personal digital assistant (PDA), lightweight, hand-held computer designed for use as a personal organizer with communications capabilities; also called a handheld. A typical PDA has no keyboard, relying instead on special hardware and pen-based computer software to enable the recognition of handwritten input, which is entered on the surface of a liquid crystal display screen. In addition to including such applications as a word processor, spreadsheet, calendar, and address book, PDAs are used as notepads, appointment schedulers, and wireless communicators for sending and receiving data, faxes, and electronic-mail messages. Introduced in 1993, PDAs achieved only modest acceptance during the remainder of the decade due to their relatively high price and limited applications, but improved software and lower prices subsequently led to more widespread use. See also palmtop.

digital-to-analog or D/A conversion, the process of changing discrete digital data into a continuously varying signal in relation to a standard or reference. There are two types of converters: electromechanical—also called shaft- or position-to-digital—and electronic. An example of electromechanical conversion is a positioning mechanism, such as a strip-chart recorder. The most common use of an electronic converter is to present the output of a digital computer as a graphic display (see computer graphics) or as audio output, as in computer-generated music. A modem, or data set, is a device that converts the digital signals produced by computers and terminals into analog signals that telephone circuits are designed to carry and then back to digital signals at the other end of the communication link. See also analog-to-digital conversion.

digital versatile disc or digital video disc (DVD), a small plastic disc used for the storage of digital data. The successor media to the compact disc (CD), a DVD can have as much as 26 times the storage capacity of a CD. When compared to CD technology, DVD also allows for better graphics and greater resolution. In the case of an audio recording, where the data to be stored is in analog rather than digital form, the sound signal is sampled at a rate of 48,000 or 96,000 times a second, then each sample is measured and digitally encoded on the 43/4-in. (12-cm) disc as a series of microscopic pits on an otherwise polished surface. The disc is covered with a protective, transparent coating so that it can be read by a laser beam. As with other optical disks nothing touches the encoded portion, and the DVD is not worn out by the playing process. Because DVD players are backward compatible to existing technologies, they can play CD and CD-ROM discs; however, CD players cannot play DVD and DVD-ROM discs.

DVD formats include DVD-Video (often simply called DVD), DVD-ROM, and DVD-Audio. DVD-Video discs hold digitized movies or video programs and are played using a DVD player hooked up to a standard television receiver. In a sense, DVD-Video players are the successors to the videocasette recorders (VCRs) that play VHS tapes. DVD-ROM [Read Only Memory] discs hold computer data and are read by a DVD-ROM drive hooked up to a computer. These disks can only be read—the disks are impressed with data at the factory but once written cannot be erased and rewritten with new data. DVD-ROM also includes recordable variations. DVD-R [Recordable] discs can be written to sequentially but only once. DVD-RAM [Random Access Memory], DVD-RW, and DVD+RW [ReWritable] discs can be written to thousands of times; they differ in their technical standards and, as a result, in the amount of information they can hold. Many DVD recorders can record in several different recordable DVD formats. Some recorders include computer hard drives that allow the user to record tens to hundreds of hours of material temporarily; the user can then select the material that will be transferred to a DVD. When DVD was released in 1996 there was no DVD-Audio format, although the audio capabilities of DVD-Video far surpassed those available from a CD; the DVD-Audio format was introduced in 1999.

digital radio, audio broadcasting in which an analog audio signal is converted into a digital signal before being transmitted; also known as digital audio broadcasting (DAB) and high-definition radio. Digital radio reception is virtually free of static and fading, pops, and hisses; overall, adjacent stations do not interfere within one another, audio clarity and volume are improved, and weather, noise, and other interference cease to be a factor. Digital radio can be land based (or terrestrial) or transmitted via satellite. In either case, a special receiver is required to decode the multiplexed signal; the receiver may contain a small display that provides information about the audio content (such as the name of the artist or title of the music).

The land-based technology was first deployed in Great Britain in 1995, and has since become established throughout Europe. The first satellite-based digital radio system was WorldSpace, which orbited the first of its three geostationary earth orbit (GEO) satellites, AfriStar, in 1998. Each satellite transmits three overlapping signal beams carrying more than 40 channels of programming; most of world (except mainly North America and Australia) is covered. The first satellite-based system to provide a mobile subscription digital radio service covering the United States was XM Satellite Radio, which orbited two GEO satellites in 2001. XM's ground station transmits digital signals to its satellites, which retransmit them directly to radio receivers on the ground. The receivers unscramble the signal, which contain up to 100 channels of digital audio. In metropolitan areas where tall buildings, overpasses, and other obstacles can interfere with the signals when, for example, the receiver is in a moving vehicle, a network of ground-based repeaters retransmit the signals. The receiver also buffers the signal briefly so that if it loses the satellite signal it can use one from a repeater to maintain a continuous broadcast. Sirius Satellite Radio, which launched national service to the United States in 2002, employs three satellites in inclined elliptical orbits instead of GEO satellites.

digital computer: see computer.

digital circuit, electronic circuit that can take on only a finite number of states. That is contrasted with analog circuits, whose voltages or other quantities vary in a continuous manner. Binary (two-state) digital circuits are the most common. The two possible states of a binary circuit are represented by the binary digits, or bits, 0 and 1. The states are also commonly referred to as "on" and "off" or "high" and "low" (see information theory). The simplest forms of digital circuits are built from logic gates, the building blocks of the digital computer. Since most of the physical variables encountered in the real world, e.g., position and temperature, exist in analog form, they are represented electrically by continuously varying currents and voltages in analog circuits. To make digital and analog circuits compatible special converters are used—either analog-to-digital or digital-to-analog depending on the direction of information flow. Digital circuits simulate continuous functions with strings of bits; the more bits that are used, the more accurately the continuous signal can be represented. For example, if 16 bits are used to represent a varying voltage, the signal can be assigned one of more than 65,000 different values. Digital circuits are more immune to noise than analog circuits, and digital signals can be stored and duplicated without degradation (see compact disc). Digital circuits can often manipulate signals more effectively—and less expensively—than analog circuits. Those reasons helped digital systems to succeed over all analog contenders for proposed high-definition television in the United States.

digital art, contemporary art in which computer technology is used in a wide variety of ways to make distinctive works. Digital art was pioneered in the 1970s but only came into its own as a viable art form with the widespread availability of computers, appropriate software, video equipment, sound mixers, and digital cameras toward the end of the 20th cent. and the subsequent development of increasingly sophisticated digital tools. A boundary-shattering style, digital art can combine and transform such elements as painting, filmmaking, photography, digital design, video, installation art, sculpture, animation, and sound.

Presented on video screens, digital works may be created of abstract or figurative forms in the artists' choice of millions of shades of color, and may be manipulated so that the images appear, combine, morph, and/or disappear. Digital art also includes works, many of them interactive, made to be viewed on the World Wide Web. Sculpture, too, can be a digital art as a result of rapid prototyping, a technique that "prints out" three-dimensional forms from computer-designed models. Contemporary digital works range from the shimmering and transforming video paintings of Jeremy Blake to the computer-modified imagery of Carl Fudge's screenprints, the shifting geometric panels of John F. Simon, Jr., and the participatory audiovisual worlds of Janet Cardiff. Among the many other artists involved in the movement, each with his or her own approach to the seemingly infinite possibilities of digital art, are Jim Campbell, Leah Gilliam, Robert Lazzarini, Jim O'Rourke, Paul Pfeiffer, Marina Rosenfeld, Elliott Sharp, Diana Thater, Inez van Lamsweerde, and Adrianne Wortzel.

analog-to-digital or A/D conversion, the process of changing continuously varying data, such as voltage, current, or shaft rotation, into discrete digital quantities that represent the magnitude of the data compared to a standard or reference at the moment the conversion is made. There are two types of converters: electromechanical—also called shaft- or position-to-digital—and electronic. The most common use is to change analog signals into a form that can be manipulated by a digital computer, as in data communications; a modem, or data set, is a device that converts the digital signals produced by computers and terminals into analog signals that telephone circuits are designed to carry and then back to digital signals at the other end of the communication link. Similarly, in digital sound recording, audio signals are transformed into digital data, which are then recorded on a magnetic or optical disk or tape; the digitized data on the recording medium then must be changed back into the analog sound signals that can be used by a stereophonic sound system. See also digital-to-analog conversion.

Computer capable of solving problems by processing information expressed in discrete form. By manipulating combinations of binary digits (see binary code), it can perform mathematical calculations, organize and analyze data, control industrial and other processes, and simulate dynamic systems such as global weather patterns. Seealso analog computer.

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Camera that captures images electronically rather than on film. The image is captured by an array of charge-coupled devices (CCDs), stored in the camera's random access memory or a special diskette, and transferred to a computer for modification, long-term storage, or printing out. Since the technology produces a graphics file, the image can be readily edited using suitable software. Models designed and priced for the mass consumer market—as opposed to costly models designed for photojournalism and industrial photography—first became available in 1996. They appeal particularly to users who want to send pictures over the Internet or to crop, combine, enhance, or otherwise modify their photographs.

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