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. 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. "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 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. 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" to generate codes of variable length. Though most commercial use of Morse code was via machinery, it was also used as a manual code, generatable by hand on a telegraph key and decipherable by ear, and persists in amateur radio use. Most codes are of fixed per-character length or variable-length sequences of fixed-length codes.
Common examples of character encoding systems include Morse code, the Baudot code, the American Standard Code for Information Interchange 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 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, Fieldata fell short of its goals and was short-lived. In 1963 the first ASCII code was released by the ASCII committee 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 ASCII67 was adopted fairly widely. ASCII67's American-centric nature was somewhat addressed in the European ECMA-6 standard, which persists today as the base encoding for the UNICODE extended encoding strings.
Somewhat historically isolated, IBM's Binary Coded Decimal 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, 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. For the most part, IBMs codes were used primarily with IBM equipment, which was more or less a closed ecosystem, and did not see much adoption outside of IBM "circles". IBM's Extended Binary Coded Decimal Interchange Code 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.
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, those additional bits were a colossal waste of then-scarce and expensive computing resources.
The compromise solution that was eventually found and developed into Unicode was to break the assumption 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 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:
; Character repertoire :
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:
Example of a code unit: Consider a string of the letters "abc" followed by . That string contains:
The convention to refer to a character in Unicode is to start with 'U+' followed by the codepoint value in hexadecimal. The range of valid code points for the Unicode standard is U+0000 to U+10FFFF, inclusive, divided in 17 planes, identified by the numbers 0 to 16. Characters in the range U+0000 to U+FFFF are in plane 0, called the Basic Multilingual Plane. This plane contains most commonly-used characters. Characters in the range U+10000 to U+10FFFF in the other planes are called supplementary characters.
The following table shows examples of code point values:
CharacterUnicode code pointGlyph
Latin AU+0041Α
Latin sharp SU+00DFß
Han for EastU+6771東
AmpersandU+0026&
Inverted exclamation markU+00A1¡
Section signU+00A7§

A code point is represented by a sequence of code units. The mapping is defined by the encoding. Thus, the number of code units required to represent a code point depends on the encoding:
and its parallel standard, the ISO/IEC 10646 Universal Character Set, together constitute a modern, unified character encoding. Rather than mapping characters directly to octets, they separately define what characters are available, corresponding natural numbers, how those numbers are encoded as a series of fixed-size natural numbers, and finally how those units are encoded as a stream of octets. The purpose of this decomposition is to establish a universal set of characters that can be encoded in a variety of ways. To describe this model correctly requires more precise terms than "character set" and "character encoding." The terms used in the modern model follow:
A character repertoire is the full set of abstract characters that a system supports. The repertoire may be closed, i.e. no additions are allowed without creating a new standard, or it may be open, allowing additions. The characters in a given repertoire reflect decisions that have been made about how to divide writing systems into basic information units. The basic variants of the Latin, Greek and Cyrillic alphabets can be broken down into letters, digits, punctuation, and a few special characters such as the space, which can all be arranged in simple linear sequences that are displayed in the same order they are read. But even with these alphabets, diacritics pose a complication: they can be regarded either as part of a single character containing a letter and diacritic, or as separate characters. The former allows a far simpler text handling system but the latter allows any letter/diacritic combination to be used in text. Ligatures pose similar problems. Other writing systems, such as Arabic and Hebrew, are represented with more complex character repertoires due to the need to accommodate things like bidirectional text and glyphs that are joined together in different ways for different situations.
A coded character set is a function that maps characters to code points. For example, in a given repertoire, the capital letter "A" in the Latin alphabet might be represented by the code point 65, the character "B" to 66, and so on. Multiple coded character sets may share the same repertoire; for example ISO/IEC 8859-1 and IBM code pages 037 and 500 all cover the same repertoire but map them to different code points.
A character encoding form is the mapping of code points to code units to facilitate storage in a system that represents numbers as bit sequences of fixed length. For example, a system that stores numeric information in 16-bit units can only directly represent code points 0 to 65,535 in each unit, but larger code points could be represented by using multiple 16-bit units. This correspondence is defined by a CEF.
Next, a character encoding scheme is the mapping of code units to a sequence of octets to facilitate storage on an octet-based file system or transmission over an octet-based network. Simple character encoding schemes include UTF-8, UTF-16BE, UTF-32BE, UTF-16LE or UTF-32LE; compound character encoding schemes, such as UTF-16, UTF-32 and ISO/IEC 2022, switch between several simple schemes by using byte order marks or escape sequences; compressing schemes try to minimise the number of bytes used per code unit.
Although UTF-32BE is a simpler CES, most systems working with Unicode use either UTF-8, which is backward compatible with fixed-width ASCII and maps Unicode code points to variable-width sequences of octets, or UTF-16BE, which is backward compatible with fixed-width UCS-2BE and maps Unicode code points to variable-width sequences of 16-bit words. See comparison of Unicode encodings for a detailed discussion.
Finally, there may be a higher level protocol which supplies additional information to select the particular variant of a Unicode character, particularly where there are regional variants that have been 'unified' in Unicode as the same character. An example is the XML attribute xml:lang.
The Unicode model uses the term character map for historical systems which directly assign a sequence of characters to a sequence of bytes, covering all of CCS, CEF and CES layers.

Character sets, character maps and code pages

Historically, the terms "character encoding", "character map", "character set" and "code page" were synonymous in computer science, as the same standard would specify a repertoire of characters and how they were to be encoded into a stream of code units – usually with a single character per code unit. But now the terms have related but distinct meanings, due to efforts by standards bodies to use precise terminology when writing about and unifying many different encoding systems. Regardless, the terms are still used interchangeably, with character set being nearly ubiquitous.
A "code page" usually means a byte-oriented encoding, but with regard to some suite of encodings, where many characters share the same codes in most or all those code pages. Well-known code page suites are "Windows" and "IBM"/"DOS", see Windows code page for details. Most, but not all, encodings referred to as code pages are single-byte encodings
IBM's Character Data Representation Architecture designates with coded character set identifiers and each of which is variously called a "charset", "character set", "code page", or "CHARMAP".
The term "code page" does not occur in Unix or Linux where "charmap" is preferred, usually in the larger context of locales.
In contrast to a "coded character set", a "character encoding" is a map from abstract characters to code words. A "character set" in HTTP parlance is the same as a character encoding.
"Legacy encoding" is a term sometimes used to characterize old character encodings, but with an ambiguity of sense. Most of its use is in the context of Unicodification, where it refers to encodings that fail to cover all Unicode code points, or, more generally, using a somewhat different character repertoire: several code points representing one Unicode character, or versa. Some sources refer to an encoding as legacy only because it preceded Unicode. All Windows code pages are usually referred to as legacy, both because they antedate Unicode and because they are unable to represent all 221 possible Unicode code points.

Character encoding translation

As a result of having many character encoding methods in use, many computer programs have been developed to translate data between encoding schemes as a form of data transcoding. Some of these are cited below.
Cross-platform:
Unix-like:
Windows: