History of Communications - The Baudot Code

The Baudot Code is an early example of a binary character code based on 5-bit values defining 32 different codewords. The invention of this code in 1870 by Emile Baudot marks a milestone in the history of telecommunications. More remarkably, radio communications had not yet been invented, nor any of the electronics that we normally associate with digital communications. Baudot envisaged an improved telegraph system where an operator would use a five-key keyboard to send individual characters efficiently by pressing combinations of the five keys. As technology advanced, the code invented by Baudot would be adapted for use throughout the next century, and his name is immortalized in the modern term baud.

The use of 5-bit codewords allows 26 alphabet letters (uppercase only); however, the clever use of the remaining codewords to include two control values FIGS and LTRS means that the same 5-bit codewords can be used to represent letters, numbers and punctuation. Whenever FIGS is encountered, all subsequent codewords are considered to be non-letters until LTRS is encountered. Baudot's invention also foreshadows the development of data compression techniques by sending messages containing more than 32 different characters while using an average of less than 6-bits per character.

In the 1930's, Baudot's code formed the basis for the CCITT ITA2 Code which became a standard for telecommunications for over 40 years. In North America, the same code with minor changes was called USTTY. The code was initially designed for use with electromechanical teleprinters sending and receiving messages at 45 bits per second, where start and stop bits accompany each character in an asychronous transmission. These devices were constructed long before the invention of transistors and data bits were shifted using mechnical timing wheels attached to electric motors. The 5-bit codewords were stored for transmission on punched paper tapes as shown below; electrical contacts extended through the punched holes as the paper was pulled through by another electric motor. Under these conditions, the use of short codewords is advantageous as timing and synchronization would not be practical with long codewords or at higher speeds.

In this example, the codeword for 'E' is the same as the codeword for '3' except that the first digit is preceded by the FIGS code. Similarly, the digit '5' and the letter 'T' share the same 5-bit codeword. Letters resume at the 'D' following the LTRS codeword.

CCITT ITA2 Code

One interesting aspect of this code is the way in which the characters were assigned to codewords. Look at your computer keyboard and note that the keys in the top row 'QWERTYUIOP' are directly below the digit keys '1234567890'. It is no coincidence that those are the LTRS and FIGS versions of these ten ITA2 codewords. Regularly spaced small holes provide a reference alignment and serve as sprocket holes so that a toothed wheel can pull the paper though a tape reader.

BINARY   PUNCHED PAPER TAPE   LETTERS   FIGURES
(USTTY)		   FIGURES
(ITA2)
00000   012534   NUL   NUL   NUL
00001   012534   E   3   3
00010   012534   LF   LF   LF
00011   012534   A   -   -
00100   012534   SP   SP   SP
00101   012534   S   '   '
00110   012534   I   8   8
00111   012534   U   7   7
01000   012534   CR   CR   CR
01001   012534   D   ENQ   $
01010   012534   R   4   4
01011   012534   J   BELL   '
01100   012534   N   ,   ,
01101   012534   F   !   !
01110   012534   C   :   :
01111   012534   K   (   (
10000   012534   T   5   5
10001   012534   Z   +   "
10010   012534   L   )   )
10011   012534   W   2   2
10100   012534   H   #   #
10101   012534   Y   6   6
10110   012534   P   0   0
10111   012534   Q   1   1
11000   012534   O   9   9
11001   012534   B   ?   ?
11010   012534   G   &   &
11011   012534   FIGS   FIGS   FIGS
11100   012534   M   .   .
11101   012534   X   /   /
11110   012534   V   ;   ;
11111   012534   LTRS   LTRS   LTRS