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A character encoding consists of a code that pairs a sequence of characters from a given character set (sometimes referred to as code page) with something else, such as a sequence of natural numbers, octets or electrical pulses, in order to facilitate the storage of text in computers and the transmission of text through telecommunication networks. In Information theory, a code is a function mapping an alphabet to non-negative real numbers satisfying a generalization of Kraft's inequality. For other uses see Character. In Computer and machine-based Telecommunications terminology a character is a unit of Code page is the traditional IBM term used to map a specific set of characters to numerical Code point values. A number is an Abstract object, tokens of which are Symbols used in Counting and measuring. In Computing, an octet is a grouping of eight Bits Octet, with the only exception noted below always refers to an entity having exactly eight Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital For other uses see Character. In Computer and machine-based Telecommunications terminology a character is a unit of A computer is a Machine that manipulates data according to a list of instructions. Common examples include Morse code, which encodes letters of the Latin alphabet as series of long and short depressions of a telegraph key; and ASCII, which encodes letters, numerals, and other symbols, as integers. Morse code is a Character encoding for transmitting telegraphic information using standardized sequences of short and long elements to represent the letters numerals Telegraph key, also known as a Morse key, are generic terms for any switching device used primarily to send Morse code. American Standard Code for Information Interchange ( ASCII) The integers (from the Latin integer, literally "untouched" hence "whole" the word entire comes from the same origin but via French

In earlier days of computing, the introduction of coded character sets such as ASCII (1963) and EBCDIC (1964) began the process of standardization. American Standard Code for Information Interchange ( ASCII) Extended Binary Coded Decimal Interchange Code ( EBCDIC) is an 8- Bit Character encoding ( Code page) used on IBM mainframe Operating The limitations of such sets soon became apparent, and a number of ad-hoc methods developed to extend them. Ad hoc is a Latin phrase which means "for this [ Purpose ]" The need to support multiple writing systems (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. A writing system is a type of Symbolic system used to represent elements or statements expressible in Language. CJK is a collective term for Chinese, Japanese, and Korean, which constitute the main East Asian languages.

Contents

Simple character sets

Conventionally character set and character encoding were considered synonymous, as the same standard would specify both what characters were available and how they were to be encoded into a stream of code units (usually with a single character per code unit). For historical reasons, MIME and systems based on it use the term charset to refer to the complete system for encoding a sequence of characters into a sequence of octets. Multipurpose Internet Mail Extensions ( MIME) is an Internet standard that extends the format of e-mail to support text in Character

Modern encoding model

Unicode and its parallel standard, ISO 10646 Universal Character Set, which together constitute the most modern character encoding, broke away from this idea, and instead separated the ideas of what characters are available, their numbering, how those numbers are encoded as a series of "code units" (limited-size numbers), and finally how those units are encoded as a stream of octets (bytes). In Computing, Unicode is an Industry standard allowing Computers to consistently represent and manipulate text expressed in most of the world's The Universal Character Set (UCS defined by the ISO / IEC 10646 International Standard, is a standard set of characters upon which The idea behind this decomposition is to establish a universal set of characters that can be encoded in a variety of ways. To correctly describe this model needs 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, that is no additions are allowed without creating a new standard (as is the case with ASCII and most of the ISO-8859 series), or it may be open, allowing additions (as is the case with Unicode and to a limited extent the Windows code pages). Windows code pages are sets of characters or Code pages (known as Character encodings in other operating systems used in Microsoft Windows systems The characters in a given repertoire reflect decisions that have been made about how to divide writing systems into linear 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 like the space, which can all be arranged in simple linear sequences that are displayed in the same order they are read. The Greek alphabet (Ελληνικό αλφάβητο is a set of twenty-four letters that has been used to write the Greek language since the late 9th or early The Cyrillic alphabet (səˈrɪlɪk also called azbuka, from the old name of the first two letters is actually a family of Alphabets, subsets of which are used by Even with these alphabets however diacritics pose a complication: they can be regarded either as part of a single character containing a letter and diacritic (known in modern terminology as a precomposed character), or as separate characters. A diacritic ( also called a diacritic or diacritical mark, point, or sign, is a small sign added to a letter to alter pronunciation The former allows a far simpler text handling system but the latter allows any letter/diacritic combination to be used in text. 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 glyph is an element of writing Two or more glyphs representing the same symbol whether interchangeable or context-dependent are called Allographs the abstract unit they

A coded character set specifies how to represent a repertoire of characters using a number of non-negative integer codes called code points. For example, in a given repertoire, a character representing the capital letter "A" in the Latin alphabet might be assigned to the integer 65, the character for "B" to 66, and so on. A complete set of characters and corresponding integers is a coded character set. Multiple coded character sets may share the same repertoire; for example ISO-8859-1 and IBM code pages 037 and 500 all cover the same repertoire but map them to different codes. ISO 8859-1, more formally cited as ISO/IEC 8859-1 is part 1 of ISO/IEC 8859, a standard Character encoding of the Latin alphabet. In a coded character set, each code point only represents one character.

A character encoding form (CEF) specifies the conversion of a coded character set's integer codes into a set of limited-size integer code values that facilitate storage in a system that represents numbers in binary form using a fixed number of bits (i. e. practically any computer system). For example, a system that stores numeric information in 16-bit units would only be able to directly represent integers from 0 to 65,535 in each unit, but larger integers could be represented if more than one 16-bit unit could be used. This is what a CEF accommodates: it defines a way of mapping single code point from a range of, say, 0 to 1. 4 million, to a series of one or more code values from a range of, say, 0 to 65,535.

The simplest CEF system is simply to choose large enough units that the values from the coded character set can be encoded directly (one code point to one code value). This works well for coded character sets that fit in 8 bits (as most legacy non-CJK encodings do) and reasonably well for coded character sets that fit in 16 bits (such as early versions of Unicode). However, as the size of the coded character set increases (e. g. modern Unicode requires at least 21 bits/character), this becomes less and less efficient, and it is difficult to adapt existing systems to use larger code values. Therefore, most systems working with later versions of Unicode use either UTF-8, which maps Unicode code points to variable-length sequences of octets, or UTF-16, which maps Unicode code points to variable-length sequences of 16-bit words. UTF-8 (8- Bit UCS / Unicode Transformation Format) is a variable-length Character encoding for Unicode. In Computing, UTF-16 (16- Bit Unicode Transformation Format is a variable-length Character encoding for Unicode, capable of encoding

Next, a character encoding scheme (CES) specifies how the fixed-size integer codes should be mapped into an octet sequence suitable for saving on an octet-based file system or transmitting over an octet-based network. With Unicode, a simple character encoding scheme is used in most cases, simply specifying whether the bytes for each integer should be in big-endian or little-endian order (even this isn't needed with UTF-8). However, there are also compound character encoding schemes, which use escape sequences to switch between several simple schemes (such as ISO 2022), and compressing schemes, which try to minimise the number of bytes used per code unit (such as SCSU, BOCU, and Punycode). ISO 2022, more formally ISO/IEC 2022 "Information Technology—Character code structure and extension techniques" is an ISO standard (equivalent to the The Standard Compression Scheme for Unicode (SCSU is a Unicode Technical Standard for reducing the number of Bytes needed to represent Unicode text especially BOCU-1 is a MIME compatible Unicode compression scheme BOCU stands for B inary O rdered C ompression for U nicode Punycode is a Computer programming encoding syntax by which a Unicode string of characters can be translated into the more-limited character set permitted

Finally, there may be a higher level protocol which supplies additional information that can be used 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. In Computing, Unicode is an Industry standard allowing Computers to consistently represent and manipulate text expressed in most of the world's An example is the XML attribute xml:lang.

History of character encodings

Early binary repertoires:

Popular character encodings

Character conversion tools

Cross-platform:

Linux:

Windows:

See also

References

  1. ^ Homepage of Michael Goerz - convert_encoding.py
  2. ^ decodeh - heuristically decode a string or text file
  3. ^ recode - GNU Project - Free Software Foundation (FSF)
  4. ^ Utrac Homepage
  5. ^ convmv - converts filenames from one encoding to another
  6. ^ Extremely Naive Charset Analyser
  7. ^ Character Set Converter

External links

Dictionary

character encoding

-noun

  1. (computing) A well-defined correspondence between characters and numbers used to represent them.
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