Digital, related to digits or the way they are represented. In computing, digitalis virtually synonymous with binary because the computers familiar to most people process information coded as combinations of binary digits (bits). One bit can represent at most two values; 2 bits, four values; 8 bits, 256 values; and so on. Values that fall between two numbers are represented as either the lower or the higher of the two. Because digital repre 14414r1716o sentation represents a value as a coded number, the range of values represented can be very wide, although the number of possible values is limited by the number of bits used. See also Computer; Digital-To-Analog Converter.
Digital-to-Analog Converter or DAC, device for converting digital data into current or voltage analogs. DACs are now widely used in compact disk (CD) players, in digital audio- and videotape players, and in digital signal processing audio and video equipment. Most DACs use some form of resistor network (see Resistance). Digital data is applied to the resistors in groups of bits. The resistances vary in definite ratios; the current flow in each one relates directly to the binary value of the bit received (see Computer).
Analog-To-Digital Converter or ADC, an electronic device for converting data from analog to digital form for use in electronic equipment such as digital computers (see Computer), digital audio and video recorders, and communication equipment. Analog or continuously varying electrical waveforms are applied to the device and are sampled at a fixed rate. Sample values are then expressed as a digital number, using a binary numbering system consisting only of 0's and 1's. The resulting digital codes can be used in various types of communications systems.
Analog Display, in computer science, a video display capable of rendering a continuous range (an infinite number) of colors or gray shades, as opposed to a digital display, which is capable of rendering only a finite number of colors. Examples of analog displays include IBM's MCGA and VGA displays. See also Analog Computer.
Digital Technology, study and development of devices that store and manipulate numbers. Digital devices can translate words and pictures into numbers for a computer to process and then translate the numbers back into pictures or words.
By contrast, analog technology is the study of devices that function without storing or using numbers. For example, an analog watch contains a complex mechanism of gears that produce the steady motion of its hands. Although numbers may be painted on the face of the watch, the mechanism does not store or use those numbers to keep time. Inside a digital watch, electronic circuitry stores and manipulates numbers representing the time, and these are displayed on the face of the watch.
Humans write numbers using ten digits, 0 through 9, and combinations of these digits. Every digit in a decimal number represents powers of ten. Modern digital devices store numbers using only two digits, 1 and 0, called bits. Such numbers are called binary numbers. Every digit in a binary number represents a power of two. For example, in the binary number 101, the 1 at the right represents 1 x 20; the 0 in the middle represents 0 x 21; and the 1 to the far left represents 1 x 22 (see Number Systems). The decimal equivalent of 101 is (1 x 22) + (0 x 21) + (1 x 20) = 4 + 0 + 1 = 5.
Processing Digital Information
Digital devices such as computers have electronic circuits that can turn on and off extremely rapidly. When a circuit is on, it represents a value of 1, and when a circuit is off, it represents a value of 0 in the binary number system. Digital devices perform mathematical operations by turning these switches on and off according to the rules of binary arithmetic.
The simplicity of binary arithmetic, combined with the rapid rate at which a computer can manipulate binary data, makes binary numbers ideally suited for computers. All instructions that direct the function of computers and all data manipulated by computers are first converted into binary numbers so they may be processed more efficiently and more quickly.
Digital devices are designed to perform mathematical operations as fast as possible. Devices that perform parallel processing complete multiple operations simultaneously. Pipelined devices behave like an assembly line, so that at any given time, several operations in various stages are being executed on different data.
Computers translate information from the computer user into binary numbers in a process called digital encoding. Letters can be encoded by replacing every letter with its numerical position (1-26) in the alphabet, and then converting these decimal numbers into binary equivalents. A sound can be encoded as a series of numbers that measure its pitch and volume at each instant in time. An image can be encoded as a sequence of numbers that represent the color and brightness of each portion of the picture. The computer is able to decode information by converting the numbers back into letters, sounds, or images.
Digital devices can process encoded information in a variety of useful ways. They can sharpen blurry images, clarify sounds, and check the spelling of words by manipulating the numbers representing information. A digital device also can compress information by identifying a recurring sequence of numbers and then representing that sequence with a single number. Compression saves space when storing the information, and it saves time when transferring the information from computer to computer.
Progress in digital technology is driven by the human need to communicate. Advanced compression technologies allow computers to find, store, and transmit massive amounts of information efficiently. Digitally encoded text, sounds, images, and video can be stored together on magnetic media and compact disks and broadcast over cable to homes and over satellites to cellular phones.
Digitize, in computer science, to convert any continuously varying source of input, such as the lines in a drawing or a sound signal, into a series of discrete units represented (in a computer) by the binary digits 0 and 1. A drawing or photograph, for example, can be digitized by a scanner that converts lines and shading into combinations of 0's and 1's by sensing different intensities of light and dark. Analog-to-digital converters are commonly used to perform this translation. See also Aliasing; Analog-To-Digital Converter.
Telecommunications, the transmission of words, sounds, images, or data in the form of electronic or electromagnetic signals or impulses. Transmission media include the telephone (using wire or optical cable), radio, television, microwave, and satellite. Data communication, the fastest growing field of telecommunication, is the process of transmitting data in digital form by wire or radio.
Digital data can be generated directly in a 1/0 binary code by a computer or can be produced from a voice or visual signal by a process called encoding. A data communications network is created by interconnecting a large number of information sources so that data can flow freely among them. The data may consist of a specific item of information, a group of such items, or computer instructions. Examples include a news item, a bank transaction, a mailing address, a letter, a book, a mailing list, a bank statement, or a computer program.
The devices used can be computers, terminals (devices that transmit and receive information), and peripheral equipment such as printers (see Computer; Office Systems). The transmission line used can be a normal or a specially purchased telephone line called a leased, or private, line (see Telephone). It can also take the form of a microwave or a communications-satellite linkage, or some combination of any of these various systems.
Hardware and Software
Each telecommunications device uses hardware, which connects a device to the transmission line; and software, which makes it possible for a device to transmit information through the line.
Hardware
Hardware usually consists of a transmitter and a cable interface, or, if the telephone is used as a transmission line, a modulator/demodulator, or modem.
A transmitter prepares information for transmission by converting it from a form that the device uses (such as a clustered or parallel arrangement of electronic bits of information) to a form that the transmission line uses (such as, usually, a serial arrangement of electronic bits). Most transmitters are an integral element of the sending device.
A cable interface, as the name indicates, connects a device to a cable. It converts the transmitted signals from the form required by the device to the form required by the cable. Most cable interfaces are also an integral element of the sending device.
A modem converts digital signals to and from the modulated form required by the telephone line to the demodulated form that the device itself requires. Modems transmit data through a telephone line at various speeds, which are measured in bits per second (bps) or as signals per second (baud). Modems can be either integral or external units. An external unit must be connected by cable to the sending device. Most modems can dial a telephone number or answer a telephone automatically.
Software
Among the different kinds of software are file-transfer, host, and network programs. File-transfer software is used to transmit a data file from one device to another. Host software identifies a host computer as such and controls the flow of data among devices connected to it. Network software allows devices in a computer network to transmit information to one another.
Applications
Three major categories of telecommunication applications can be discussed here: host-terminal, file-transfer, and computer-network communications.
Host-Terminal
In these types of communications, one computer-the host computer-is connected to one or more terminals. Each terminal transmits data to or receives data from the host computer. For example, many airlines have terminals that are located at the desks of ticket agents and connected to a central, host computer. These terminals obtain flight information from the host computer, which may be located hundreds of kilometers away from the agent's site.
The first terminals to be designed could transmit data only to or from such host computers. Many terminals, however, can now perform other functions such as editing and formatting data on the terminal screen or even running some computer programs. Manufacturers label terminals as "dumb," "smart," or "intelligent" according to their varying capabilities. These terms are not strictly defined, however, and the same terminal might be labeled as dumb, smart, or intelligent depending upon who is doing the labeling and for what purposes.
File-Transfer
In file-transfer communications, two devices are connected: either two computers, two terminals, or a computer and a terminal. One device then transmits an entire data or program file to the other device. For example, a person who works at home might connect a home computer to an office computer and then transmit a document stored on a diskette to the office computer.
An outgrowth of file transfer is electronic mail. For example, an employee might write a document such as a letter, memorandum, or report on a computer and then send the document to another employee's computer.
Computer-Network
In computer-network communications, a group of devices is interconnected so that the devices can communicate and share resources. For example, the branch-office computers of a company might be interconnected so that they can route information to one another quickly. A company's computers might also be interconnected so that they can all share the same hard disk.
The three kinds of computer networks are local area networks (LAN), private branch exchange (PBX) networks, and wide area networks (WAN). LANs interconnect devices with a group of cables; the devices communicate at a high speed and must be in close proximity. A PBX network interconnects devices with a telephone switching system; in this kind of network, the devices must again be in close proximity. In wide-area networks, on the other hand, the devices can be at great distances from one another; such networks usually interconnect devices by means of telephone.
Telecommunication Services
Public telecommunication services are a relatively recent development in telecommunications. The four kinds of services are network, information-retrieval, electronic-mail, and bulletin-board services.
Network
A public network service leases time on a WAN, thereby providing terminals in other cities with access to a host computer. Examples of such services include Telenet, Tymnet, Uninet, and Datapac. These services sell the computing power of the host computer to users who cannot or do not wish to invest in the purchase of such equipment.
Information-Retrieval
An information-retrieval service leases time on a host computer to customers whose terminals are used to retrieve data from the host. An example of this is CompuServe, whose host computer is accessed by means of the public telephone system. This and other such services provide general-purpose information on news, weather, sports, finances, and shopping.
Other information-retrieval services may be more specialized. For example, Dow Jones News Retrieval Services provide general-purpose information on financial news and quotations, corporate-earning estimates, company disclosures, weekly economic survey updates, and Wall Street Journal highlights. Newsnet provides information from about 200 newsletters in 30 different industries; Dialog Information Services, BRS Bibliographic Retrieval Services, and Orbit Information Retrieval Services provide library information; and Westlaw provides legal information to its users. See Database.
Electronic-Mail
By means of electronic mail, terminals transmit documents such as letters, reports, and telexes to other computers or terminals. To gain access to these services, most terminals use a public network. Source Mail (available through The Source) and EMAIL (available through CompuServe) enable terminals to transmit documents to a host computer. The documents can then be retrieved by other terminals. MCI Mail Service and the U.S. Postal ECOM Service (also available through The Source) let terminals transmit documents to a computer in another city. The service then prints the documents and delivers them as hard copy. ITT Timetran, RCA Global Communications, and Western Union Easylink let terminals send telexes to other cities.
Bulletin-Board
By means of a bulletin board, terminals are able to facilitate exchanges and other transactions. Many bulletin boards do not charge a fee for their services. Users of these services simply exchange information on hobbies, buy and sell goods and services, and exchange computer programs.
Ongoing Developments
Certain
telecommunication methods have become standard in the telecommunications
industry as a whole, because if two devices use different standards they are
unable to communicate properly. Standards are developed in two ways: (1) the
method is so widely used that it comes to dominate; (2) the method is published
by a standard-setting organization. The most important organization in this
respect is the International Telecommunication Union, a specialized agency of
the United Nations, and one of its operational entities, the International
Telegraph and Telephone Consultative Committee (CCITT). Other organizations in
the area of standards are the American National Standards Institute, the
Other developments in the industry are aimed at increasing the speed at which data can be transmitted. Improvements are being made continually in modems and in the communications networks. Some public data networks support transmission of 56,000 bits per second (bps), and modems for home use (see Microcomputer) are capable of as much as 28,800 bps.
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