Karsten Behrend

Etech 153

Jean-Maurice-Émile Baudot

1845 - 1903

Jean-Maurice-Émile Baudot (b. 1845, Magneux, France--d. March 28, 1903, Sceaux), engineer who, in 1874, received a patent on a telegraph code that by the mid-20th century had supplanted Morse Code as the most commonly used telegraphic alphabet.

In Baudot's code, each letter was represented by a five-unit combination of current-on or current-off signals of equal duration; this represented a substantial economy over the Morse system of short dots and long dashes. Thus, 32 permutations were provided, sufficient for the Roman alphabet, punctuation signs, and control of the machine's mechanical functions.

Baudot also invented (1894) a distributor system for simultaneous (multiplex) transmission of several messages on the same telegraphic circuit or channel.

Modern versions of the Baudot Code usually use groups of seven or eight "on" and "off" signals. Groups of seven permit transmission of 128 characters; with groups of eight, one member may be used for error correction or other function.

Source: Britannica Online

The French engineer Jean-Maurice-Émile Baudot (1845-1903), was employed by the French Telegraph Administration, to improve the efficiency of the then very slow telegraphic transmission. He had to overcome many difficulties, especially the avoidance of timing errors between phases of transmission and reception. In 1875 he devised a system which used only five keys to represent every letter of the alphabet. This was in comparison to the then current David Hughes system which used 28!

In Baudot's alphabet (not code) the signals which created the letters differed not only in length but also in their respective position. Emitted signals controlled five magnets in any receiver: signals which occupied only one unit controlled only one magnet; two units, controlled two successive magnets, etc. The keys similar to those of a piano, were lowered according to a metronome. Baudot's system also permitted two or four transmissions, depending on the importance of the line.

The Baudot Code

As telegraph traffic grew over the 19th century, efforts were made to improve on this invention. The duplex circuit made it possible for messages to travel simultaneously in opposite directions on the same line. Thomas Edison even invented a way in 1874 to send 4 messages (quadruplex) at the same time. In 1871 the Frenchman Jean-Maurice-Émile Baudot invented the first device for time-division multiplexing. It consisted of a brush arm that traveled around a copper ring. The ring was divided into equal sectors and in each sector there were five contacts that would be opened or closed--yielding 32 possible combinations. Each five-bit combination encoding a letter or symbol. As the brush arm moved in its circle it would pick up a code combination from one sector and then move on. As many messages as there were sectors could be sent simultaneously.

Baudot modified the Morse code to meet the requirements of his invention. This five-bit Baudot (character) code became the basis of all early 20th Century teleprinters. Later on, this code was standardized as the CCITT International Alphabet #1. In 1903 Donald Murray improved Baudot's time-division multiplex system and its five-level code by inventing a system for punching tape, creating a system that transmitted telegrams rapidly without the need of a key operator. Murray also modified the original Baudot code and his modifications became the CCITT International Alphabet #2. This is the version of the Baudot code that was the grandfather of all modern character codes such as the ASCII code and IBM's EBCDIC code. This is how today's character code standards evolved.

The Baudot code uses 5 bits to represent a character and does not include a parity check bit. 2⁵ provides us with 32 unique codes, which was all that Baudot's multiplexer could handle. But that's clearly not enough to represent all the letters A-Z and numbers 0-9. To handle this problem, Baudot code assigns two of the 32 codes as "shift" signals: the lower (letter) shift and the upper (number) shift. This is the first application to use the concept of an escape code. An escape code is any code that results in a change in the way the system interprets following codes--escape codes are essential in the operations of all modern computer systems. Four of Baudot's remaining thirty codes were assigned for blank, space, carriage return and line feed codes are interpreted with the same meaning same in both upper and lower shift modes, leaving 26 remaining character combinations available: the letters of the alphabet. In the upper (number) shift these 26 codes serve as the numbers 0-9 and other symbols.

Most teletypewriters built in the 20^th century were constructed to operate on this Baudot code standard. The limited number of available codes did not permit lower case characters, so all teletypes using the Baudot code printed only in upper case characters. Special teletypes were also constructed which used the upper (figure) shift to portray weather symbols--these are still used today for weather observations. The Baudot code technology is also still used for some technologies used to communicate with the hearing impaired. The standard continued to evolve within the constraints of the technological history. Baudot's code had considerable influence on the later design of communication devices and character codes. Changing to another code would have required redesigning everyone's equipment. To insure compatibility the early design and evolution of data communications equipment was often standardized through the use of the Baudot code.