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DISPLAY MONITORS & VIDEO ADAPTERS 2

INTELLIGENT TERMINAL:
A terminal (monitor and keyboard) that contains processing power. Intelligent terminals include memory and a processor to perform special display operations. In contrast, a dumb terminal has no processing capabilities; it must rely entirely on the central computer. A smart terminal has some processing capabilities, but not as much as an intelligent terminal.

INTERLACING:
A display technique that enables a monitor to provide more resolution inexpensively. With
interlacing monitors, the electron guns draw only half the horizontal lines with each pass (for example, all odd lines on one pass and all even lines on the next pass). Because an interlacing monitor refreshes only half the lines at one time, it can display twice as many lines per refresh cycle, giving it greater resolution. Another way of looking at it is that interlacing provides the same resolution as noninterlacing, but less expensively.
A shortcoming of interlacing is that the reaction time is slower, so programs that depend on quick refresh rates (animation and video, for example), may experience flickering or streaking. Given two monitors that offer the same resolution, the noninterlacing one will generally be better.
Preparing a graphic image so that alternating rows are displayed in separate passes. Interlaced images give a nice effect because the entire image is displayed quickly and then details are filled in gradually. They are especially prevalent on the World Wide Web because of the slow transmission speed. Web pages with interlaced GIFs appear in a browser more quickly than pages with normal GIF images. The interlaced GIFs look blurry at first, but then become sharp as the rows are filled in.

LCD:
Abbreviation of liquid crystal display, a type of display used in digital watches and many portable computers. LCD displays utilize two sheets of polarizing material with a liquid crystal solution between them. An electric current passed through the liquid causes the crystals to align so that light cannot pass through them. Each crystal, therefore, is like a shutter, either allowing light to pass through or blocking the light.
Monochrome LCD images usually appear as blue or dark gray images on top of a grayish-white background. Color LCD displays use two basic techniques for producing color: Passive matrix is the less expensive of the two technologies. The other technology, called thin film transistor (TFT) or active-matrix, produces color images that are as sharp as traditional CRT displays, but the technology is expensive. Recent passive-matrix displays using new CSTN and DSTN technologies produce sharp colors rivaling active-matrix displays.
Most LCD screens used in notebook computers are backlit to make them easier to read in bright environments.

MASK PITCH:
In color monitors, the distance between phosphors of the same color. The mask pitch is generally about 30 millimeters (mm). The tighter the mask pitch, the sharper the image.

MCGA:
Abbreviation of multicolor/graphics array (or memory controller gate array), the graphics system built into some older PCs. It provides graphics capabilities equal to or greater than MDA and CGA, but it is not as powerful as EGA or VGA. Like VGA, MCGA uses analog signals.

MDA:
Abbreviation of monochrome display adapter, an old monochrome video standard for PCs. MDA supports high-resolution monochrome text but does not support graphics or colors. The resolution for text is 720 by 350 pixels.

MONOCHROME:
One color. Monitors, for example, can be monochrome, grayscale or color. Monochrome monitors actually use two colors, one for the display image (the foreground) and one for the background. Graphic images can also be monochrome, grayscale, or color.

MULTIFREQUENCY MONITOR:
A type of video monitor capable of accepting signals at more than one frequency range. This enables the monitor to support several different video standards. Typically, multifrequency monitors for PCs support MDA, Hercules, CGA, VGA, and sometimes SVGA.
Multifrequency monitors differ somewhat from multiscanning monitors. Multiscanning monitors can support video signals at any frequency level within its range, whereas multifrequency monitors support only a select number of frequency levels. However, because almost all video signals conform to one of a handful of video standards, the greater potential of multiscanning monitors is generally not utilized.

MULTISCANNING MONITOR:
A type of monitor that automatically adjusts to the signal frequency of the video display board to which it is connected. Consequently, multiscanning monitors can display images based on almost any graphics display system, including MDA, Hercules, EGA, VGA, and SVGA.
In contrast, fixed-frequency monitors respond to only one, or a few, frequencies, so they can connect to a limited number of video display boards. However, fixed-frequency monitors are less expensive than multiscanning monitors and sometimes produce sharper images.
Multiscanning monitors are also called multisync, multifrequency, and variable-frequency monitors. Increasingly, however, the term multifrequency monitor is reserved for monitors that support a fixed number of video frequencies. In contrast, multiscanning monitors scan the incoming signals and set themselves to whatever frequency range they are receiving. In practice, there is little difference between the two types of monitors because most video signals conform to one of a handful of video standards.

NON-INTERLACED:
Refers to monitors and video standards that do not use interlacing techniques to improve resolution. Although interlacing increases resolution, it also increases screen flicker and reduces reaction time.

PAPER-WHITE DISPLAY:
A high-quality monochrome monitor that displays characters in black against a white background. Such monitors are popular for desktop publishing because they most closely mimic real paper with black type. Some manufacturers make a distinction between normal white-background monitors and paper-white monitors, where the background is slightly tinted to look more like bonded paper.

PASSIVE-MATRIX DISPLAY:
A common type of flat-panel display consisting of a grid of horizontal and vertical wires. At the intersection of each grid is an LCD element which constitutes a single pixel, either letting light through or blocking it. A higher quality and more expensive type of display, called an active-matrix display, uses a transistor to control each pixel.
In the mid-90s, it appeared that passive-matrix displays would eventually become extinct due to the higher quality of active-matrix displays. However, the high cost of producing active-matrix displays, and new technologies such as DSTN and CSTN that improve passive-matrix displays, have cause passive-matrix displays to make a surprising comeback.

PIXEL:
Short for Picture Element, a pixel is a single point in a graphic image. Graphics monitors display pictures by dividing the display screen into thousands (or millions) of pixels, arranged in rows and columns. The pixels are so close together that they appear connected.
The number of bits used to represent each pixel determines how many colors or shades of gray can be displayed. For example, an 8-bit color monitor uses 8 bits for each pixel, making it possible to display 2 to the 8th power (256) different colors or shades of gray.
On color monitors, each pixel is actually composed of three dots -- a red, a blue, and a green one. Ideally, the three dots should all converge at the same point, but all monitors have some convergence error that can make color pixels appear fuzzy.
The quality of a display monitor largely depends on its resolution, how many pixels it can display, and how many bits are used to represent each pixel. VGA monitors display 640 by 480, or about 300,000 pixels. In contrast, SVGA monitors display 1,024 by 768, or nearly 800,000 pixels. True Color monitors use 24 bits per pixel, allowing them to display more than 16 million different colors.

PLASMA DISPLAY:
A type of flat-panel display that works by sandwiching an ionized gas between two wired panels. In one panel the wires are placed in vertical rows, and in the other they are placed in horizontal rows. Together, the two panels form a grid. An individual point (pixel) can then be charged by passing a current through the appropriate x-coordinate and y-coordinate wires. When the gas is charged, it glows a bright orange.

RESOLUTION:
Refers to the sharpness and clarity of an image. The term is most often used to describe monitors, printers, and bit-mapped graphic images. In the case of dot-matrix and laser printers, the resolution indicates the number of dots per inch. For example, a 300-dpi (dots per inch) printer is one that is capable of printing 300 distinct dots in a line 1 inch long. This means it can print 90,000 dots per square inch.
For graphics monitors, the resolution signifies the number of dots (pixels) on the entire screen. For example, a 640-by-480 pixel screen is capable of displaying 640 distinct dots on each of 480 lines, or about 300,000 pixels. This translates into different dpi measurements depending on the size of the screen. For example, a 15-inch VGA monitor (640x480) displays about 50 dots per inch.
Printers, monitors, scanners, and other I/O devices are often classified as high resolution, medium resolution, or low resolution. The actual resolution ranges for each of these grades is constantly shifting as the technology improves

REVERSE VIDEO:
A display method that causes a portion of the display to appear like a negative of the regular display. If the display screen normally displays light images against a dark background, putting it in reverse video mode will cause it to display dark images against a light background.
Many programs use reverse video to highlight items, such as selected text or menu options. Also, some systems allow you to change the mode for all displays, so you can choose the display that is most comfortable for you. Some people prefer dark images on a light background, while others prefer light images on a dark background.

RGB MONITOR:
Short for red, green, blue monitor, a monitor that requires separate signals for each of the three colors. This differs from color televisions, for example, which use composite video signals, in which all the colors are mixed together. All color computer monitors are RGB monitors.
An RGB monitor consists of a vacuum tube with three electron guns -- one each for red, green, and blue -- at one end and the screen at the other end. The three electron guns fire electrons at the screen, which contains a phosphorous coating. When the phosphors are excited by the electron beams, they glow. Depending on which beam excites them, they glow either red, green, or blue. Ideally, the three beams should converge for each point on the screen so that each pixel is a combination of the three colors.

SCREEN FLICKER:
The phenomenon whereby a display screen appears to flicker. Screen flicker results from a variety of factors, the most important of which is the monitor's refresh rate, the speed with which the screen is redrawn. If the refresh rate is too slow, the screen will appear to glimmer. Another factor that affects screen flicker is the persistence of the screen phosphors. Low-persistence phosphors fade more quickly than high-persistence monitors, making screen flicker more likely. Screen flicker can also be affected by lighting. Finally, screen flicker is a subjective perception that affects people differently. Some people perceive screen flicker where others do not.

SCREEN SAVER:
A small program that takes over the display screen if there are no keystrokes or mouse movements for a specified duration. Screen savers were originally developed to prevent ghosting, the permanent etching of a pattern on a display screen. For older monochrome monitors, ghosting often occurred if the same pattern was displayed on a display screen for a long period of time. Screen savers would prevent this by either blanking out the screen entirely or by displaying a constantly moving image.
Modern display screens do not suffer so much from this problem. Today, therefore, screen savers are mostly an adornment, a way to liven up the computer. Many screen savers provide another benefit, hiding a user’s work from would-be snoopers. These screen savers fill the display with an image or animation until the user enters a password.

SCROLL:
To view consecutive lines of data on the display screen. The term scroll means that once the screen is full, each new line appears at the edge of the screen and all other lines move over one position. For example, when you scroll down, each new line appears at the bottom of the screen and all the other lines move up one row, so that the top line disappears. The term vertical scrolling refers to the ability to scroll up or down. Horizontal scrolling means that the image moves sideways.
In theory, the display should move smoothly, as if it were a piece of paper being moved up, down, or sideways. In practice, however, scrolling is not always so smooth.The scrolling method of viewing documents does not recognize page boundaries. One advantage to scrolling, therefore, is that you can look at the end of one page and the beginning of the next page at the same time. Another method of viewing data is called paging, whereby an entire page is displayed at once. Each subsequent page replaces the previous page on the screen.

SMART TERMINAL:
A terminal that has some processing capabilities, but not as many as an intelligent terminal. Smart terminals have built-in logic for performing simple display operations, such as blinking and boldface. In contrast, a dumb terminal has no processing capabilities at all.

SUPERTWIST:
A technique for improving LCD display screens by twisting light rays. In addition to normal supertwist displays, there also exist double supertwist and triple supertwist displays. In general, the more twists, the higher the contrast.Supertwist displays are also known as supertwist nematic (STN) displays.

SVGA:
Short for Super VGA, a set of graphics standards designed to offer greater resolution than VGA. There are several varieties of SVGA, each providing a different resolution:
· 800 by 600 pixels
· 1024 by 768 pixels
· 1280 by 1024 pixels
· 1600 by 1200 pixels
All SVGA standards support a palette of 16 million colors, but the number of colors that can be displayed simultaneously is limited by the amount of video memory installed in a system. One SVGA system might display only 16 simultaneous colors while another displays the entire palette of 16 million colors. The SVGA standards are developed by a consortium of monitor and graphics manufacturers called VESA.

TERMINAL:
A device that enables you to communicate with a computer. Generally, a terminal is a combination of keyboard and display screen. Terminals are sometimes divided into three classes based on how much processing power they contain:
intelligent terminal: A stand-alone device that contains main memory and a CPU.smart terminal: Contains some processing power, but not as much as an intelligent terminal.dumb terminal: Has no processing capabilities. It relies entirely on the computer's processor.
In networking, a terminal is a personal computer or workstation connected to a mainframe. The personal computer usually runs terminal emulation software that makes the mainframe think it is like any other mainframe terminal.

TIGA:
A high-resolution graphics specification designed by Texas Instruments. Unlike other graphics standards, TIGA does not specify a particular resolution or number of colors. Instead, it defines an interface between software and graphics processors. Programs written for TIGA, therefore, should be able to run on future systems that conform to the TIGA standard, regardless of resolution and color specifics.
Currently, the only graphics standard that conforms to TIGA is TI 34010, which defines a resolution of 1,024 by 768, with 256 simultaneous colors. Two competing standards with the same resolution are 8514/A from IBM and SVGA from VESA.

TFT:
Abbreviation of thin film transistor, a type of LCD flat-panel display screen, in which each pixel is controlled by from one to four transistors. The TFT technology provides the best resolution of all the flat-panel techniques, but it is also the most expensive. TFT screens are sometimes called active-matrix LCDs.

TI 34010:
A video standard from Texas Instruments that supports a resolution of 1,024 by 768. TI 34010 conforms to TI's Graphics Architecture (TIGA). Unlike IBM's 8514/A, which supports the same resolution, TI 34010 is noninterlaced.

TRUE COLOR:
Refers to any graphics device or software that uses at least 24 bits to represent each dot or pixel. Using 24 bits means that more than 16 million unique colors can be represented. Since humans can only distinguish a few million colors, this is more than enough to accurately represent any color image.

TOUCH SCREEN:
A type of display screen that has a touch-sensitive transparent panel covering the screen. Instead of using a pointing device such as a mouse or light pen, you can use your finger to point directly to objects on the screen. Although touch screens provide a natural interface for computer novices, they are unsatisfactory for most applications because the finger is such a relatively large object. It is impossible to point accurately to small areas of the screen. In addition, most users find touch screens tiring to the arms after long use.

TRUE COLOR:
Refers to any graphics device or software that uses at least 24 bits to represent each dot or pixel. Using 24 bits means that more than 16 million unique colors can be represented. Since humans can only distinguish a few million colors, this is more than enough to accurately represent any color image.

TTL MONITOR:
TTL stands for transistor-transistor logic and refers to a special type of digital circuit. More commonly, however, TTL is used to designate any type of digital input or device. A TTL monitor, therefore, is a monitor that accepts digital input. TTL monitors are consistent with older graphics standards such as MDA, but all newer graphics standards, including VGA, require analog signals. Some monitors can accept both types of signal.

VDT RADIATION:
The radiation emitted by video display terminals. Like televisions, computer monitors emit various types of radiation. Since the late 1980s, there has been a public debate about whether this radiation poses a health problem. To date, however, there is no conclusive evidence to settle the question once and for all.

VESA:
Short for Video Electronics Standards Association, a consortium of video adapter and monitor manufacturers whose goal is to standardize video protocols. VESA has developed a family of video standards that offer greater resolution and more colors than VGA. These standards are known collectively as Super VGA (SVGA).

VGA:
Abbreviation of video graphics array, a graphics display system for PCs developed by IBM. VGA has become one of the de facto standards for PCs. In text mode, VGA systems provide a resolution of 720 by 400 pixels. In graphics mode, the resolution is either 640 by 480 (with 16 colors) or 320 by 200 (with 256 colors). The total palette of colors is 262,144.
Unlike earlier graphics standards for PCs -- MDA, CGA, and EGA -- VGA uses analog signals rather than digital signals. Consequently, a monitor designed for one of the older standards will not be able to use VGA.Since its introduction in 1987, several other standards have been developed that offer greater resolution and more colors (see SVGA, 8514/A graphics standard, and XGA), but VGA remains the lowest common denominator. All PCs made today support VGA, and possibly some other more advanced standard.

VIDEO ADAPTER:
A board that plugs into a personal computer to give it display capabilities. The display capabilities of a computer, however, depend on both the logical circuitry (provided in the video adapter) and the display monitor. A monochrome monitor, for example, cannot display colors no matter how powerful the video adapter. Many different types of video adapters are available for PCs. Most conform to one of the video standards defined by IBM or VESA.
Each adapter offers several different video modes. The two basic categories of video modes are text and graphics. In text mode, a monitor can display only ASCII characters. In graphics mode, a monitor can display any bit-mapped image. Within the text and graphics modes, some monitors also offer a choice of resolutions. At lower resolutions a monitor can display more colors.
Most modern video adapters contain memory, so that the computer's RAM is not used for storing displays. In addition, some adapters have their own graphics coprocessor for performing graphics calculations. These adapters are often called graphics accelerators. Video adapters are also called video cards, video boards, and video display boards.

VIDEO OVERLAY:
The placement of a full-motion video window on the display screen. There are various techniques used to display video on a computer's screen, depending on whether the video source has been digitized or is still in analog NTSC format.Since computer monitors are generally analog, NTSC video can be merged with signals coming from the display adapter. Increasingly, faster computer buses (PCI, VL-bus, etc.) and faster video busses (Advanced Feature Connector, VM Channel, etc.), allow for analog video to be digitized and stored with other binary data for output. Then the display adapter turns it into analog scan lines for the monitor.

VIDEO STANDARDS:
There are a variety of video standards that define the resolution and colors for displays. Support for a graphics standard is determined both by the monitor and by the video adapter. The monitor must be able to show the resolution and colors defined by the standard, and the video adapter must be capable of transmitting the appropriate signals to the monitor. Listed here, in approximate order of increasing power and sophistication, are the more popular video standards for PCs. Note that many of these numbers represent only the minimums specified in the standards. Many suppliers of video adapters provide greater resolution and more colors. For more information, refer to the entries for the specific graphics systems.

WRAM:
Short for Window RAM, a type of RAM developed by Samsung Electronics that supports two ports. This enables a graphics adapter to fetch the contents of memory for display at the same time that new bytes are being pumped into memory. This results in much faster display than is possible with conventional single-port RAM.WRAM is similar to VRAM, but achieves even faster performance at less cost.

XGA:
Short for extended graphics array, a high-resolution graphics standard introduced by IBM in 1990. XGA was designed to replace the older 8514/A video standard. It provides the same resolutions (640 by 480 or 1024 by 768 pixels), but supports more simultaneous colors (65 thousand compared to 8514/A’s 256 colors). In addition, XGA allows monitors to be non-interlaced.

DISPLAY MONITORS & VIDEO ADAPTERS 1

8514/A:
A high-resolution video standard for PCs developed by IBM in 1987. It is designed to extend the capabilities of VGA. The 8514/A standard provides a resolution of 1,024 by 768 pixels, which gives it about 2.5 times the pixels of VGA (640 by 480). Like VGA, 8514/A provides a palette of 262,000 colors, of which 256 can be displayed at one time. On monochrome displays, 8514/A provides 64 shades of gray.
In its original version, 8514/A relies on interlacing, a technique that makes it possible to provide resolution at low cost. Interlacing, however, carries a performance penalty, so many manufacturers produce noninterlaced 8514/A clones.
In 1990, IBM released the Extended Graphics Array (XGA) standard, which supersedes 8514/A.

ACTIVE MATRIX DISPLAY:
A type of flat-panel display in which the screen is refreshed more frequently than in conventional passive-matrix displays. The most common type of active-matrix display is based on a technology known as TFT (thin film transistor). The two terms, active matrix and TFT, are often used interchangeably.

ANALOG MONITOR:
The traditional type of color display screen that has been used for years in televisions. In reality, all monitors based on CRT technology (that is, all monitors except flat-panel displays) are analog. Some monitors, however, are called digital monitors because they accept digital signals from the video adapter. EGA monitors, for example, must be digital because the EGA standard specifies digital signals. Digital monitors must nevertheless translate the signals into an analog form before displaying images. Some monitors can accept both digital and analog signals.
Most analog monitors are designed to accept signals at a precise frequency. They are therefore called fixed-frequency monitors. However, a special type of monitor, called a multiscanning or multisync monitor, automatically adjusts itself to the frequency of the signals being sent to it. A third type of monitor, called a multifrequency monitor, is designed to accept signals at two or more preset frequency levels.

ASPECT RATIO:
In computer graphics, the relative horizontal and vertical sizes. For example, if a graphic has an aspect ratio of 2:1, it means that the width is twice as large as the height. When resizing graphics, it is important to maintain the aspect ratio to avoid stretching the graphic out of proportion.

AUTOSIZING:
Refers to a monitor's ability to accept signals at one resolution and display the image at a different resolution. For example, a VGA video card outputs images at a resolution of 640 by 480. An SVGA monitor, however, can display images at a resolution of only 800 by 600, or even 1,024 by 768. If the monitor supports autosizing, it automatically adjusts the size of the image so that the proportions are correct. Without autosizing, you need to adjust an image manually so that it fills the screen properly.

BACKLIGHTING:
A technique used to make flat-panel displays easier to read. A backlit display is illuminated so that the foreground appears sharper in contrast with the background.

CGA:
Abbreviation of color/graphics adapter, an old graphics system for PCs. Introduced in 1981 by IBM, CGA was the first color graphics system for IBM PCs. Designed primarily for computer games, CGA does not produce sharp enough characters for extended editing sessions. CGA's highest-resolution mode is 2 colors at a resolution of 640 by 200.
CGA has been superseded by VGA systems, which are backward compatible with CGA.

CODEC:
Short for compressor/decompressor, a codec is any technology for compressing and decompressing data. Codecs can be implemented in software, hardware, or a combination of both. Some popular codecs for computer video include MPEG, Indeo, Cinepak, QuickTime, and Video for Windows.
In telecommunications, a device that encodes or decodes a signal. For example, telephone companies use codecs to convert binary signals transmitted on their digital networks to analog signals converted on their analog networks.
The translation of a binary value into a voltage that can be transmitted over a wire.

COLOR DEPTH:
The number of distinct colors that can be represented by a piece of hardware or software. Color depth is sometimes referred to as bit depth because it is directly related to the number of bits used for each pixel. A 24-bit video adapter, for example, has a color depth of 2 to the 24th power (about 16.7 million) colors.

COLOR MONITOR:
A display monitor capable of displaying many colors. In contrast, a monochrome monitor can display only two colors -- one for the background and one for the foreground. Color monitors implement the RGB color model by using three different phosphors that appear red, green, and blue when activated. By placing the phosphors directly next to each other, and activating them with different intensities, color monitors can create an unlimited number of colors. In practice, however, the real number of colors that any monitor can display is controlled by the video adapter.

COMPOSITE VIDEO:
A type of video signal in which all information -- the red, blue, and green signals (and sometimes audio signals as well) -- are mixed together. This is the type of signal used by televisions in the United States (see NTSC).
In contrast, most computers use RGB video, which consists of three separate signals for red, green, and blue. In general, RGB video produces sharper images than composite video does.

CONVERGENCE:
The coming together of two or more disparate disciplines or technologies. For example, the so-called fax revolution was produced by a convergence of telecommunications technology, optical scanning technology, and printing technology.
In graphics, convergence refers to how sharply an individual color pixel on a monitor appears. Each pixel is composed of three dots -- a red, blue, and green one. If the dots are badly misconverged, the pixel will appear blurry. All monitors have some convergence errors, but they differ in degree.

CRT:
Abbreviation of cathode-ray tube, the technology used in most televisions and computer display screens. A CRT works by moving an electron beam back and forth across the back of the screen. Each time the beam makes a pass across the screen, it lights up phosphor dots on the inside of the glass tube, thereby illuminating the active portions of the screen. By drawing many such lines from the top to the bottom of the screen, it creates an entire screenful of images.

CSTN:
Short for color super-twist nematic, an LCD technology developed by Sharp Electronics Corporation. Unlike TFT, CSTN is based on a passive matrix, which is less expensive to produce. The original CSTN displays developed in the early 90's suffered from slow response times and ghosting. Recent advances in the technology, however, have made CSTN a viable alternative to active-matrix displays. New CSTN displays offer 100ms response times, a 140 degree viewing angle, and high-quality color rivaling TFT displays - all at about half the cost.

DDC:
Short for Display Data Channel, a VESA standard for communication between a monitor and a video adapter. Using DDC, a monitor can inform the video card about its properties, such as maximum resolution and color depth. The video card can then use this information to ensure that the user is presented with valid options for configuring the display.

DIGITAL MONITOR:
A monitor that accepts digital rather than analog signals. All monitors (except flat-panel displays) use CRT technology, which is essentially analog. The term digital, therefore, refers only to the type of input received from the video adapter. A digital monitor then translates the digital signals into analog signals that control the actual display.
Although digital monitors are fast and produce clear images, they cannot display continuously variable colors. Consequently, only low-quality video standards, such as MDA, CGA, and EGA, specify digital signals. VGA and SVGA, on the other hand, require an analog monitor. Some monitors are capable of accepting either analog or digital signals.

DISP SCREEN:
The display part of a monitor. Most display screens work under the same principle as a television, using a cathode ray tube (CRT). Consequently, the term CRT is often used in place of display screen.

DLP:
Short for Digital Light Processing, a new technology developed by Texas Instruments used for projecting images from a monitor onto a large screen for presentations. Prior to the development of DLP, most computer projection systems were based on LCD technology which tends to produce faded and blurry images. DLP uses tiny mirrors housed on a special kind of microchip called a Digital Micromirror Device (DMD). The result is sharp images that can be clearly seen even in a normally lit room.

DOT PITCH:
A measurement that indicates the vertical distance between each pixel on a display screen. Measured in millimeters, the dot pitch is one of the principal characteristics that determines the quality of display monitors. The lower the number, the crisper the image. The dot pitch of color monitors for personal computers ranges from about 0.22 mm to 0.42 mm.

DSTN:
Short for double-layer supertwist nematic, a passive-matrix LCD technology that uses two display layers to counteract the color shifting that occurs with conventional supertwist displays.

DUAL -SCAN DISPLAY:
A type of passive-matrix LCD display that provides faster refresh rates that conventional passive-matrix displays by dividing the screen into two sections that are refreshed simultaneously. Dual-scan displays are not as sharp or bright as active-matrix displays, but they consume less power.

EGA:
Abbreviation of enhanced graphics adapter, a graphics display system for PCs introduced by IBM in 1984. EGA supports 16 colors from a palette of 64 and provides a resolution of 640 by 350. This is better than CGA but not as good as VGA.

ELD:
A technology used to produce a very thin display screen, called a flat-panel display, used in some portable computers. An ELD works by sandwiching a thin film of phosphorescent substance between two plates. One plate is coated with vertical wires and the other with horizontal wires, forming a grid. When an electrical current is passed through a horizontal and vertical wire, the phosphorescent film at the intersection glows, creating a point of light, or pixel.
Other types of flat-panel displays include LCD displays and gas-plasma displays, both of which are more common than ELDs.

ELF EMISSION:
ELF stands for extremely low frequency, and ELF emissions are magnetic fields generated by common electrical appliances. There is considerable debate about whether ELF emissions from computer monitors pose a threat. Some European countries have adopted regulations controlling the amount of allowable emission. The most well-know regulation is Sweden’s MPR II standard. You can play it safe by buying MPR II-compliant monitors.

FIXED FREQUENCY MONITOR:
A monitor that can only accept signals in one frequency range. In contrast, multiscanning monitors automatically adjust themselves to the frequency at which data is being sent.

FLAT TECHNOLOGY MONITOR:
Often abbreviated as FTM, flat technology monitors are monitors that have a flat display screen to reduce glare. Conventional display screens are curved, which makes them more susceptible to reflections from external light sources.
Do not confuse flat technology monitors with flat-panel displays. Flat-panel displays are the display screens used in laptops and other portable computers.

FLAT PANEL DISPLAY:
A very thin display screen used in portable computers. Nearly all modern flat-panel displays use LCD technologies. Most LCD screens are backlit to make them easier to read in bright environments.

GAS PLASMA DISPLAY:
A type of thin display screen, called a flat-panel display, used in some portable computers. A gas-plasma display works by sandwiching neon gas between two plates. Each plate is coated with a conductive print. The print on one plate contains vertical conductive lines and the other plate has horizontal lines. Together, the two plates form a grid. When electric current is passed through a horizontal and vertical line, the gas at the intersection glows, creating a point of light, or pixel. You can think of a gas-plasma display as a collection of very small neon bulbs. Images on gas-plasma displays generally appear as orange objects on top of a black background.
Although gas-plasma displays produce very sharp monochrome images, they require much more power than the more common LCD displays.

GENLOCK:
Short for generator locking device, a genlock is a device that enables a composite video machine, such as a TV, to accept two signals simultaneously. A genlock locks one set of signals while it processes the second set. This enables you to combine graphics from a computer with video signals from a second source such as a video camera.

GRAPHICS ACCELERATOR:
A special type of video adapter that contains its own processor to boost performance levels. These processors are specialized for computing graphical transformations, so they achieve better results than the general-purpose CPU used by the computer. In addition, they free up the computer’s CPU to execute other commands while the graphics accelerator is handling graphics computations.
The popularity of graphical applications, and especially multimedia applications, has made graphics accelerators not only a common enhancement, but a necessity. Most computer manufacturers now bundle a graphics accelerator with their mid-range and high-end systems.
Aside from the graphics processor used, the other characteristics that differentiate graphics accelerators are:
memory: Graphics accelerators have their own memory, which is reserved for storing graphical representations. The amount of memory determines how much resolution and how many colors can be displayed. Some accelerators use conventional DRAM, but others use a special type of video RAM (VRAM), which enables both the video circuitry and the processor to simultaneously access the memory.
bus: Each graphics accelerator is designed for a particular type of video bus. As of 1995, most are designed for the PCI bus.
register width: The wider the register, the more data the processor can manipulate with each instruction. 64-bit accelerators are already becoming common, and we can expect 128-bit accelerators in the near future.

GRAPHICS COPROCESSOR:
A microprocessor specially designed for handling graphics computations. Most graphics accelerators include a graphics coprocessor.

GRAPHICS MODE:
Many video adapters support several different modes of resolution, all of which are divided into two general categories: character mode and graphics mode. In character mode, the display screen is treated as an array of blocks, each of which can hold one ASCII character. In graphics mode, the display screen is treated as an array of pixels. Characters and other shapes are formed by turning on combinations of pixels.
Of the two modes, graphics mode is the more sophisticated. Programs that run in graphics mode can display an unlimited variety of shapes and fonts, whereas programs running in character mode are severely limited. Programs that run entirely in graphics mode are called graphics-based programs.

GRAY SCALING:
The use of many shades of gray to represent an image. Continuous-tone images, such as black-and-white photographs, use an almost unlimited number of shades of gray. Conventional computer hardware and software, however, can only represent a limited number of shades of gray (typically 16 or 256). Gray-scaling is the process of converting a continuous-tone image to an image that a computer can manipulate.
While gray scaling is an improvement over monochrome, it requires larger amounts of memory because each dot is represented by from 4 to 8 bits. At a resolution of 300 dpi, you would need about 6 megabytes to represent a single 8½ by 11-inch page using 256 shades of gray. This can be reduced somewhat through data compression techniques, but gray scaling still requires a great deal of memory.
Many optical scanners are capable of gray scaling, using from 16 to 256 different shades of gray. However, gray scaling is only useful if you have an output device -- monitor or printer -- that is capable of displaying all the shades. Most color monitors are capable of gray scaling, but the images are generally not as good as on dedicated gray-scaling monitors.
Note that gray scaling is different from dithering. Dithering simulates shades of gray by altering the density and pattern of black and white dots. In gray scaling, each individual dot can have a different shade of gray.

HERCULES GRAPHICS:
A graphics display system for PCs developed by Van Suwannukul, founder of Hercules Computer Technology. Suwannukul developed the system so that he could produce his doctoral thesis on PC equipment using his native Thai alphabet.
First offered in 1982, the original Hercules system filled a void left by IBM's MDA (monochrome display adapter) system. MDA produces high-resolution monochrome text but cannot generate graphics. Hercules systems generate both high-resolution text and graphics for monochrome monitors. The resolution is 720 by 348.

DISK STORAGE 2

HALF HEIGHT:
PCs support both full-height and half-height bays for disk drives and other mass storage devices. The half-height models take up less space but usually have less storage capacity.

HALF CARD:
A hard disk drive and controller on an expansion card. Unlike most disk drives that are either external to the computer or fit in one of the disk drive bays, a hard card slips into an expansion slot. Hard cards are often faster than conventional disk drives, and easier to install. Their storage capacities, however, are more limited.

HARD DISK:
A magnetic disk on which you can store computer data. The term hard is used to distinguish it from a soft, or floppy, disk. Hard disks hold more data and are faster than floppy disks. A hard disk, for example, can store anywhere from 10 megabytes to several gigabytes, whereas most floppies have a maximum storage capacity of 1.4 megabytes.
A single hard disk usually consists of several platters. Each platter requires two read/write heads, one for each side. All the read/write heads are attached to a single access arm so that they cannot move independently. Each platter has the same number of tracks, and a track location that cuts across all platters is called a cylinder. For example, a typical 84 megabyte hard disk for a PC might have two platters (four sides) and 1,053 cylinders.
In general, hard disks are less portable than floppies, although it is possible to buy removable hard disks. There are two types of removable hard disks: disk packs and removable cartridges.

HARD DISK DRIVE:
The mechanism that reads and writes data on a hard disk. Hard disk drives for PCs generally have access times of about 15 milliseconds or less. Many disk drives improve their performance through a technique called caching.
The disk drives for hard disks are called hard disk drives or Winchester drives, Winchester being the name of one of the first popular hard disk drive technologies developed by IBM in 1973.
There are several interface standards for passing data between a hard disk and a computer. The most common are:
ST-506: The standard interface on all PC/XT and PC/AT computers. The ST-506 standard supports both MFM and RLL encoding formats.
SCSI (Small Computer System Interface): The standard interface for Apple Macintoshes, SCSI is also popular on UNIX systems and is available on many PC compatibles.IDE (Integrated Drive Interface): Not as fast as SCSI, but faster than ST-506.
Enhanced IDE: A new version of the IDE interface that supports data transfer rates comparable to SCSI.

HARD DISK TYPE:
A number that indicates important features of a hard disk, such as the number of platters and cylinders. Hard disk manufacturers have agreed on a numbering scheme so that it is possible to install and configure a new hard disk simply by entering the disk’s type number into the BIOS setup.

HEAD:
The mechanism that reads data from or writes data to a magnetic disk or tape. If the head becomes dirty, it will not work properly. This is one of the first things to check if your disk drive or tape drive begins to malfunction.
The head is sometimes called a read/write head. Double-sided floppy disk drives have two heads, one for each side of the disk. Hard disk drives have many heads, two for each platter.

HEAD CRASH:
A serious disk drive malfunction. A head crash usually means that the head has scratched or burned the disk. In a hard disk drive, the head normally hovers a few microinches from the disk. If the head becomes misaligned or if dust particles come between it and the disk, it can touch the disk. When this happens, you usually lose much of the data on the hard disk and will need to replace both the head and the disk. For this reason, it is important to operate disk drives, particularly hard disk drives, in as clean an environment as possible. Even smoke particles can cause a head crash.
Head crashes are less common for floppy disks because the head touches the disk anyway under normal operation.
Another term for head crash is disk crash.

HELICAL SCAN CARTRIDGE:
A type of magnetic tape that uses the same technology as VCR tapes. The term helical scan usually refers to 8 mm tapes, although 4 mm tapes (called DAT tapes) use the same technology. The 8 mm helical-scan tapes have data capacities from 2.5GB to 5GB.

HIGH DENSITY DISK:
A high-quality floppy disk capable of holding more data than a double-density disk. High-density 5¼-inch disks for PCs can hold 1.2MB (megabytes) of data. High-density 3½-inch disks can store 1.44MB.

IDE INTERFACE:
Abbreviation of either Intelligent Drive Electronics or Integrated Drive Electronics, depending on who you ask. An IDE interface is an interface for mass storage devices, in which the controller is integrated into the disk or CD-ROM drive. It is a low-cost alternative to SCSI interfaces.
The original IDE interface supports data transfer rates of about 3.3 megabytes per second and has a limit of 538 megabytes per device. However, a recent version of IDE, called enhanced IDE (EIDE) or Fast IDE, supports data transfer rates of about 12 megabytes per second and storage devices of up to 8.4 gigabytes. These numbers are comparable to what SCSI offers. To take advantage of EIDE, you need both an EIDE mass storage device and an EIDE interface installed in your computer.

INTERLEAVING:
Refers to the way sectors on a disk are organized. In one-to-one interleaving, the sectors are placed sequentially around each track. In two-to-one interleaving, sectors are staggered so that consecutively numbered sectors are separated by an intervening sector.
The purpose of interleaving is to make the disk drive more efficient. The disk drive can access only one sector at a time, and the disk is constantly spinning beneath the read/write head. This means that by the time the drive is ready to access the next sector, the disk may have already spun beyond it. If a data file spans more than one sector and if the sectors are arranged sequentially, the drive will need to wait a full rotation to access the next chunk of the file. If instead the sectors are staggered, the disk will be perfectly positioned to access sequential sectors.
The optimum interleaving factor depends on the speed of the disk drive, the operating system, and the application. The only way to find the best interleaving factor is to experiment with various factors and various applications.

JAZ DRIVE:
A removable disk drive developed by Iomega Corporation. The Jaz drive has a 12-ms average seek time and a transfer rate of 5.5Mbps. The removable cartridges hold 1GB of data. The fast data rates and large storage capacity make it a viable alternative for backup storage as well as everyday use.

LBA:
A removable disk drive developed by Iomega Corporation. The Jaz drive has a 12-ms average seek time and a transfer rate of 5.5Mbps. The removable cartridges hold 1GB of data. The fast data rates and large storage capacity make it a viable alternative for backup storage as well as everyday use.

LOSSLESS COMPRESION:
Refers to data compression techniques in which no data is lost. The PKZIP compression technology is an example of lossless compression. For most types of data, lossless compression techniques can reduce the space needed by only about 50%. For greater compression, one must use a lossy compression technique.

LOSSY COMPRESSION:
Refers to data compression techniques in which some amount of data is lost. Lossy compression technologies attempt to eliminate redundant or unnecessary information. Most video compression technologies, such as MPEG, use a lossy technique.

LOW-LEVEL FORMAT:
Hard disks must be formatted twice before they can be used. The first format, called a low-level or physical format, sets the interleave factor and prepares the disk for a particular type of disk controller. This is generally performed at the factory.

MO DRIVE:
A type of disk drive that combines magnetic disk technologies with CD-ROM technologies. Like magnetic disks, MO disks can be read and written to. And like floppy disks, they are removable. However, their storage capacity can be more than 200 megabytes, much greater than magnetic floppies. In terms of data access speed, they are faster than floppies and CD-ROMs, but not as fast as hard disk drives.

MASS STORAGE:
Refers to various techniques and devices for storing large amounts of data. The earliest storage devices were punched paper cards, which were used as early as 1804 to control silk-weaving looms. Modern mass storage devices include all types of disk drives and tape drives. Mass storage is distinct from memory, which refers to temporary storage areas within the computer. Unlike main memory, mass storage devices retain data even when the computer is turned off.
The main types of mass storage are:
floppy disks: Relatively slow and have a small capacity, but they are portable, inexpensive, and reliable.
hard disks: Very fast and with more capacity than floppy disks, but also more expensive. Some hard disk systems are portable (removable cartridges), but most are not.optical disks: Unlike floppy and hard disks, which use electromagnetism to encode data, optical disk systems use a laser to read and write data. Optical disks have very large storage capacity, but they are not as fast as hard disks. In addition, the inexpensive optical disk drives are read-only. Read/write varieties are very expensive.
tapes: Relatively inexpensive and can have very large storage capacities, but they do not permit random access of data.
Mass storage is measured in kilobytes (1,024 bytes), megabytes (1,024 kilobytes), and gigabytes (1,024 megabytes).

MEDIA:
Objects on which data can be stored. These include hard disks, floppy disks, CD-ROMs, and tapes.
In computer networks, media refers to the cables linking workstations together. There are many different types of transmission media, the most popular being twisted-pair wire (normal electrical wire), coaxial cable (the type of cable used for cable television), and fiber optic cable (cables made out of glass).
The form and technology used to communicate information. Multimedia presentations, for example, combine sound, pictures, and videos, all of which are different types of media.

MFM:
Abbreviation of modified frequency modulation, an encoding scheme used by some PC disk drives. A competing scheme, known as RLL (run length limited), produces faster data access speeds and can increase a disk's storage capacity by up to 50 percent.
Technically, any disk drive can use MFM or RLL. The one it uses depends on the disk controller.

MICROFLOPPY DISK:
A small floppy disk. Microfloppy disks come enclosed in hard, 3½ square-inch casings. Although smaller than older-style floppies, microfloppies have greater storage capacity.
PCs support two types of microfloppies:
· double-density microfloppies hold 720K.
· high-density microfloppies can store 1.44MB (megabytes).
For Macintosh computers, which have always used microfloppies, there are three sizes:
· Single-sided standard microfloppies hold 400K.
· Double-sided standard microfloppies hold 800 K.
· Double-sided, high-density microfloppies hold 1.44 megabytes.

MOUNT:
To make a mass storage device available. In Macintosh environments, for example, inserting a floppy disk into the drive is called mounting the floppy.
To install a device, such as a disk drive or expansion board.

MTBF:
Short for mean time between failures. MTBF ratings are measured in hours and indicate the sturdiness of hard disk drives and printers.
Typical disk drives for personal computers have MTBF ratings of about 40,000 hours. This means that at least half of the disk drives with such a rating will fail once in the first 40,000 hours of operation. Most working conditions are not ideal, so MTBF ratings can be considered as only approximate guidelines for judging the hardiness of disk drives. The fact that MTBF ratings exist at all, however, underscores the fact that every disk drive will eventually fail if run long enough.

OPTICAL DISK:
A storage medium from which data is read and to which it is written by lasers. Optical disks can store much more data -- up to 6 gigabytes (6 billion bytes) -- than magnetic media, such as floppies and hard disks. There are three basic types of optical disks:
CD-ROM: Like audio CDs, CD-ROMs come with data already encoded onto them. The data is permanent and can be read any number of times, but CD-ROMs cannot be modified.
WORM: Stands for write-once, read-many. With a WORM disk drive, you can write data onto a WORM disk, but only once. After that, the WORM disk behaves just like a CD-ROM.erasable: Optical disks that can be erased and loaded with new data, just like magnetic disks. These are often referred to as EO (erasable optical) disks.
These three technologies are not compatible with one another; each requires a different type of disk drive and disk. Even within one category, there are many competing formats, although CD-ROMs are relatively standardized.

OPTIMIZE:
In programming, to fine-tune a program so that it runs more quickly or takes up less space.
When applied to disks, the term means the same as defragment. See under fragmentation.

PARK:
To lock the read/write head of a hard disk drive in a safe position so that the disk will not be damaged while moving the drive. Parking the disk is particularly important for portable computers, which are moved frequently. The disk will automatically unpark itself once you turn the power on.
Most modern disk drives support automatic head parking, in which the drive automatically parks the head whenever the power is turned off.

PARTITION:
(v) To divide memory or mass storage into isolated sections. In DOS systems, you can partition a disk, and each partition will behave like a separate disk drive. Partitioning is particularly useful if you run more than one operating system. For example, you might reserve one partition for DOS and another for UNIX.In addition, partitioning on DOS and Windows machines can improve disk efficiency. This is because the FAT system used by these operating systems automatically assigns cluster size based on the disk size: the larger the disk, the larger the cluster. Unfortunately, large clusters can result in a wasted disk space. There is an entire sector of the software industry devoted to building utilities that let you partition your hard disk.
On Apple Macintosh computers, there are two types of partitioning: hard and soft. Hard partitioning is the same as DOS partitioning -- the disk is physically divided into different sections. Soft partitioning, on the other hand, does not physically affect the disk at all, but it fools the Finder into believing that the disk is partitioned. The advantage of this is that you can partition the disk without affecting the data on it. With hard partitioning, it is usually necessary to reformat the entire disk.
(n) A section of main memory or mass storage that has been reserved for a particular application.

PHASE CHANGE DISK:
A type of rewritable optical disk that employs the phase change recording method. Using this technique, the disk drive writes data with a laser that changes spots on the disk between amorphous and crystalline states. An optical head reads data by detecting the difference in reflected light from amorphous and crystalline spots. A medium-intensity pulse can then restore the original crystalline structure.Magneto-optical and dye-polymer technologies offer similar capabilities for developing re-writable optical disks.

PLATTER:
A round magnetic plate that constitutes part of a hard disk. Hard disks typically contain two, four, or eight platters. Each platter requires two read/write heads, one for each side.

QIC:
The abbreviation for quarter-inch cartridge. QIC tapes are among the most popular tapes used for backing up personal computers. QIC tapes are divided into two general classes: full-size and minicartridge. The full-size cartridges are often referred to as DC 6000 cartridges, and minicartridges are called DC 2000 cartridges.
The QIC-40 and QIC-80 standards are sometimes referred to as floppy tape standards because they are designed to use a personal computer's existing floppy disk drive controller instead of requiring a customized controller.
The various QIC standards are controlled by a consortium of manufacturers called the Quarter-Inch Cartridge Drive Standards, Inc. The term QIC, therefore, is used to refer both to the type of tape and to the standards -producing organization.

QUAD-SPEED CE-ROM DRIVE:
A CD-ROM drive designed to run four times as fast as original models. Often denoted as 4X CD players, they provide data transfer rates of 600K per second and data access times as low as 125 milliseconds (ms).

RAID:
Short for Redundant Array of Independent Disks, a category of disk drives that employ two or more drives in combination for fault tolerance and performance. RAID disk drives are used frequently on servers but aren’t generally necessary for personal computers.There are number of different RAID levels. The three most common are 0, 3, and 5:
Level 0: Provides data striping (spreading out blocks of each file across multiple disks) but no redundancy. This improves performance but does not deliver fault tolerance.
Level 3: Same as Level 3, but also reserves one dedicated disk for error correction data. It provides good performance and some level of fault tolerance.
Level 5: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance.

RAM DISK:
Refers to RAM that has been configured to simulate a disk drive. You can access files on a RAM disk as you would access files on a real disk. RAM disks, however, are approximately a thousand times faster than hard disk drives. They are particularly useful, therefore, for applications that require frequent disk.
Because they are made of normal RAM, RAM disks lose their contents once the computer is turned off. To use a RAM disk, therefore, you need to copy files from a real hard disk at the beginning of the session and then copy the files back to the hard disk before you turn the computer off. Note that if there is a power failure, you will lose whatever data is on the RAM disk. (Some RAM disks come with a battery backup to make them more stable.)

READ-ONLY:
Capable of being displayed, but not modified or deleted. All operating systems allow you to protect objects (disks, files, directories) with a read-only attribute that prevents other users from modifying the object.

REMOVABLE HARD DISK:
A type of disk drive system in which hard disks are enclosed in plastic or metal cartridges so that they can be removed like floppy disks. Removable disk drives combine the best aspects of hard and floppy disks. They are as fast as hard disks and have the portability of floppy disks. The storage capacity of a typical removable hard disk can range from 60MB (megabytes) to 250MB, much more than the capacity of floppy disks.

RLL:
Abbreviation of run length limited, an encoding scheme used to store data on some PC hard disks. Although RLL produces fast data access times and increases a disk's storage capacity, it is not as prevalent as another encoding scheme called MFM (modified frequency modulation).
Technically, any disk drive can use either encoding method. The one used depends on the disk controller.

SCSI:
Abbreviation of small computer system interface. Pronounced scuzzy, SCSI is a parallel interface standard used by Apple Macintosh computers, some PCs, and many UNIX systems for attaching peripheral devices to computers. All Apple Macintosh computers starting with the Macintosh Plus come with a SCSI port for attaching devices such as disk drives and printers.
SCSI interfaces provide for faster data transmission rates (up to 40 megabytes per second) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface.Although SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. For example, SCSI supports several types of connectors.
While SCSI is the only standard interface for Macintoshes, PCs support a variety of interfaces in addition to SCSI. These include IDE, enhanced IDE and ESDI for mass storage devices, and Centronics for printers. You can, however, attach SCSI devices to a PC by inserting a SCSI board in one of the expansion slots. Many high-end new PCs come with SCSI built in. Note, however, that the lack of a single SCSI standard means that some devices may not work with some SCSI boards.
The following varieties of SCSI are currently implemented:
SCSI: Uses an 8-bit bus, and supports data rates of 4 MBps
Fast SCSI: Uses an 8-bit bus, and supports data rates of 10 MBps.
Ultra SCSI: Uses an 8-bit bus, and supports data rates of 20 MBps.
Fast Wide SCSI: Uses a 16-bit bus and supports data rates of 20 MBps.
Ultra Wide SCSI: Uses a 16-bit bus and supports data rates of 40 MBps. Also called SCSI-3.

SECTOR:
The smallest unit that can be accessed on a disk. When you format a disk, the operating system divides it into tracks and sectors. The tracks are concentric circles around the disk and the sectors are segments within each circle. For example, a formatted disk might have 40 tracks, with each track divided into 10 sectors. The operating system and disk drive keep tabs on where information is stored on the disk by noting its track and sector number.
Generally, every track has the same number of sectors, but some high-capacity hard disk drives use a technique called zoned-bit recording in which tracks on the outside of the disk contain more sectors than those on the inside. A sector that cannot be used due to a physical flaw on the disk is called a bad sector.

SEEK TIME:
Refers to the time a program or device takes to locate a particular piece of data. For disk drives, the terms seek time and access time are often used interchangeably.

SEQUENTIAL ACCESS:
Refers to reading or writing data records in sequential order, that is, one record after the other. To read record 10, for example, you would first need to read records 1 through 9. This differs from random access, in which you can read and write records in any order.Some programming languages and operating systems distinguish between sequential-access data files and random-access data files, allowing you to choose between the two types. Sequential-access files are faster if you always access records in the same order. Random-access files are faster if you need to read or write records in a random order.
Devices can also be classified as sequential access or random access. For example, a tape drive is a sequential-access device because to get to point q on the tape, the drive needs to pass through points a through p. A disk drive, on the other hand, is a random-access device because the drive can access any point on the disk without passing through all intervening points.

SINGLE-DENSITY DISK:
A low-density floppy disk. All modern floppies are double-density or high-density.

ST-506 INTERFACE:
A standard interface for connecting hard disk drives to PCs. Newer standards, such as enhanced IDE and SCSI, support faster data transfer rates. ST-506 is sometimes referred to as MFM, which is the most prevalent encoding scheme used on ST-506 disk drives. ST-506 also supports the RLL encoding format.

SUPER DRIVE:
The common name for the FDHD (floppy disk, high density) disk drive that comes with all models of the Apple Macintosh computer. The SuperDrive can read and write to all three Macintosh disk sizes (400K, 800K, and 1.2MB) as well as the two PC 3½-inch disk sizes (720K and 1.44MB).

TAPE:
A magnetically coated strip of plastic on which data can be encoded. Tapes for computers are similar to tapes used to store music. Some personal computers, in fact, enable you to use normal cassette tapes.Storing data on tapes is considerably cheaper than storing data on disks. Tapes also have large storage capacities, ranging from a few hundred kilobytes to several gigabytes. Accessing data on tapes, however, is much slower than accessing data on disks. Tapes are sequential-access media, which means that to get to a particular point on the tape, the tape must go through all the preceding points. In contrast, disks are random-access media because a disk drive can access any point at random without passing through intervening points.
Because tapes are so slow, they are generally used only for long-term storage and backup. Data to be used regularly is almost always kept on a disk. Tapes are also used for transporting large amounts of data.Tapes come in a variety of sizes and formats.Tapes are sometimes called streamers or streaming tapes.

TAPE DRIVE:
A device, like a tape recorder, that reads data from and writes it onto a tape. Tape drives have data capacities of anywhere from a few hundred kilobytes to several gigabytes. Their transfer speeds also vary considerably. Fast tape drives can transfer as much as 20MB (megabytes) per minute.The disadvantage of tape drives is that they are sequential-access devices, which means that to read any particular block of data, you need to read all the preceding blocks. This makes them much too slow for general-purpose storage operations. However, they are the least expensive media for making backups.

TPI:
Short for tracks per inch, the density of tracks on a disk. For example, double-density 5.25-inch floppies have a TPI of 48, while high-density floppies record 96 TPI. High-density 3.5-inch diskettes are formatted with 135 TPI.

TRACK:
A ring on a disk where data can be written. A typical floppy disk has 80 (double-density) or 160 (high-density) tracks. For hard disks, each platter is divided into tracks, and a single track location that cuts through all platters is called a cylinder. Hard disks can have anywhere from 400 to more than 1000 cylinders. Each track is further divided into a number of sectors. The operating system and disk drive remember where information is stored by noting its track and sector numbers.
The density of tracks (how close together they are) is measured in terms of tracks per inch (TPI).

ULTRA ATA:
The newest version of the AT Attachment (ATA) standard, which supports burst mode data transfer rates of 33.3 MBps. To take advantage of these high speeds, your computer must also be equipped with Ultra DMA, a protocol that supports faster data transfer rates to and from hard disk drives.

ULTRA DMA:
A protocol developed by Quantum Corporation and Intel that supports burst mode data transfer rates of 33.3 MBps. This is twice as fast as the previous disk drive standard for PCs, and is necessary to take advantage of new, faster Ultra ATA disk drives.The official name for the protocol is Ultra DMA/33.

VOLUME:
A fixed amount of storage on a disk or tape. The term volume is often used as a synonym for the storage medium itself, but it is possible for a single disk to contain more than one volume or for a volume to span more than one disk.

WINCHESTER DISK DRIVE:
Another term for hard disk drive. The term Winchester comes from an early type of disk drive developed by IBM that stored 30MB (megabytes) and had a 30-millisecond access time; so its inventors called it a Winchester in honor of the.30-caliber rifle of the same name. Although modern disk drives are faster and hold more data, the basic technology is the same, so Winchester has become synonymous with hard.

WORM:
Refers to an optical disk technology that allows you to write data onto a disk just once. After that, the data is permanent and can be read any number of times. A single WORM disk can store from 600MB (megabytes) (megabytes) to over 3GB (gigabytes) of data. Unlike CD-ROMs, there is no single standard for WORM disks, which means that they can only be read by the same type of drive that wrote them. This has hampered their acceptance, although they have found a niche market as an archival media.

ZIP DRIVE:
A high-capacity floppy disk drive developed by Iomega Corporation. Zip disks are slightly larger than conventional floppy disks, and about twice as thick. They can hold 100MB of data. Because they're relatively inexpensive and durable, they have become a popular media for backing up hard disks and for transporting large files.

DATA STORAGE 1

3480,3490:
The IBM designation for families of half-inch magnetic tape drives typically used on mainframes and AS/400s. The 3480 drives use 18-track cartridges at 38000 bpi to yield 200MB. The 3490 uses built-in compression to obtain 400MB. The 3490e records 36 tracks and uses longer tape to hold 800MB. Tape libraries are available that hold from a handful to thousands of cartridges.The 3490 drives provide the fastest transfer rates in the tape backup business, generally at least 3MB/sec. Drives are available that use the ESCON and Fast SCSI-2 interfaces to obtain up to 20MB/sec.

ACCESS TIME:
The time a program or device takes to locate a single piece of information and make it available to the computer for processing. DRAM (dynamic random access memory) chips for personal computers have access times of 50 to 150 nanoseconds (billionths of a second). Static RAM (SRAM) has access times as low as 10 nanoseconds. The access time of memory should be fast enough to keep up with the CPU. If not, the CPU will waste a certain number of clock cycles, which makes it slower.
Note, however, that reported access times can be misleading because most memory chips, especially DRAM chips, require a pause between back-to-back accesses. This is one reason why SRAM is so much faster than DRAM, even when the reported access times are equivalent; SRAM requires fewer refreshes, so the pause between back-to-back accesses is smaller. A more important measurement of a chip’s speed, therefore, is its cycle time, which measures how quickly two back-to-back accesses can be made.
Access time is also frequently used to describe the speed of disk drives. Disk access times are measured in milliseconds (thousandths of a second), often abbreviated as ms. Fast hard disk drives for personal computers boast access times of about 9 to 15 milliseconds. This means that the drive can make about 500 data accesses per second. Note that this is about 200 times slower than average DRAM.
The access time for disk drives (also called the seek time) refers to the time it actually takes for the read/write head to locate a sector on the disk. This is an average time since it depends on how far away the head is from the desired data. The performance of disk drives can be improved through special techniques such as caching and interleaving.

AREAL DENSITY:
The amount of data that can be packed onto a storage medium. Areal densities are generally measured in megabits per square inch. The term is useful for comparing different types of media, such as magnetic disks and optical disks. Current magnetic disks have an areal density of about 200 megabits per square inch. Optical disks, on the other hand, have areal densities of more than one gigabit per square inch.

ATA:
Short for AT Attachment, a disk drive implementation that integrates the controller on the disk drive itself. There are several versions of ATA, all developed by the Small Form Factor (SFF) Committee:
ATA: Known also as ATAPI (AT Attachment Packet Interface) and IDE.ATA-2: Known as EIDE (Western Digital's implementation) and Fast-ATA (Seagate's implementation)ATA-3: Also known as Ultra ATA, supports data transfer rates of 33 MBps.

BACKUP:
To copy files to a second medium (a disk or tape) as a precaution in case the first medium fails. One of the cardinal rules in using computers is
Back up your files regularly.
Even the most reliable computer is apt to break down eventually. Many professionals recommend that you make two, or even three, backups of all your files. To be especially safe, you should keep one backup in a different location from the others.
You can back up files using operating system commands, or you can buy a special-purpose backup utility. In general, backup programs are much faster than operating system shell commands because they use a DMA channel. In addition, the backup programs often compress the data so that backups require fewer disks. (n) (1) The act of backing up.
A substitute or alternative. The term backup usually refers to a disk or tape that contains a copy of data.

BAD SECTOR:
A portion of a disk that cannot be used because it is flawed. When you format a disk, the operating system identifies any bad sectors on the disk and marks them so they will not be used. If a sector that already contains data becomes damaged, you will need special software to recover the data.
Almost all hard disks come with bad sectors (sectors damaged during the manufacturing process), so do not be alarmed if a utility reports that your hard disk has bad sectors. However, additional bad sectors should occur only infrequently if your drive is functioning properly. Floppy disks should not have any bad sectors.

BERNOULLI DISK DRIVE:
Named after a Swiss scientist who discovered the principle of aerodynamic lift, the Bernoulli disk drive is a special type of floppy disk drive from Iomega Corporation that is faster and has greater storage capacity than traditional floppy drives.
A Bernoulli drive is really a cross between a hard disk drive and floppy drive. Like the platters in hard disk drives, Bernoulli disks float between the read/write heads, so there is no actual contact between the disk and the heads. But the disk itself is flexible and removable like a floppy disk. Because the disk is flexible, it is less susceptible than a hard disk to head crashes. Bernoulli disk drives, however, are not as fast as hard disk drives.
A Bernoulli disk drive is sometimes called a Bernoulli box.

CAV:
Short for Constant Angular Velocity, a technique for accessing data off CD-ROMs. With CAV, the disk rotates at a constant speed regardless of what area of the disk is being accessed. This differs from Constant Linear Velocity (CLV), which rotates the disk faster for inner tracks.The advantage of CAV is that it is much simpler to design and produce because the motor doesn't need to change speed. In addition, CLV runs into problems for very high-speed CD-ROMs because there's a brief latency whenever the drive needs to change the rotational speed. The drawback to CAV is that data on inner tracks is accessed more slowly than data on outside tracks. And unfortunately, most CDs are laid out with data on the inner tracks first.

CD-I:
A software and hardware standard developed jointly by Philips International and Sony Corporation for storing video, audio, and binary data on compact optical disks. It supports 552MB (megabytes) of binary data and specifies several different types of video and audio encoding formats. Unlike conventional CD-ROM drives, CD-I drives have a built-in microprocessor to handle many of the computing functions. It is sometimes referred to as the Green Book standard.
Although there are some CD-I devices and titles, the format has not yet become widely accepted.

CD-R DRIVE:
Short for Compact Disk-Recordable drive, a type of disk drive that can create CD-ROMs and audio CDs. This allows users to “master” a CD-ROM or audio CD for publishing. Until recently, CD-R drives were quite expensive, but prices have dropped dramatically. A feature of many CD-R drives, called multisession recording, enables you to keep adding data to a CD-ROM over time. This is extremely important if you want to use the CD-R drive to create backup CD-ROMs.
To create CD-ROMs and audio CDs, you'll need not only a CD-R drive, but also a CD-R software package. Often, it is the software package, not the drive itself, that determines how easy or difficult it is to create CD-ROMs.
CD-R drives can also read CD-ROMs and play audio CDs.

CD-ROM:
Pronounced see-dee-rom, abbreviation of Compact Disc-Read-Only Memory. A type of optical disk capable of storing large amounts of data -- up to 1GB, although the most common size is 630MB (megabytes). A single CD-ROM has the storage capacity of 700 floppy disks, enough memory to store about 300,000 text pages.
CD-ROMs are recorded by the vendor, and once recorded, they cannot be erased and filled with new data. To read a CD, you need a CD-ROM player. Almost all CD-ROMs conform to a standard size and format, so it is usually possible to load any type of CD into any ROM player. In addition, most CD-ROM players are capable of playing audio CDs, which share the same technology.
CD-ROMs are particularly well-suited to information that requires large storage capacity. This includes color graphics, sound, and especially video. In recent years, as the prices of CD-ROM players have decreased, and the tools for creating new CD-ROM titles have improved, the CD-ROM industry has been expanding rapidly. To date, the most popular CD-ROM titles have been computer games and multimedia reference works.

CD-ROM PLAYER:
Also called a CD-ROM drive, a CD-ROM player is a device that can read information from a CD-ROM. CD-ROM players can be either internal, in which case they fit in a bay, or external, in which case they generally connect to the computer’s parallel port. Parallel CD-ROM players are easier to install, but they have several disadvantages: They’re somewhat more expensive than internal players, they use up the parallel port which means that you can’t use that port for another device such as a printer, and the parallel port itself may not be fast enough to handle all the data pouring through it.
There are a number of features that distinguish CD-ROM players, the most important of which is probably their speed. CD-ROM players are generally classified as single-speed or some multiple of single-speed. For example, a 4X player access data at four times the speed of a single-speed player. Within these groups, however, there is some variation. Also, you need to be aware of whether the CD-ROM uses the CLV or CAV technology. The reported speeds of players that use CAV are generally not accurate because they refer only to the access speed for outer tracks. Inner tracks are accessed more slowly.
Two more precise measurements are the drive’s seek time and data transfer rate. The seek time, also called the access time, measures how long, on average, it takes the drive to access a particular piece of information. The data transfer rate measures how much data can be read and sent to the computer in a second.
Aside from its speed, another important feature of a CD-ROM player is its compatibility with existing standards. If you plan to run CD-ROMs in a windows environment, you need a player that conforms to the MPC III standard. If you want to be able to view photographs stored on CD-ROM, make sure your player conforms to Kodak’s PhotoCD format.
Finally, you should consider how the player connects to your computer. Most CD-ROMs connect via a SCSI bus. If your computer doesn’t already contain such an interface, you will need to install one. Other CD-ROMs connect to an IDE or Enhanced IDE interface, which is the one used by the hard disk drive; still others use a proprietary interface.

CD-RW DISK:
Short for CD-ReWritable disk, a new type of CD disk that enables you to write onto it in multiple sessions. One of the problems with CD-R disks is that you can only write to them once. With CD-RW drives and disks, you can treat the optical disk just like a floppy or hard disk, writing data onto it multiple times. The first CD-RW drives became available in mid-1997. They can read CD-ROMs and can write onto today's CD-R disks, but they cannot write on CD-ROMs. Many experts believe that they'll be a popular storage medium until DVD devices become widely available.

CLUSTER:
A group of disk sectors. The operating system assigns a unique number to each cluster and then keeps track of files according to which clusters they use. Occasionally, the operating system marks a cluster as being used even though the file that is supposedly using it no longer exists. This is called a lost cluster. You can free up disk space by deleting lost clusters, but you should first make sure that the clusters do not, in fact, contain valuable data.
DOS and Windows keep track of clusters with the File Allocation Table (FAT). The size of each cluster depends on the disk's partition size.

CLV:
Short for Constant Linear Velocity, a method used by most CD-ROM players to access data. With CLV, the rotation speed of the disk changes based on how close to the center of the disk the data is. For tracks near the center, the disk rotates faster, and for data on the outside, the disk rotates slower. The purpose of CLV is to ensure a constant data rate regardless of where on the disk the data is being accessed. Because less data can fit on the inside tracks, the disk needs to rotate faster for these areas
An alternative technology, which is becoming increasingly popular, is CAV (Constant Angular Velocity).

CD:
Known by its abbreviation, CD, a compact disk is a polished metal platter capable of storing digital information. The most prevalent types of compact disks or those used by the music industry to store digital recordings and CD-ROMs used to store computer data. Both of these types of compact disk are read-only, which means that once the data has been recorded onto them, they can only be read, or played.
Another type of compact disk, generally called an erasable optical disk, can have its data erased and overwritten by new data. Currently, erasable optical storage is too slow to be used as a computer’s main storage facility, but as the speed improves and the cost comes down, optical storage devices are becoming a popular alternative to tape systems as a backup method.

CYLINDER:
A single track location on all the platters making up a hard disk. For example, if a hard disk has four platters, each with 600 tracks, then there will be 600 cylinders, and each cylinder will consist of 8 tracks (2 for each platter).

DAT:
Acronym for digital audio tape, a type of magnetic tape that uses an ingenious scheme called helical scan to record data. A DAT cartridge is slightly larger than a credit card and contains a magnetic tape that can hold from 2 to 24 gigabytes of data. It can support data transfer rates of about 2 MBps. Like other types of tapes, DATs are sequential-access media.
The most common format for DAT cartridges is DDS (digital data storage).

DATA RECOVERY:
Salvaging data stored on damaged media, such as magnetic disks and tapes. There are a number of software products that can help recover data damaged by a disk crash or virus. In addition, there are companies that specialize in data recovery. Of course, not all data is recoverable, but data recovery specialists can often restore a surprisingly high percentage of the data on damaged media.

DDS:
Abbreviation of Digital Data Storage, the industry standard for digital audio tape (DAT) formats. The latest format, DDS-3, specifies tapes that can hold 24GB (the equivalent of over 40 CD-ROMs) and support data transfer rates of 2 MBps.

DEFRAGMENT:
To optimize a disk by unfragmenting files.

DENSITY:
How tightly information is packed together on a storage medium (tape or disk). A higher density means that data are closer together, so the medium can hold more information. Floppy disks can be single-density, double-density, high-density, or extra-high-density. To use a double-density, high-density, or extra-high-density disk, you must have a disk drive that supports the density level. Density, therefore, can refer both to the media and the device.
The table shows the storage capacities of double- and high-density floppies on the PC and the Apple Macintosh. Note that the only difference between double-density and high-density disks is that the high-density disks were found to be higher quality during the testing process. High-quality disks are sold as high-density, and lower-quality disks are sold as double-density. It is often possible, therefore, to format a double-density disk as a high-density disk, but this practice is not encouraged.

DISK:
A round plate on which data can be encoded. There are two basic types of disks: magnetic disks and optical disks.
A magnetic disk is like a phonograph record, except that the data is encoded as microscopic magnetized needles on the disk's surface rather than as grooves in the vinyl. Also, you can record and erase data on a magnetic disk any number of times, just as you can with a cassette tape. Magnetic disks come in a number of different forms:
floppy disk: A typical 5¼-inch floppy disk can hold 360K or 1.2MB (megabytes). Microfloppies (3½-inch floppies) normally store 720K, 1.2MB or 1.44MB of data. hard disk: Hard disks can store anywhere from 20MB to more than 3GB. Hard disks are also from 2 to 20 times faster than floppy disks.removable cartridge: Removable cartridges are hard disks encased in a metal or plastic cartridge, so you can remove them just like a floppy disk. Removable cartridges are very fast, often faster than fixed hard disks. A typical cartridge has a capacity of about 80MB.
Optical disks record data by burning microscopic holes in the surface of the disk with a laser. To read the disk, another laser beam shines on the disk and detects the holes by changes in the reflection pattern.
Optical disks have a much larger data capacity than magnetic disks, but they are slower.
Optical disks come in three forms:
CD-ROM: Most optical disks are read-only. When you purchase them, they are already filled with data. You can read the data from a CD-ROM, but you cannot modify, delete, or write new data.WORM: Stands for write-once, read-many. WORM disks can be written on once and then read any number of times; however, you need a special WORM disk drive to write data onto a WORM disk.
erasable optical (EO): EO disks can be read to, written to, and erased just like magnetic disks.
The machine that spins a disk is called a disk drive. Within each disk drive is one or more heads (often called read/write heads) that actually read and write data.
Accessing data from a disk is not as fast as accessing data from main memory, but disks are cheaper and more stable. Unlike RAM, disks hold on to data even when the computer is turned off. Consequently, disks are the storage medium of choice for most types of data. Another storage medium is the magnetic tape. But tapes are used only for backup and archiving because they are sequential-access devices (to access data in the middle of a tape, the tape drive must pass through all the preceding data).
A new disk, called a blank disk, has no data on it. Before you can store data on a blank disk, however, you must format it. In DOS systems, disks are formatted with the FORMAT command. When you execute the FORMAT command, the operating system allocates space for directories and other bookkeeping information. Every operating system formats disks somewhat differently, so a disk formatted by one operating system usually cannot be read by another operating system.

DISK CACHE:
A portion of RAM used to speed up access to data on a disk. The RAM can be part of the disk drive itself (sometimes called a hard disk cache) or it can be general-purpose RAM in the computer that is reserved for use by the disk drive (sometimes called a soft disk cache). Hard disk caches are more effective, but they are also much more expensive.
In both cases, a disk cache works by storing the most recently accessed data in the RAM cache. When a program needs to access new data, the operating system first checks to see if the data is in the cache before reading it from the disk. Because computers can access data from RAM much faster than from a disk, disk caching can significantly increase performance. Many cache systems also attempt to predict what data will be requested next so they can place that data in the cache ahead of time.
Although caching improves performance, there is some risk involved. If the computer crashes (due to a power failure, for example), the system may not have time to copy the cache back to the disk. In this case, whatever changes you made to the data will be lost. Usually, however, the cache system updates the disk frequently so that even if you lose some data, it will not be much. A special type of disk cache, called a write-thru cache, removes the risk of losing data because it only caches data for read operations; write operations are always sent directly to the disk.

DISK CONTROLLER:
A chip and associated circuitry that is responsible for controlling a disk drive. There are different controllers for different interfaces. For example, an IDE interface requires an IDE controller and a SCSI interface requires a SCSI controller. On Macintosh computer systems, the disk controller is built into the motherboard. On PCs, the disk controller is sometimes housed on a separate card.

DISK DRIVE:
A machine that reads data from and writes data onto a disk. A disk drive resembles a stereo turntable in that it rotates the disk very fast. It has one or more heads that read and write data.
There are different types of disk drives for different types of disks. For example, a hard disk drive reads and writes hard disks, and a floppy drive accesses floppy disks. A magnetic disk drive reads magnetic disks, and an optical drive reads optical disks.
Disk drives can be either internal (housed within the computer) or external (housed in a separate box that connects to the computer).

DISK MIRRORING:
A technique in which data is written to two duplicate disks simultaneously. This way if one of the disk drives fails, the system can instantly switch to the other disk without any loss of data or service. Disk mirroring is used commonly in on-line database systems where it's critical that the data be accessible at all times.

DISK OPTIMIZER:
A program that makes a disk more efficient by defragmenting the disk. Fragmentation occurs naturally when a disk is used often.

DISK STRIPING:
A technique for spreading data over multiple disk drives. Disk stripping can speed up operations that retrieve data from disk storage. The computer system breaks a body of data into units and spreads these units across the available disks. Systems that implement disk striping generally allow the user to select the data unit size or stripe width. Disk striping is available in two types. Single user striping uses relatively large data units, and improves performance on a single-user workstation by allowing parallel transfers from different disks. Multi-user striping uses smaller data units and improves performance in a multi-user environment by allowing simultaneous (or overlapping) read operations on multiple disk drives.
Disk striping stores each data unit in only one place and does not offer protection from disk failure such a RAID technology.

DOUBLE-DENSITY DISK:
A floppy disk that has twice the storage capacity of a single-density floppy. Single-density disks are now practically obsolete. Double-density 5¼-inch disks for PCs can hold 360K of data. Double-density 3½-inch disks can hold 720K.
Although high-density disk drives can format both high-density and double-density disks, double-density disks formatted by a high-density drive may not be readable by a double-density drive. Similarly, high-density drives may not be able to read disks that have been formatted by a double-density disk drive.

DOUBLE-SIDED DISK:
A floppy disk with both sides prepared for recording data. You can store twice as much data on a double-sided disk, but you need to use a double-sided disk drive. Nearly all modern disks and disk drives are double-sided.

DRIVE BAY:
An area of reserved space in a personal computer where hard or floppy disk drives (or tape drives) can be installed. The number of drive bays in a computer determines the total number of internal mass storage devices it can handle.

DVD:
Short for digital versatile disk or digital video disk, a new type of CD-ROM that holds a minimum of 4.7GB (gigabytes), enough for a full-length movie. Many experts believe that DVD disks, called DVD-ROMs, will eventually replace CD-ROMs, as well as VHS video cassettes and laser discs. Currently, however, DVD is more promise than reality. There are only a few DVD devices currently on the market and even fewer DVD-ROM titles.
The DVD specification supports disks with capacities of from 4.7GB to 17GB and access rates of 600KBps to 1.3 MBps. One of the best features of DVD drives is that they are backward-compatible with CD-ROMs. This means that DVD players can play old CD-ROMs, CD-I disks, and video CDs, as well as new DVD-ROMs. DVD players cannot, however, read CD-R disks.DVD uses MPEG-2 to compress video data.

DVD-RAM:
A new type of rewritable compact disc that provides much greater data storage than today's CD-RW systems. The specifications for DVD-RAMs are still being hammered out by the DVD Consortium.

ERASABLE OPTICAL DISK:
A type of optical disk that can be erased and loaded with new data, just like magnetic disks. In contrast, most optical disks, called CD-ROMs, are read-only.
Although the technology is still young, erasable optical disks seem destined to become the future medium of choice. A single 5¼-inch optical disk can store 650 megabytes of data, about 1,000 times more than a typical floppy disk. And unlike hard disks, optical disks are portable.
The data -access speed of optical disks varies considerably, from about 20 to 200 milliseconds. Though comparable to floppy disks, they are not yet as fast as magnetic hard disk drives.

FAT 32:
A new version of the file allocation table (FAT) available in Windows 95 OSR 2. FAT32 increases the number of bits used to address clusters and also reduces the size of each cluster to 4KB. This result is that it can support larger disks (up to 2 terabytes) and better storage efficiency.

FDHD:
Short for floppy drive, high density, and pronounced fud-hud. FDHD refers to 3½-inch disk drives for Macintosh computers that can accept double-density or high-density 3½-inch floppy disks. FDHDs can also read DOS-formatted floppy disks, which enables Macintosh computers and PCs to share data. FDHD drives are often called SuperDrives.

FIBRE CHANNEL:
A data transfer architecture developed by a consortium of computer and mass storage device manufacturers and now being standardized by ANSI. The most prominent Fibre Channel standard is Fibre Channel Arbitrated Loop (FC-AL). FC-AL was designed for new mass storage devices and other peripheral devices that require very high bandwidth. Using optical fiber to connect devices, FC-AL supports full-duplex data transfer rates of 100MBps. FC-AL is compatible with, and is expected to eventually replace, SCSI for high-performance storage systems. In addition, FC-AL serves as the physical specification for Gigabit Ethernet.

FLOPPY DISK:
A soft magnetic disk. It is called floppy because it flops if you wave it (at least, the 5¼-inch variety does). Unlike most hard disks, floppy disks (often called floppies or diskettes) are portable, because you can remove them from a disk drive. Disk drives for floppy disks are called floppy drives. Floppy disks are slower to access than hard disks and have less storage capacity, but they are much less expensive. And most importantly, they are portable.
Floppies come in two basic sizes:
5¼-inch: The common size for PCs made before 1987. This type of floppy is generally capable of storing between 100K and 1.2MB (megabytes) of data. The most common sizes are 360K and 1.2MB.3½-inch: Floppy is something of a misnomer for these disks, as they are encased in a rigid envelope. Despite their small size, microfloppies have a larger storage capacity than their cousins -- from 400K to 1.4MB of data. The most common sizes for PCs are 720K (double-density) and 1.44MB (high-density). Macintoshes support disks of 400K, 800K, and 1.2MB.

FORMAT:
To prepare a storage medium, usually a disk, for reading and writing. When you format a disk, the operating system erases all bookkeeping information on the disk, tests the disk to make sure all sectors are reliable, marks bad sectors (that is, those that are scratched), and creates internal address tables that it later uses to locate information. You must format a disk before you can use it.
Note that reformatting a disk does not erase the data on the disk, only the address tables. Do not panic, therefore, if you accidentally reformat a disk that has useful data. A computer specialist should be able to recover most, if not all, of the information on the disk. You can also buy programs that enable you to recover a disk yourself.
The previous discussion, however, applies only to high-level formats, the type of formats that most users execute. In addition, hard disks have a low-level format, which sets certain properties of the disk such as the interleave factor. The low-level format also determines what type of disk controller can access the disk (e.g., RLL or MFM).
Almost all hard disks that you purchase have already had a low-level format. It is not necessary, therefore, to perform a low-level format yourself unless you want to change the interleave factor or make the disk accessible by a different type of disk controller. Performing a low-level format erases all data on the disk.To specify the properties, particularly visible properties, of an object. For example, word processing applications allow you to format text, which involves specifying the font, alignment, margins, and other properties.