Week Nine | Part One: Character Encoding.

What the *$! are Character Sets?

I suspect that many of you have hit some kind of snag whem you validated your Web pages for Strict DTD. I suspect further that this snag was the result of the validator having to chose the character set for your page because no set was specified

But what are these marvelous things call character sets?

I confess that, until I needed to teach this course, I never gave them any thought. But since the absence of them seems to cause some validation problems for both of us, I thought a brief examination of what character sets was in order.

The following is from Wikipedia. Enjoy!

An Introduction to Character Sets.

"A character encoding or character set (sometimes referred to as code page) consists of a code that pairs a sequence of characters from a given set 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. 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, both as integers and as 7-bit binary versions of those integers, generally extended with an extra zero-bit to facilitate storage in 8-bit bytes (octets)."

In earlier days of computing, the introduction of character sets such as ASCII (1963) and EBCDIC (1964) began the process of standardization. The limitations of such sets soon became apparent, and a number of ad-hoc methods developed to extend them. The need to support multiple writing systems, 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."

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."

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). 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). 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."

"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. In a coded character set, each code point only represents one character."

"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."

There! Now you know!