Early in the twentieth century, detailed commercial codes (systems of rules for replacing a piece of information such as a letter, word, or phrase with an arbitrary symbolic equivalent) were developed for communications purposes. One such code was the Baudot code (pronounced baw-DOH), a simple five-bit alphanumeric coding scheme. It was originally developed for telegraphy (a system of communication using an electrical apparatus to transmit and receive pulses), but principally used for telex transmissions (a system for sending and receiving typed messages electronically). The Baudot code mapped complete phrases to single code words (five-bit groups). However, this type of code later proved inadequate for radio and other, more advanced forms of communications that were soon developed.
Baudot code is a character set (a mapping between characters and bit strings) that preceded the more sophisticated character set EBCDIC (Extended Binary Coded Decimal Interchange Code), which was developed in the 1960s by International Business Machines (IBM) Corporation. During the late nineteenth century, French engineer and telegrapher Jean-Maurice-Émile Baudot (1845-1903) developed Baudot code, for which he received a patent in 1874. By the mid-twentieth century it had replaced Morse code (the original telegraphic alphabet that American inventor Samuel F. B. Morse created with the help of other scientists) as the most commonly used telegraphic alphabet. Baudot code was one of the first standards for international telegraphy.
In Baudot code, characters are expressed using 5 bits per character. A bit is the smallest unit of information, having a value of either 1 or 0 (or "true" or "false," or any other binary pair). Since 5 bits can only produce 32 unique codes (25 = 32), two special codes (named FIGS and LTRS) are used to shift between a "figures" subset (containing 31 "figure" characters) and a "letters" subset (containing 31 "letter" characters). Transmission of the FIGS code word (11011) signifies that following words are to be interpreted as being in the FIGS subset, which includes numerals and certain other characters: occurrence of the LTRS word (11111) signifies that following words are to be interpreted as being in the LTRS subset, which includes the alphabet. Occurrence of FIGS always switches interpretation to figures, and occurrence of LTRS always switches it back to letters. Thus most 5-bit words have two meanings in Baudot code, depending on whether the FIGS or LTRS code word has appeared most recently: 10101, say, represents either Y (LTRS mode) or 6 (FIGS mode). A sample Baudot code string is as follows:
The resulting output is MAY_06,_DAY. Note that the code word FIGS (11011) switches interpretation to numbers, and that LTRS (11111) switches it back to letters. Note also that the word 00100 is interpreted as a space character in both the LTRS and FIGS modes, so that modes need not be switched to transmit a space character (the most common character in ordinary text).
Baudot code was an improvement over the earlier Morse code system of short dots and long dashes. Its 32 binary words, including the FIGS and LTRS subsets, were sufficient to represent the alphabet and several punctuation signs and even to control certain mechanical functions. Modern versions of the Baudot code usually use groups of 7 or 8 bits. Groups of 7 bits permit transmission of 128 characters (27 = 128). Groups of 8 permit transmission of 256 characters (28 = 256), allowing for even more complex functions, including the use of error-correction codes.. Today, Baudot code survives primarily in TDDs (Telecommunications Devices for the Deaf) and some ham (amateur) radio applications.
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