and public information kiosks. It has a higher clarity than Resistive technology, but it only responds to finger contact and will not work with a gloved hand or pen stylus. Capacitive touch screens can also support Multitouch. A good example of this is Apple Inc.'s iPhone and iPod touch.

Projected Capacitance

Projected Capacitance Touch technology is a type of capacitive technology which involves the relationship between an XY array of sensing wires embedded within two layers of non-metallic material, and a third object. In touchscreen applications the third object can be a human finger. Capacitance forms between the user’s fingers and projected capacitance from the sensing wires. A touch is made, precisely measured, then passed on to the controller system which is connected to a computer running a software application. This will then calculate how the user’s touch relates to the computer software.^[5]

Visual Planet’s ViP Interactive Foil is an example of a product that uses Projected Capacitance Touch technology. This technology allows a gloved hand to make the touch, resulting in Projected Capacitance Touch technology now being common in external "through window" touch applications (i.e. those where no direct physical contact with the touchscreen is made).

Strain gauge

In a strain gauge configuration the screen is spring-mounted on the four corners and strain gauges are used to determine deflection when the screen is touched.^[6] This technology can also measure the Z-axis. Typically used in exposed public systems such as ticket machines due to their resistance to vandalism.

Optical imaging

A relatively-modern development in touchscreen technology, two or more image sensors are placed around the edges (mostly the corners) of the screen. Infrared backlights are placed in the camera's field of view on the other sides of the screen. A touch shows up as a shadow and each pair of cameras can then be triangulated to locate the touch or even measure the size of the touching object (see visual hull). This technology is growing in popularity, due to its scalability, versatility, and affordability, especially for larger units.

Dispersive signal technology

Introduced in 2002, this system uses sensors to detect the mechanical energy in the glass that occurs due to a touch. Complex algorithms then interpret this information and provide the actual location of the touch. The technology claims to be unaffected by dust and other outside elements, including scratches. Since there is no need for additional elements on screen, it also claims to provide excellent optical clarity. Also, since mechanical vibrations are used to detect a touch event, any object can be used to generate these events, including fingers and stylus. A downside is that after the initial touch the system cannot detect a motionless finger.

Acoustic pulse recognition

This system uses more than two piezoelectric transducers located at some positions of the screen to turn the mechanical energy of a touch (vibration) into an electronic signal.^[7] The screen hardware then uses an algorithm to determine the location of the touch based on the transducer signals. This process is similar to triangulation used in GPS. The touchscreen itself is made of ordinary glass, giving it good durability and optical clarity. It is usually able to function with scratches and dust on the screen with good accuracy. The technology is also well suited to displays that are physically larger. As with the Dispersive Signal Technology system, after the initial touch, a motionless finger cannot be detected.

Building touch screens

There are several principal ways to built a touch screen. The key goals are to recognize one or more fingers touching a display, to interpret the command that this represents, and to communicate the command to the appropriate application.

In the most popular techniques called the capacitive or resistive approach, manufactures coat the screen with a thin, transparent metallic layer. When a user touches the surface, the system records the change in the electrical current that flows through the display.

Dispersive-signal technology which [3M] created in 2002, measures the piezoelectric effect- the voltage generated when mechanical force is applied to a material- that occurs chemically when a strengthened glass substrate is touched.

There are two infrared-based approaches. In one, any array of sensors detects finger touching or almost touching the display, there by interrupting light beams projected over the screen. In the other, bottom-mounted infrared cameras record screen touches.

In each case, the system determines the intended command based on the controls showing on the screen at the time and the potation of the touch.

Development

Virtually all of the significant touchscreen technology patents were filed during the 1970s and 1980s and have expired. Touchscreen component manufacturing and product design are no longer encumbered by royalties or legalities with regard to patents and the manufacturing of touchscreen-enabled displays on all kinds of devices is widespread.

The development of multipoint touch screens facilitated the tracking of more than one finger on the screen, thus operations that require more than one finger are possible. These devices also allow multiple users to interact with the touchscreen simultaneously.

With the growing acceptance of many kinds of products with an integral touchscreen interface the marginal cost of touchscreen technology is routinely absorbed into the products that incorporate it and is effectively eliminated. As typically occurs with any technology, touchscreen hardware and software has sufficiently matured and been perfected over more than three decades to the point where its reliability is unassailable. As such, touchscreen displays are found today in airplanes, automobiles, gaming consoles, machine control systems, appliances and handheld display devices of every kind. With the influence of the multi touch-enabled iPhone, the touchscreen market for mobile devices is projected to produce US$5 billion in 2009.^[8]

The ability to accurately point on the screen itself is taking yet another step with the emerging graphics tablet/screen hybrids.

Ergonomics and usage

Finger stress

An ergonomic problem of touchscreens is their stress on human fingers when used for more than a few minutes at a time, since significant pressure can be required and the screen is non-flexible. This can be alleviated with the use of a pen or other device to add leverage, but the introduction of such items can sometimes be problematic depending on the desired use case (for example, public kiosks such as ATMs). Also, fine motor control is better achieved with a stylus,. a finger being a rather broad and ambiguous point of contact with the screen.>>

Fingernail as stylus

These ergonomic issues of direct touch can be bypassed by using a different technique, provided that the user's fingernails are either short or sufficiently long. Rather than pressing with the soft skin of an outstretched fingertip, the finger is curled over, so that the top of the forward edge of a fingernail can be used instead. (The thumb is optionally used to provide support for the finger or for a long fingernail, from underneath.)

The fingernail's hard, curved surface contacts the touchscreen at a single very small point. Therefore, much less finger pressure is needed, much greater precision is possible (approaching that of a stylus, with a little experience), much less skin oil is smeared onto the screen, and the fingernail can be silently moved across the screen with very little resistance, allowing for selecting text, moving windows, or drawing lines.

The human fingernail consists of keratin which has a hardness and smoothness similar to the tip of a stylus (and so will not typically scratch a touchscreen). Alternately, very short stylus tips are available, which slip right onto the end of a finger; this increases visibility of the contact point with the screen. Oddly, with capacitive touchscreens, the reverse problem applies in that individuals with long nails have reported problems getting adequate skin contact with the screen to register keystrokes (note that ordinary styli do not work on capacitive touchscreens nor do gloved fingers).

The concept of using a fingernail trimmed to form a point, to be specifically used as a stylus on a writing tablet for communication, appeared in the 1950 science fiction short story Scanners Live in Vain.

Fingerprints

Touch screens also suffer from the problem of fingerprints on the display. This can be mitigated by the use of materials with optical coatings designed to reduced the visible effects of fingerprint oils.

"Gorilla arm"

Gorilla arm was a side-effect that destroyed vertically-oriented touch-screens as a mainstream input technology despite a promising start in the early 1980s. Designers of touch-menu systems failed to notice that humans are not built to hold their arms at waist- or head-height, making small and precise motions. After a short period of time, cramp may begin to set in, and arm movement becomes painful and clumsy — the operator looks like a gorilla while using the touch screen and feels like one afterwards. This is now considered a classic cautionary tale to human-factors designers; "Remember the gorilla arm!" is shorthand for "How is this going to fly in real use?".^[9] Gorilla arm is not a problem for specialist short-term-use devices such as ATMs, since they only involve brief interactions which are not long enough to cause gorilla arm. Gorilla arm also can be mitigated by the use of horizontally-mounted screens such as those used in Tablet PCs, but these need to account for the user's need to rest their hands on the device. This can increase the amount of dirt deposited on the device, and occludes the user's view of the screen.

References

^ Evolution of portable consumer electronics from the Electronic Organizer to Apple's iPhone.
^ Patschon, Mark (1988-03-15), Acoustic touch technology adds a new input dimension, Computer Design, pp. 89-93, http://rwservices.no-ip.info:81/pens/biblio88.html#Platshon88
^ Kable, Robert G. (1986-07-15), Electrographic Apparatus, United States Patent 4,600,807, http://rwservices.no-ip.info:81/pens/biblio86.html#Kable86
^ Kable, Robert G. (1986-07-15), Electrographic Apparatus, United States Patent 4,600,807 (full image), http://www.freepatentsonline.com/4600807.pdf
^ Truetouch Projective Capacitance Technology
^ Minsky, M.R. (1984-08-15), Manipulating simulated objects with real-world gestures using a force and position sensitive screen, Computer Graphice, Vol 18 No 3, pp. 195-203, http://rwservices.no-ip.info:81/pens/biblio85.html#Minsky84
^ Acoustic Pulse Recognition Touchscreens, Elo Touch Systems, 1888-07-31, http://www.elotouch.com/Products/Touchscreens/AcousticPulseRecognition/default.asp, retrieved on 25 August 2008
^ http://www.abiresearch.com/press/1231-Touch+Screens+in+Mobile+Devices+to+Deliver+$5+Billion+Next+Year
^ "Jargon File - Gorilla Arm". www.catb.org. Retrieved on 2008-11-17.

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

Andreas Holzinger: Finger Instead of Mouse: Touch Screens as a means of enhancing Universal Access, In: Carbonell, N.; Stephanidis C. (Eds): Universal Access, Theoretical Perspectives, Practice, and Experience. Lecture Notes in Computer Science. Vol. 2615. Berlin, Heidelberg, New York: Springer, 2003, ISBN 3-540-00855-1, 387–397.

Touchscreen

2009-01-16T16:50:00.000-08:00

History

One of the the Nintendo DS' main selling points is its second screen on the bottom, which is a touch screen.

Technologies

There are a number of types of touchscreen technology.

Resistive

Surface acoustic wave

Capacitive

Projected Capacitance

Strain gauge

Optical imaging

Dispersive signal technology

Acoustic pulse recognition

Building touch screens

In each case, the system determines the intended command based on the controls showing on the screen at the time and the potation of the touch.

Development

The ability to accurately point on the screen itself is taking yet another step with the emerging graphics tablet/screen hybrids.

Ergonomics and usage

Finger stress

Fingernail as stylus

Fingerprints

"Gorilla arm"

References

^ Evolution of portable consumer electronics from the Electronic Organizer to Apple's iPhone.
^ Patschon, Mark (1988-03-15), Acoustic touch technology adds a new input dimension, Computer Design, pp. 89-93, http://rwservices.no-ip.info:81/pens/biblio88.html#Platshon88
^ Kable, Robert G. (1986-07-15), Electrographic Apparatus, United States Patent 4,600,807, http://rwservices.no-ip.info:81/pens/biblio86.html#Kable86
^ Kable, Robert G. (1986-07-15), Electrographic Apparatus, United States Patent 4,600,807 (full image), http://www.freepatentsonline.com/4600807.pdf
^ Truetouch Projective Capacitance Technology
^ Minsky, M.R. (1984-08-15), Manipulating simulated objects with real-world gestures using a force and position sensitive screen, Computer Graphice, Vol 18 No 3, pp. 195-203, http://rwservices.no-ip.info:81/pens/biblio85.html#Minsky84
^ Acoustic Pulse Recognition Touchscreens, Elo Touch Systems, 1888-07-31, http://www.elotouch.com/Products/Touchscreens/AcousticPulseRecognition/default.asp, retrieved on 25 August 2008
^ http://www.abiresearch.com/press/1231-Touch+Screens+in+Mobile+Devices+to+Deliver+$5+Billion+Next+Year
^ "Jargon File - Gorilla Arm". www.catb.org. Retrieved on 2008-11-17.

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

Andreas Holzinger: Finger Instead of Mouse: Touch Screens as a means of enhancing Universal Access, In: Carbonell, N.; Stephanidis C. (Eds): Universal Access, Theoretical Perspectives, Practice, and Experience. Lecture Notes in Computer Science. Vol. 2615. Berlin, Heidelberg, New York: Springer, 2003, ISBN 3-540-00855-1, 387–397.

Computer and Video Games

2009-01-16T16:44:00.001-08:00

Computer and Video Games (CVG) is a video game magazine and website published in the United Kingdom. Initially published monthly between November 1981 and October 2004 and solely web-based from 2004 onwards^[1], the magazine was one of the first publications to capitalise on the growing home computing market, although it also covered arcade games. The magazine saw many changes over the course of its life, and by the mid 1990's had switched focus to concentrate entirely on games consoles.

The magazine was "suspended" in 2004 after Future Publishing bought the magazine (alongside PC Zone) from Dennis Publishing Ltd who had themselves purchased it from the magazine's original publishers EMAP. Future, who also published CVG's main rival, GamesMaster, subsequently decided to publish their magazine as opposed to keeping CVG in operation. Subscribers received a copy of GamesMaster in place of CVG, along with a letter claiming the magazine had been suspended to allow the staff a break and would return in a few months.

The magazine returned in a new form, titled CVG Presents, on 16 April 2008 with a bi-monthly release schedule.^[2] The new format concentrates the whole magazine on a single subject. The first issue of the new format concentrated on the history of the Grand Theft Auto series of games.

In the meantime, the magazine's website has continued to flourish, and recently Future incorporated the forums of many of its other games related publications to ComputerAndVideoGames.com in addition to devoting sections to those that did not previously have a formal website, such as PC Gamer.

1 Previous editors

Previous editors

Magazine

Terry Pratt
Tim Metcalfe
Eugene Lacey
Graham Taylor
Julian Rignall
Tim Boone
Paul Rand
Mark Patterson
Paul Davies
Alex Simmons

Website

Gareth Ramsay
Patrick Garratt (2002/2003)
Johnny Minkley (early 2004)
Stuart Bishop (acting Ed 2004)
John Houlihan (late 2004)
Gavin Ogden (2006)

References

Computer and Video Games

2009-01-16T16:44:00.000-08:00

1 Previous editors

Previous editors

Magazine

Terry Pratt
Tim Metcalfe
Eugene Lacey
Graham Taylor
Julian Rignall
Tim Boone
Paul Rand
Mark Patterson
Paul Davies
Alex Simmons

Website

Gareth Ramsay
Patrick Garratt (2002/2003)
Johnny Minkley (early 2004)
Stuart Bishop (acting Ed 2004)
John Houlihan (late 2004)
Gavin Ogden (2006)

References

Nesting Technology Leader Optimation Helps Conveyor Firm Emi Corp

2008-10-04T20:37:00.001-07:00

Headquartered in Jackson Center, Ohio, EMI Corp. is the nation's largest maker of conveyors and conveyor systems for plastic products. The company, which sells productivity enhancement to its customers, is keenly aware of the need to keep costs competitive yet still produce quality products faster.

EMI initially purchased Optimation software shortly after installing a new CL-707 laser. The company started with manual nesting, then moved to batch nesting, and is now toward a fully automated system in which orders can be entered into its AS400 computer system and placed directly to the Optimation software.

Michael D. Lundy, P.E., President & CEO of Optimation noted that, “In addition to its line of standard conveyors, EMI often also designs and produces tailored systems to meet special needs for individual customers. One of the primary advantages of the Optimation solution is the ability to tailor nests 'on the fly' to meet immediate production schedules.”

For 30 years Optimation® has been the world leader in Part Nesting for Optimized Material and Labor utilization. Consistently providing product advancement leadership to industry, the company’s new Nesting Technology, AxiomVE make previous nesting methods obsolete. The new technology, Vision Emulation™, allows the system to “see” the shape of parts, just as human eyes would view them. When a person looks at a part, they see the whole shape and any special features on the part. This information is then used to determine if the part will fit in an area of the nest. Vision Emulation eliminates excessive trial and error as well as excessive rotation.

Optimation (www.optimation.com) offers benchmarking against any other method of part nesting; recent benchmarks have show improvements up to 15% in material efficiency alone. As lean efficiencies are essential, the most advanced nesting technology guarantees the most important parts are always nested in the next available machine, providing advanced information to integrated costing, labor reporting, and material inventory systems. Because Optimation is strong financially and has the most advanced technology; the company offers direct financing from its own capital resources.

Optimation

www.optimation.com

Michael D Lundy P.E.

Opti1@optinest.com

877-827-2100

The Benefits of Free Screensaver Downloads

2008-10-04T20:36:00.001-07:00

One of the more popular things that people do online is search for free things. They may search for products that they can order over the Internet that comes as a free sample. Other people look for something to download that they can run on their computer which is handy. One of the most popular of these items are free screensaver downloads, something that people have been doing ever since the Internet has been around. Why do people enjoy screensaver's so much.

Whenever a computer sits idle for a short period of time, the screensaver usually kicks in in an attempt to save the monitor from damage. Even though this is not as important as it used to be, many individuals still like to run screensavers during this time. The reason why it is not as important is because monitors have really changed since the early ones that were used with computers. Whenever one of these old monitors was left alone for too long of a period of time, it would usually burn an image of whatever was left on the screen while it was running directly into the screen. By having a screensaver, your monitor did not have the opportunity to burn that image because it was usually animated. Fortunately, computer monitors have outgrown that behavior.

Whenever you are looking for free screensaver downloads, it is best to go to a website that is relatively well-known. One of the reasons why this is the case is because many of the screen savers will come bundled with something additional. At times, it is just a relatively harmless program that they may use as advertising but at others, it may actually have viruses or some kind of spyware attached. As long as you scan it, however, you can do as many of these free screensaver downloads as you want without running into problems.

Another reason why it's good to go to a larger free screensaver downloads website is because the one that you want will be easy to find. If you're looking for something specific, you can usually search for it right from the main page. If, however, you're unsure of exactly what you want you can usually browse through the different categories until you find one that will work perfect for you. While you're there, you might as well go ahead and download several so that you can have a variety whenever it's time for your computer to go to sleep.

Welcome Windows 7, Seventh Heaven for Microsoft

2008-10-04T20:35:00.000-07:00

Microsoft tries to make new innovation after launching Windows Vista, although the innovation will launch in 2010, but few issues is heard through Internet. The innovation of Windows 7 was created when Bill Gates retired from Microsoft big company last June 30, 2008. Bill Gates will take responsibility in management, not responsibility in operational. The retiring of Bill Gates as founder and visionary leader is not right time when Microsoft is struggling to face a lot of competitors.

Based on Microsoft commitment, Windows Vista must implement until 2010, although the users complain about the product, because of needing much power to operate it.
Windows 7 will finish in 2010 after trying and error, and expect more responsive than Windows Vista.

Through Internet information, we find many issues that we can make conclusion like below,

1.Windows 7 will give facilities as, report mechanism so that developer can send their experiences, order and bug to Microsoft.
2.PC in Windows 7 will connect to workgroups and network when installation is processing.
3.You will see interesting “Windows meeting Space” when you are starting Menu, after installation process.
4.We can share document and folder with peer to peer on the net.
5.Users can share photo, music and file with others users after setting.
6.To use all services above, users must register without Windows Live ID.
7.Users can manage desktop, windows size, font and background desktop.
8.Users can use new facility “ Windows Health Center” Summary of PC condition”
(update condition, access user account, and risks that will happen) Alarm will active when Windows find the risks.
9.Easy recovery center, making easy restoration back up.
10. “Touchable Paint” Users can draw without mouse. With multi touch function, users can play game and move file without mouse.
11. Software and driver have been running in Windows Vista, guarantee and must be running in Windows 7.

The new system above will be finished by 2000 developers and planning to finish in October. 2000 tester will try bug in Professional Developers Conference at Los Angeles.

Choosing the Right Hard Drive

2008-10-04T20:27:00.001-07:00

How much memory and storage space do I need for my new PC? Should I opt for removable storage instead? What is a gigabyte anyway? If you find yourself asking these questions, don't get discouraged. Buying the right hard drive requires research, thought, and a lot of patience. There are a lot of hard drives out there - this article will help you narrow down your search and pick the hard drive that's right for you.

First, you must determine whether you need an internal or external hard drive. If you're looking for the easiest way to add data storage capacity to your computer, then you should go with an external hard drive. They're great for backing up your PC, and they allow you to easily share photos, videos, and songs with others. Ideal for those who travel, external hard drives are physically very small so you can take them with you wherever you go.

On the other hand, internal hard drives are designed for replacing or expanding the storage of a single PC. They offer massive storage capabilities, the highest performance, and the lowest cost per gigabyte. Most desktop PC cases have at least one internal drive bay (the place where you can mount extra hard drives). However, before you purchase an additional drive for your system, make sure your case has enough room. If you have a smaller, low-profile case, you won't be able to use the old and new drives simultaneously. You'll also have to choose between the two interfaces: PATA (Parallel Advanced Technology Attachment, also known as IDE drives) and SATA (Serial Advanced Technology Attachment). In most cases, SATA drives are a better choice for a few reasons—they're slightly faster, they're easier to connect, and they don't require you to configure jumpers as PATA drives do. Nonetheless, the performance tends to be similar.

The next thing you need to do is determine the size of the hard drive, which simply refers to its data storage capacity. For the most part, the size of the hard drive depends on what you plan to do with your computer. If you’re just browsing the web or doing a little word processing, you probably don't need more than 8-10 gigabytes. The lower capacity drives are typically less expensive and should only be used to handle basic computing needs. But if you plan on storing large amounts of data, music, or pictures, you'll want to go with a larger hard drive to avoid running out of space.

Next, you must choose the speed of the hard drive, which is measured in revolutions per minute (RPM). Hard drives consist of a disc that rotates and a needle which reads/writes data to this disc. The faster the disc spins, the faster the data is read and written. For the average user, 5400 RPM is perfectly adequate. But if you want your system to be as fast as possible, then choose a hard drive with 7200 RPM.

General Tips:

Shop around. Hard drives come in different sizes to suit different storage needs, and they're priced very competitively. So spend a little time searching for a killer deal, and you'll certainly be glad you did.

Look out for warranties. In general, you should get at least a 3-year warranty on your hard drive.

Consider buying a hard-drive kit, which includes mounting hardware, cables, detailed instructions, and software that eases installation. A kit may also include an application for cloning the contents of your old hard drive onto the new one, which then becomes your new main drive.

If you've outgrown your existing storage, it may be easier and cheaper to upgrade a drive instead of buying an entirely new one. And if you're an avid gamer, opt for a drive with approximately 10,000 RPM.

Finding the Best Deals on Pc Parts

2008-10-04T20:26:00.001-07:00

Finding the Best Deal on Computer Components

I have now been working with a computer component ecommerce store for over two years – unfortunately it does not make a decent profit. One thing it has done (and continues doing so) is to teach me about ecommerce and the Internet as well as web site development.

The most important thing I have learnt is that competition within the computer component market, as well as most other markets out there is immense. Even being within the first page of results within Google, for many popular computer component related search terms, does not automatically mean orders will flood in. There is so many others offering similar computer products at similar or lower prices.

This led me to thinking how good this could be for the computer consumer. Anyone who has used the internet to buy products knows that there is huge savings to be made when comparing this to the average computer store. Again this is down to the huge competition – and people perhaps do not realise and take advantage of this fact.

When shopping around for a particular computer component you will find that prices vary greatly from store to store. This comes down to the fact that particular computer suppliers will make deals direct with the component manufacturers as well as perhaps buying a specific component in bulk which naturally lowers the price.

In the market of computer components there are literally hundreds of manufacturers across the world these then supply distributors of which there are many, and they then supply the suppliers. This means price fluctuation occurs on pretty much every product line.

For the general consumer looking to buy computer components, this can be a time consuming issue, and generally, unless the purchase is of a large value, it is difficult to justify the time spent trawling through the countless computer suppliers to locate the best deal on the computer component.

This is where price comparison searches such as ComponentCompare.com can really help save you time and money.

The way this works is very simple. Type in the part number or name of the computer product you want to buy into the search field. This then brings up a list of all the suppliers that supply your chosen computer part. Alongside is the price, the cost of shipping and the availability. This quickly allows you to see who can offer you the best deal and how quickly you can obtain your goods.

Comparing the market in the manner suits everyone involved so if you are looking for the best deals on computer parts then get searching now!

Video Game Tester - 3 Things You Must Know Before Looking For A Job

2008-10-04T20:25:00.001-07:00

Gaming companies across the world are frequently looking for keen video game testers to find bugs and issues in their unreleased video games. The gaming industry is a multi billion dollar monster that keeps on getting fed and growing and growing. Scam artists and internet entrepreneurs have become wise to this fact and have started setting up scam websites. Wannabe game testers you need to be aware of a few things before you hand over your cash for a video game tester job guide - if that is in fact the path you are heading.

1 - Not All Jobs Pay Well

Game testers don't be misled by the hype. Not every job you are going to apply for is going to pay well. Only the latest - and more expensive gaming consoles will pay more to beta test their unreleased video games. For example Xbox 360 games pay quite handsomely, online PC games do not on the other hand. Do some research on the video game tester job guide and seek testimonials.

2 - Own Multiple Gaming Consoles

Game testers need to own multiple gaming consoles, if they want to make a substantial income from testing video games. Why? Its pretty simple really. The more games you can test, the more jobs you will get. The more jobs you get, the more money you will make. Again I would suggest you have the most recent gaming consoles such as PS3 and Xbox 360 at least. This will definitely open some doors for you.

3 - Treat It Like A Career Not A Hobby

A major flaw a lot of game testers show is that their game testing is more of a hobby than a career. I know its awesome that you can get paid to play video games, but for gaming companies this is serious business. So come across like a professional. Respond immediately to job offers, make sure you give as much information as possible when testing the video games, and always and I mean always meet your deadlines. You will be surprised how much you will be rewarded for being professional.

Conclusion

Wannabe game testers follow the above tips and you should have no problems making decent money testing video games. Most game testers do it for a while, get lazy and give up. Treat it like a proper job, and there is no reason why you should have any issues, and be able to achieve long lasting success.

COMPUTER GATE

Hard Disk Drive (HDD)

Contents

[edit] History

[edit] Technology

[edit] Magnetic recording

[edit] Components

[edit] Error handling

[edit] Future development

[edit] Capacity

[edit] Capacity measurements

[edit] Form factors

[edit] Current hard disk form factors

[edit] Obsolete hard disk form factors

[edit] Performance characteristics

[edit] Access time

[edit] Seek time

[edit] Latency

[edit] Data transfer rate

[edit] Power consumption

[edit] Power management

[edit] Audible noise

[edit] Shock resistance

[edit] Access and interfaces

[edit] Disk interface families used in personal computers

[edit] Integrity

[edit] Actuation of moving arm

[edit] Landing zones and load/unload technology

[edit] Disk failures and their metrics

[edit] External removable drives

[edit] Market segments

[edit] Sales

[edit] Icons

[edit] Manufacturers

LCD (Liquid Crystal Display)

Contents

[edit] Overview

[edit] Specifications

[edit] Brief history

[edit] Color displays

[edit] Passive-matrix and active-matrix addressed LCDs

[edit] Active matrix technologies

[edit] Twisted nematic (TN)

[edit] In-plane switching (IPS)

[edit] Advanced Fringe Field Switching (AFFS)

[edit] Vertical alignment (VA)

[edit] Blue Phase mode

[edit] Quality control

[edit] Zero-power (bistable) displays

[edit] Drawbacks

[edit] Energy Efficiency

[edit] See also

[edit] Related technology

[edit] Other display technologies

[edit] Display applications

[edit] Manufacturers

[edit] References

[edit] External links - Tutorials

[edit] General information

Touchscreen

Contents

History

Technologies

Resistive

Surface acoustic wave

Capacitive

Projected Capacitance

Strain gauge

Optical imaging

Dispersive signal technology

Acoustic pulse recognition

Building touch screens

Development

Ergonomics and usage

Finger stress

Fingernail as stylus

Fingerprints

"Gorilla arm"

See also

References