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code, in communications
code, in communications, set of symbols and rules for their manipulation by which the symbols can be made to carry information. By this extended definition all written and spoken languages are codes. While these are sufficient and actually quite efficient in transmission of information, they are at times ambiguous and are highly inefficient for telecommunications. For example, a circuit capable of carrying a voice message, e.g., a telephone circuit, could carry several times as much information if that information were represented as telegraphic code.
Generally speaking, information theory shows that for any particular application there is an optimum code; it does not, unfortunately, tell how to devise the code. Morse code, consisting of a series of dots and dashes, or marks and spaces, is commonly used in telegraphy. In a computer, information is digitally encoded as strings of binary digits or bits. ASCII, the American Standard Code for Information Interchange, and Unicode are two ways representing alphanumeric characters in a binary form.
Special error-detecting codes are used extensively in digital systems to ensure the successful transfer of data. One method uses an extra bit, called a parity-check bit; if each bit is considered as a 1 or 0 (depending on whether or not it is set), the sum of a fixed number of bits can be made even (or odd) by properly setting the parity bit to a one or zero. Errors are detected on the receiving end simply by checking whether each received word is even (or odd). Audio data on a compact disc is digitally encoded and a special error correcting code is used to detect and correct errors that may have been introduced through manufacturing error or are created during the reading or playing process.
Certain arbitrary codes are used to ensure secrecy of communication; merely the message, without the rules by which the symbols are associated, will not provide an eavesdropper with an understandable version of it (see cryptography). See also signaling.
See P. Lunde, ed., The Book of Codes (2009).
code, in law
codethe differential usage of a system or collection of SIGNS, marking differential social memberships. Codes may be conscious or unconscious. An influential example is BERNSTEIN's notion of differential usages of English, by which fundamental status and class differences are communicated and reproduced (see ELABORATED AND RESTRICTED CODES). Other examples include dress and fashion codes, with identities claimed or refused by items selected, and by the selection of terms, actions or items used as identification by formal and informal special interest groups, secret societies, sexual minorities or drug users.
in digital computers, a preset system of symbols representing the information in the computer. Each code uses the characters of its own alphabet. For most codes the alphabet has two symbols or consists of letters from a two-symbol alphabet. The physical form of a code depends on the nature of the data carrier used, and there may be several versions for the same computer. For example, in written documents the code is in the form of numerals and (or) letters of the Russian or Latin alphabet; on punched cards it is a combination of perforated and unperforated areas; on magnetic drums and disks it is in the form of configurations of magnetized sections; in the cells of an internal memory it is in the form of groups of magnetic cores, each of which is in one of two possible states. The main symbols used in a digital computer are 0 and 1.
The codes used in a computer usually include a symbolic (alpha-numeric) code to represent textual information and programs written in algorithmic languages, an instruction code to represent programs in machine language, and a number code to represent the numerical data. The plan of the code, in which all its principal parts and the number of binary symbols in each of its parts are indicated, is called the code format.
A symbolic code is a series of groups composed of identical numbers of binary symbols (eight in most modern digital computers). Each group designates one symbol (a letter, arbitrary sign, or number). The number of groups in a code depends on the length of the coded text. The instruction code includes as a principal part an “operation code,” which determines the computer’s response to a certain instruction, and the structure of the remainder of the code, which may contain the address (operand) codes, the desired results, and also sometimes the codes of the operands themselves and the codes for other parts of the instruction. A number code depends on the type of numerical representation in the computer. A number with a fixed point is represented by one of three codes (the true representation, the one’s complement, and the complement). A number code that is represented by a form with a floating point is written as an ordered pair consisting of the mantissa and order codes; both the mantissa and the order can be represented by one of the three codes indicated above.
A true representation is usually used to store numbers in a memory unit; the one’s complement and complement are used to execute arithmetic and certain other operations. When sending numbers from the memory unit to the arithmetic unit and vice versa, they are recoded. All three codes are made up of a sign code (the number of digits assigned is /), an integer code (m), and a fraction code (n). The sum d = l + m + n is called the code length. In a digital computer or its hardware, l,m, and n are usually fixed. For integers n = 0, for proper fractions usually m = 0, and when all the numbers have the same sign, l = 0.
For positive numbers the digit code is denoted by a series of zeros; for negative numbers, by a series of ones. For positive numbers the true representation, one’s complement, and complement coincide. In the true representation, the representation of negative numbers differs from that of positive numbers only in the sign code; in the one’s complement the digits of the number are replaced by their complements with respect to 1 (that is, 0 is replaced by 1, and vice versa). The complement of a negative number differs from the one’s complement in that after the inversion of the digits the result is added to a d-digit number having zeros in all places except the lowest, without a carryover operation. For example, a binary-system number is equal to + 11.01. Let l = 2, m = 3, and n = 4; by adding zeros to the integral and fractional parts we will write the number in the form +011.0100. The true representation, one’s complement, and complement of the given number are identical—that is, 00 011 0100. For the negative number — 11.01 the true representation has the form 11 011 0100, and the one’s complement has the form 11 100 1011, and the complement code is 11 100 1100. The choice between the one’s complement and the complement depends on the design and logic of the computer.
REFERENCESKitov, A. I., and N. A. Krinitskii. Elektronnye tsifrovye mashiny i programmirovanie, 2nd ed. Moscow, 1961.
Krinitskii, N. A., G. A. Mironov, and G. D. Frolov. Programmirovanie, 2nd ed. Moscow, 1966.
N. A. KRINITSKII
a system of preset characters for transmitting, processing, and storing (retrieving) a variety of information. A finite sequence of coded characters is called a word. The number of different characters used in the words of a given code is known as its base; for example a code with base 2 is called a binary code. If all the words have the same length or number of elements n, it is a uniform n-unit code. If the words are of variable length, the code is called nonuniform—for example, Morse code. A code is considered to be complete when it is impossible to add another new code combination without impairing its intelligibility. A complete, uniform n-unit code contains mn words, where m is the base of the code. A code containing code combinations that are used to isolate one message from another is called a code with delimiters; one in which all code combination are used only to designate message elements is called a code without delimiters. The code combinations that form delimiters can be made up either of special code characters or of the same code characters that form the code combinations corresponding to definite elements of a message. Sometimes it is convenient to divide the message elements into several groups and to construct a code for each group; the signal for shifting from one code to another is supplied by special code combinations (addresses). A set of codes for each of the group of elements together with the address code combinations is known as a multiple-address or multiple-program code.
Codes are usually transcribed by digits and numbers (0, 1, 2, . . . , 57, 9276, and so on) or signs, such as + (plus), — (minus), · (dot), or — (dash). In engineering each code character is a predetermined designation for some elementary signal having certain physical parameters (signal characteristics) that can take on different values. For electrical signals these characteristics may be the amplitude of the current or voltage, the polarity or duration of electric pulses (or pulse trains), or their repetition rate.
The codes used in remote control, in communications and automatic control systems, and in computer technology are a set of combinations made up of electric pulses and the spaces between them, which is equivalent to a representation of the coded values in the form of binary numbers—that is, sets composed of 0’s and 1’s. The number of pulses in the combinations or bits in the equivalent binary number determines the number of units in the code.
The selection of a code depends on the conditions of transmission, processing, or storage of the information and is associated mainly with the most efficient use of communications channels and the provision of the required noise immunity during transmission. Codes are made more complicated in order to improve their noise immunity: extra checking digits are added to the information characters. Codes that detect and correct errors are constructed using this principle.
in remote control, a system of electric or pneumatic signals used for transmitting binary or binary-coded decimal messages over a communications channel. Signals that differ in such characteristics as amplitude, frequency, polarity, phase, and duration are used to represent and transmit single elements of a code. Thus, in a binary code using polarity indicators, the element “0” is coded by a pulse of negative polarity and the element “1” is coded by a pulse of positive polarity. Width indicators may mean a difference in the duration of a pulse or in the duration of the interval between pulses. If not all possible combinations of elements are used for message transmissions, the use of special methods becomes feasible, making possible during reception the detection and correction of distortions (errors) in the transmitted elements of a code. Such methods improve the reliability of data transmission.
The choice of a coding system, the means of transmission, and the methods for improving the reliability of transmitted information depend on the actual operating conditions of the remote-control system, the importance of objectives to be achieved, the properties of communications channels, and the apparatus being used.
REFERENCESShastova, G. A. Kodirovanie i pomekhoustoichivost’ peredachi telemekhanicheskoi informatsii. Moscow-Leningrad, 1966.
Il’in, V. A. Telekontrol’ i teleupravlenie. Moscow, 1969.
M. M. GEL’MAN
a set of arbitrary symbols used by members of the diplomatic service for secret correspondence with their government and by the armed forces for transmission of orders, directives, and messages. Encipherment is achieved by substituting in varying combinations numerals or letters of the alphabet for entire phrases or for words, syllables, or letters in accordance with a predetermined system, which, consequently, is the key for deciphering the text. Texts written in double codes must be deciphered twice and require two keys. Codes do not always guarantee secrecy of correspondence, since the key to even a very complex code can be derived from calculations, from computations involving the recurrence of individual symbols, and by other means.
code (secondary surveillance radar, or SSR code)
The word "code" is often used to distinguish instructions from data (e.g. "The code is marked 'read-only'") whereas the word "software" is used in contrast with "hardware" and may consist of more than just code.
code(1) A set of machine symbols that represents data or instructions. See data code and machine language.
(2) Any representation of one set of data for another. For example, a parts code is an abbreviated name of a product, product type or category. A discount code is a percentage.
(3) To write a program. See coding, source code and line of code.
(4) To encode for security purposes. See cryptography and coding.