Performance

There are three primary factors that determine hard drive performance: seek time, latency and internal data transfer rate:

• Seek time is a measure of the speed with which the drive can position its read/write heads over any particular data track. Because neither the starting position of the head nor the distance from there to the desired track is fixed, seek time varies greatly, and it is almost always measured as an average seek time, though full-track (the longest possible) and track-to-track (the shortest possible) seeks are also quoted sometimes. The standard way to measure seek time is to time a large number of disk accesses to random locations, subtract the latency (see below) and take the mean.

• All drives have rotational latency: the time that elapses between the moment when the read/write head settles over the desired data track and the moment when the first byte of the required data appears under the head.

• The internal data rate is the speed with which the drive's internal read channel can transfer data from the magnetic media. (Or, less commonly, in the reverse direction.) Previously a very important factor in drive performance, it remains significant but less so than in prior years, as all modern drives have very high internal data rates. Internal data rates are normally measured in Megabits per second (Mbit/s).

Computer display

A computer display, monitor or screen is a computer peripheral device capable of showing still or moving images generated by a computer and processed by a graphics card. Monitors generally conform to one or more display standards. Sometimes the name "display" is preferred to the word "monitor", as the latter is perceived to be ambiguous alongside the other senses of "monitor" meaning "machine-level debugger" or "thread synchronization mechanism". Computer displays are sometimes called heads, especially when talking about how many are connected to a computer. Computer displays have also been known as visual display units or VDUs.

Technologies

As with television, several different hardware technologies exist for displaying the actual image:

• Cathode ray tube (CRT)

• Liquid crystal display (LCD). They can receive TV and computer bands (SVGA, PAL, SECAM; NTSC).

• Plasma display (rarely seen)

• Video projector

A modern CRT display has considerable flexibility: it can often handle all resolutions from 640 by 480 pixels (640×480) up to 2048 by 1536 pixels (2048×1536) with 32-bit colour and a variety of refresh rates.

The sharpness of a display is described by its dot pitch. In general, the lower the dot pitch, (e.g. .24), the sharper the picture will be.

Some technical circles prefer the name "display" to the word "monitor" (perceived as ambiguous alongside the other senses of "monitor" meaning "machine-level debugger" or "thread synchronization mechanism"). Computer displays have also been known as visual display units or VDUs.

Early CRT-based VDUs without graphics capabilities gained the label 'glass teletypes', because of the similarity to their electromechanical predecessors.

Black and white displays can only display one colour either as on or off. Monochrome displays can show only levels of a single colour. In both cases the display usually uses green, orange (amber) or gray (white).

Colour monitors may show either digital colour (each of the red, green and blue signals may be either on or off, giving eight possible colours: black, white, red, green, blue, cyan, magenta and yellow) or analog colour (red, green and blue signals are continuously variable allowing the display of any combination). Digital monitors are sometimes known as TTL because the voltages on the red, green and blue inputs are compatible with TTL logic chips.

Most modern computer displays can show thousands or millions of different colours in the RGB colour space by combining red, green, and blue dots in varying intensities.

Some display technologies (especially LCD) have an inherent misregistration of the colour planes, that is, the centers of the red, green, and blue dots do not line up perfectly. In 2001, software designers began to exploit the misregistration to produce sharper images: Microsoft's ClearType™ provides an example.

Moving texts can appear in italics, even when the display resolution is too low to show static italics: a fractional time delay causes an apparent corresponding shift of a fraction of a pixel.

Alphanumeric keyboards include typewriter and computer keyboards. An alphanumeric keyboard is a device with many keys (usually marked with the letters of the alphabet, the numerical digits, and various extra keys.)

After punchcards and paper tape, interaction via teletype-style keyboards became the main input device for computers. During the 1980s and 1990s almost all computers came equipped with them as the main form of interaction, and most users are familiar with using them.

There are different types of keyboard technologies.

The layout of keys on the modern-day English keyboard is called the QWERTY design, based on the most popular typewriter keyboard layout. This has been further extended to the standard 101-key PC keyboard layout, with the addition of cursor keys, a calculator-style numeric keypad, and two groups of special function keys, and keys for the Windows menu (on IBM and clones) or Apple menu (on Macintoshes). In the late 1990s, computer manufacturers, such as Dell, add keys specifically related to the Internet and e-mail, but these have not yet become standard.

There is another kind of computer keyboard known as a chorded keyboard. These are rarely used.

Despite the development of alternative input devices such as the mouse, touch sensitive screens, pen devices, character recognition, voice recognition, and improvements in computer speed and memory size, the keyboard remains the most commonly used and most versatile device used for direct human input into computers.

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