Baudot code
Five-bit telegraph code that preceded ITA2 and ASCII.
The Baudot code, pronounced [bodo] in French, is an early character encoding system for telegraphy, created by Émile Baudot in the 1870s. It paved the way for the International Telegraph Alphabet No. 2 (ITA2), the most widely used teleprinter code before ASCII. In this system, each character is encoded as a sequence of five bits, transmitted over a telegraph wire or radio signal using asynchronous serial communication. The unit of symbol rate, the baud, takes its name from Baudot.
Baudot first built a multiplexed telegraph in 1872 and received a patent for it in 1874. By 1876, he had switched from a six-bit code to a five-bit one, following a suggestion made by Carl Friedrich Gauss and Wilhelm Weber in 1834. This five-bit code used equal on and off intervals and could represent the Roman alphabet, along with punctuation and control signals. Only the machine was patented, not the code itself, because French patent law does not allow concepts to be patented.
Baudot’s original five-bit code was designed for a manual keyboard with five piano-like keys—operated with two fingers of the left hand and three of the right. After pressing the keys, they locked until a distributor unit’s mechanical contacts passed over the keyboard’s sector, then unlocked with an audible click (the “cadence signal”) to alert the operator. Operators had to keep a steady rhythm, typically achieving 30 words per minute. No teleprinter equipment was ever built that used Baudot’s code in its original form. The code allocation used by the British Post Office for continental and inland services assigned code elements 1, 2, and 3 to the right hand’s first three fingers, and elements 4 and 5 to the left hand’s first two fingers. In the inland version, some continental characters were replaced by fractions. This code became known as International Telegraph Alphabet No. 1 (ITA1) and is no longer in use.
In 1901, Donald Murray modified Baudot’s code, driven by his invention of a typewriter-like keyboard. Murray’s system introduced an intermediate step: an operator used a keyboard perforator to punch a paper tape, which a transmitter then read to send the message. At the receiving end, a printer produced a paper tape, and a reperforator could create a punched copy.
- Inventor
- Émile Baudot
- Year of invention
- 1870s
- Bits per character
- 5
- Predecessor to
- International Telegraph Alphabet No. 2 (ITA2)
- Derived unit
- baud
Lore & Background
Baudot developed his first multiplexed telegraph in 1872 and patented it in 1874. In 1876, he changed from a six-bit code to a five-bit code, as suggested by Carl Friedrich Gauss and Wilhelm Weber in 1834, with equal on and off intervals, allowing transmission of the Roman alphabet, punctuation, and control signals. The code itself was not patented because French patent law does not allow concepts to be patented. Baudot's 5-bit code was adapted to be sent from a manual keyboard with five piano-type keys, operated using two fingers of the left hand and three fingers of the right hand. Operators had to maintain a steady rhythm, and the usual speed was 30 words per minute. The code became known as the International Telegraph Alphabet No. 1 (ITA1) and is no longer used.
In 1901, Baudot's code was modified by Donald Murray, prompted by his development of a typewriter-like keyboard. The Murray system used an intermediate step: an operator used a keyboard perforator to punch a paper tape and then a transmitter to send the message from the punched tape. Murray designed the code to minimize wear on the machinery by assigning the code combinations with the fewest punched holes to the most frequently used characters. The Murray code also introduced control characters such as CR (Carriage Return) and LF (Line Feed). Early British Creed machines also used the Murray system.
Murray's code was adopted by Western Union, which used it until the 1950s with a few changes, including an explicit SPC (space) character, a BEL code, and a WRU (Who aRe yoU?) code. In 1932, the CCITT introduced the International Telegraph Alphabet No. 2 (ITA2) as an international standard, based on the Western Union code with minor changes. The US standardized on a version called the American Teletypewriter code (US TTY), which was the basis for 5-bit teletypewriter codes until the debut of 7-bit ASCII in 1963.
Reader's Guide
The Baudot code is significant as the foundation for later teleprinter codes, particularly ITA2, which became the most common teleprinter code before ASCII. Its five-bit structure and asynchronous serial communication method influenced the development of data transmission. The code's evolution through Murray's modifications and Western Union's adaptations led to the international standard ITA2, which introduced control characters like CR, LF, and BEL. ITA2 is still used in telecommunications devices for the deaf (TDD), Telex, and some amateur radio applications such as radioteletype (RTTY). The Baudot code also gave its name to the baud, a unit of symbol rate measurement. The article notes that the code itself was not patented due to French patent law, and that Baudot's original form was never used in teleprinter equipment. The Murray system's use of punched paper tape and its focus on minimizing machinery wear represented a practical advance. The code's legacy includes its role in early multiplexed telegraphy and its continued use in niche applications today.
Did You Know?
- The Baudot code was invented by Émile Baudot in the 1870s and uses five bits per character.
- The symbol rate measurement known as baud is derived from the name Baudot.
- ITA2, based on Baudot's code, is still used in amateur radio radioteletype (RTTY) and TDD devices.
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