|
In computing, a character encoding is used to represent a repertoire of characters by some kind of an encoding system.〔http://techterms.com/definition/characterencoding〕 Depending on the abstraction level and context, corresponding code points and the resulting code space may be regarded as bit patterns, octets, natural numbers, electrical pulses, etc. A character encoding is used in computation, data storage, and transmission of textual data. Terms such as character set, character map, codeset or code page are sometimes used as near synonyms; however, these terms have related but distinct meanings. Early character codes associated with the optical or electrical telegraph could only represent a subset of the characters used in written languages, sometimes restricted to upper case letters, numerals and some punctuation only. The low cost of digital representation of data in modern computer systems allows more elaborate character codes (such as Unicode) which represent more of the characters used in many written languages. Character encoding using internationally accepted standards permits worldwide interchange of text in electronic form. ==History== Early binary repertoires include Bacon's cipher, Braille, International maritime signal flags, and the 4-digit encoding of Chinese characters for a Chinese telegraph code (Hans Schjellerup, 1869). Common examples of character encoding systems include Morse code, the Baudot code, the American Standard Code for Information Interchange (ASCII) and Unicode. Morse code was introduced in the 1840s and is used to encode each letter of the Latin alphabet, each Arabic numeral, and some other characters via a series of long and short presses of a telegraph key. Representations of characters encoded using Morse code varied in length. The Baudot code, a five-bit encoding, was created by Émile Baudot in 1870, patented in 1874, modified by Donald Murray in 1901, and standardized by CCITT as International Telegraph Alphabet No. 2 (ITA2) in 1930. Fieldata, a six- or seven-bit code, was introduced by the U.S. Army Signal Corps in the late 1950s. IBM's Binary Coded Decimal (BCD) was a six-bit encoding scheme used by IBM in as early as 1959 in its 1401 and 1620 computers, and in its 7000 Series (for example, 704, 7040, 709 and 7090 computers), as well as in associated peripherals. BCD extended existing simple four-bit numeric encoding to include alphabetic and special characters, mapping it easily to punch-card encoding which was already in widespread use. It was the precursor to EBCDIC. ASCII was introduced in 1963 and is a seven-bit encoding scheme used to encode letters, numerals, symbols, and device control codes as fixed-length codes using integers. IBM's Extended Binary Coded Decimal Interchange Code (usually abbreviated as EBCDIC) is an eight-bit encoding scheme developed in 1963. The limitations of such sets soon became apparent, and a number of ad hoc methods were developed to extend them. The need to support more writing systems for different languages, including the CJK family of East Asian scripts, required support for a far larger number of characters and demanded a systematic approach to character encoding rather than the previous ad hoc approaches. The frustrating dilemma that researchers in this field encountered in the 1980s as they tried to develop universally interchangeable character encodings was that on the one hand, it seemed to be necessary to add more bits to accommodate additional characters. On the other hand, for the users of the relatively small character set of the Latin alphabet (who still constituted the majority of computer users at the time), those additional bits were a colossal waste of then-scarce and expensive computing resources (as they would always be zeroed out for such users). The compromise solution that was eventually hit upon with Unicode, as further explained below, was to break the longstanding assumption (dating back to the old telegraph codes) that each character should always directly correspond to a particular pattern of encoded bits. Instead, characters would be first mapped to an intermediate stage in the form of abstract numbers known as code points. Those code points would then be encoded in a variety of ways and with various default numbers of bits per character (code units) depending upon context. To encode code points higher than the length of the code unit, such as above 256 for 8-bit units, the solution was to implement variable-width encodings where an escape sequence would signal that subsequent bits should be parsed as a higher code point. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「character encoding」の詳細全文を読む スポンサード リンク
|