:For other senses of the word "code", see code (disambiguation).
In communications, a code is a rule for converting a piece of information (for example, a letter, word, or phrase) into another form or representation, not necessarily of the same sort. In communications and information processing, encoding is the process by which a source ( object) performs this conversion of information into data, which is then sent to a receiver ( observer), such as a data processing system. Decoding is the reverse process of converting data, which has been sent by a source, into information understandable by a receiver. A codecCodec is a portmanteau of co der dec oder", which describes a device or program capable of performing transformations on a data stream or signal. Codecs can both put the stream or signal into an encoded form (often for transmission, storage or encryption) is an implementation of that rule (or algorithm) for coding and decoding, for example MP3MP3 (or, more precisely, MPEG-1/2 Audio Layer 3 is an audio compression algorithm capable of greatly reducing the amount of data required to reproduce audio, while sounding like a faithful reproduction of the original uncompressed audio to the listener., which may be a hardwareHardware is equipment such as fasteners, keys, locks, hinges, wire, chains, plumbing supples, tools, utensils, cutlery and machine parts, especially when they are made of metal. In the United States, hardware has been traditionally sold in " hardware stor implementation or a software implementation , and which may include compressionThere are several types of compression physical compression data compression multimedia compression image compression audio compression video compression bandwidth compression audio level compression compression (functional analysis)..
One reason for coding is to enable communication in places where ordinary spoken or written language is difficult or impossible. For example, a cable code replaces words (eg, ship or invoice) into shorter words, allowing the same information to be sent with fewer charactersFor alternate meanings, see character. In computer terminology, a character is a unit of information that roughly corresponds to a grapheme, or written symbol, of a natural language, such as a letter, numeral, or punctuation mark. The concept also include, more quickly, and most important, less expensively. Another example is the use of semaphore flagsThe semaphore line was a signalling system invented by the Chappe brothers in France. It is different from the naval semaphore system that uses hand-held flags, which was invented later. Claude Chappe began development when he and his four brothers lost t, where the configuration of flags held by a signaller or the arms of a semaphore tower encodes parts of the message, typically individual letters and numbers. Another person standing a great distance away can interpret the flags and reproduce the words sent.
In the history of cryptography, codes were once common for ensuring the confidentiality of communications, although ciphers are now used instead. See code (cryptography).
1 Codes in communication used for brevity
Code can be used for brevity . When telegraph messages were the state of the art in rapid long distance communication, elaborate commercial codes which encoded complete phrases into single words (commonly five-letter groups) were developed, so that telegrapher s became conversant with such "words" as BYOXO ("Are you trying to weasel out of our deal?"), LIOUY ("Why do you not answer my question?"), BMULD ("You're a skunk!"), or AYYLU ("Not clearly coded, repeat more clearly."). Code words were chosen for various reasons: length, pronouncability , etc. Meanings were chosen to fit perceived needs: commercial negotiations, military terms for military codes, diplomatic terms for diplomatic codes, any and all of the preceding for espionage codes, ... Codebooks and codebook publishers proliferated, including one run as a front for the American Black Chamber run by Herbert Yardley between WWI and WWII. The purpose of most of these codes was to save on cable costs.
In the computer era since World War II, there are also "codes" for data compression, e.g. Huffman coding, which uses short codes for frequent symbols and longer codes for seldom used symbols - the same principle is used in the Morse code. It and the Baudot code which uses the same length representation for all symbols and characters, both go back to telegraph days. The latter was a primary ancestor of the ASCII character code widely used in computers.
