Character encoding
In computing, data storage, and data transmission, character encoding is used to represent a repertoire of characters by some kind of encoding system that assigns a number to each character for digital representation.[1] 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, or anything of the like. A character encoding is used in computation, data storage, and transmission of textual data. "Character set", "character map", "codeset" and "code page" are related, but not identical, terms.
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 most of the characters used in many written languages. Character encoding using internationally accepted standards permits worldwide interchange of text in electronic form.
History[]
The history of character codes illustrates the evolving need for machine-mediated character-based symbolic information over a distance, using once-novel electrical means. The earliest codes were based upon manual and hand-written encoding and cyphering systems, such as Bacon's cipher, Braille, International maritime signal flags, and the 4-digit encoding of Chinese characters for a Chinese telegraph code (Hans Schjellerup, 1869). With the adoption of electrical and electro-mechanical techniques these earliest codes were adapted to the new capabilities and limitations of the early machines. The earliest well-known electrically-transmitted character code, Morse code, introduced in the 1840s, used a system of four "symbols" (short signal, long signal, short space, long space) to generate codes of variable length. Though some commercial use of Morse code was via machinery, it was often used as a manual code, generated by hand on a telegraph key and decipherable by ear, and persists in amateur radio and aeronautical use. Most codes are of fixed per-character length or variable-length sequences of fixed-length codes (e.g. Unicode). [2]
Common examples of character encoding systems include Morse code, the Baudot code, the American Standard Code for Information Interchange (ASCII) and Unicode. Unicode, a well defined and extensible encoding system, has supplanted most earlier character encodings, but the path of code development to the present is fairly well known.
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. The name "baudot" has been erroneously applied to ITA2 and its many variants. ITA2 suffered from many shortcomings and was often "improved" by many equipment manufacturers, sometimes creating compatibility issues. In 1959 the U.S. military defined its Fieldata code, a six-or seven-bit code, introduced by the U.S. Army Signal Corps. While Fieldata addressed many of the then-modern issues (e.g. letter and digit codes arranged for machine collation), Fieldata fell short of its goals and was short-lived. In 1963 the first ASCII (American Standard Code for Information Interchange) code was released (X3.4-1963) by the ASCII committee (which contained at least one member of the Fieldata committee, W. F. Leubbert) which addressed most of the shortcomings of Fieldata, using a simpler code. Many of the changes were subtle, such as collatable character sets within certain numeric ranges. ASCII63 was a success, widely adopted by industry, and with the follow-up issue of the 1967 ASCII code (which added lower-case letters and fixed some "control code" issues) ASCII67 was adopted fairly widely. ASCII67's American-centric nature was somewhat addressed in the European ECMA-6 standard.[3]
Herman Hollerith invented punch card data encoding in the late 19th century to analyze census data. Initially, each hole position represented a different data element, but later, numeric information was encoded by numbering the lower rows 0 to 9, with a punch in a column representing its row number. Later alphabetic data was encoded by allowing more than one punch per column. Electromechanical tabulating machines represented date internally by the timing of pulses relative to the motion of the cards through the machine. When IBM went to electronic processing, starting with the IBM 603 Electronic Multiplier, it used a variety of binary encoding schemes that were tied to the punch card code.
IBM's Binary Coded Decimal (BCD) was a six-bit encoding scheme used by IBM as early as 1953 in its 702[4]: p. 80 and 704 computers, and in its later 7000 Series and 1400 series, as well as in associated peripherals. Since the punched card code then in use only allowed digits, upper-case English letters and a few special characters, 6 bits were sufficient. 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. IBMs codes were used primarily with IBM equipment; other computer vendors of the era had their own character codes, often 6 bit, but usually had the ability to read tapes produced on IBM equipment. BCD was the precursor IBM's Extended Binary Coded Decimal Interchange Code (usually abbreviated as EBCDIC), an eight-bit encoding scheme developed in 1963 for the IBM System/360 that featured a larger character set, including lower case letters.
The limitations of such sets soon became apparent,[to whom?] 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 , required support for a far larger number of characters and demanded a systematic approach to character encoding rather than the previous ad hoc approaches.[citation needed]
In trying to develop universally interchangeable character encodings, researchers in the 1980s faced the dilemma that, on the one hand, it seemed necessary to add more bits to accommodate additional characters, but 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), those additional bits were a colossal waste of then-scarce and expensive computing resources (as they would always be zeroed out for such users). In 1985, the average personal computer user's hard disk drive could store only about 10 megabytes, and it cost approximately US$250 on the wholesale market (and much higher if purchased separately at retail),[5] so it was very important at the time to make every bit count.
The compromise solution that was eventually found and developed into Unicode was to break the assumption (dating back to telegraph codes) that each character should always directly correspond to a particular sequence of bits. Instead, characters would first be mapped to a universal intermediate representation in the form of abstract numbers called code points. Code points would then be represented in a variety of ways and with various default numbers of bits per character (code units) depending on 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.
Terminology[]
- Terminology related to character encoding
- A character is a minimal unit of text that has semantic value.
- A character set is a collection of characters that might be used by multiple languages. Example: The Latin character set is used by English and most European languages, though the Greek character set is used only by the Greek language.
- A coded character set is a character set in which each character corresponds to a unique number.
- A code point of a coded character set is any allowed value in the character set or code space.
- A code space is a range of integers whose values are code points
- A code unit is a bit sequence used to encode each character of a repertoire within a given encoding form. This is referred to as a code value in some documents.[6]
- Character repertoire (the abstract set of characters)
The character repertoire is an abstract set of more than one million characters found in a wide variety of scripts including Latin, Cyrillic, Chinese, Korean, Japanese, Hebrew, and Aramaic.
Other symbols such as musical notation are also included in the character repertoire. Both the Unicode and GB18030 standards have a character repertoire. As new characters are added to one standard, the other standard also adds those characters, to maintain parity.
The code unit size is equivalent to the bit measurement for the particular encoding:
- A code unit in US-ASCII consists of 7 bits;
- A code unit in UTF-8, EBCDIC and GB18030 consists of 8 bits;
- A code unit in UTF-16 consists of 16 bits;
- A code unit in UTF-32 consists of 32 bits.
Example of a code unit: Consider a string of the letters "abc" followed by U+10400