1874

Baudot Code

Jean-Maurice-Émile Baudot (1845–1903), Donald Murray (1865–1945)

Early telegraph systems relied on human operators to encode and transmit the sender’s message, and then to perceive, decode, and transcribe the message on paper upon receipt. Relying on human operators limited the maximum speed at which a message could be sent and required operator skills that were not easily available.

Émile Baudot developed a better approach. A trained French telegraph operator, Baudot devised a system that used a special keyboard with five keys (two for the left hand and three for the right) to send each character. Thirty-one different combinations arise from pressing one or more of the five keys together; Baudot assigned each code to a different letter of the alphabet. To send a message, the operator would type the codes in sequence as the machine clicked, roughly four times a second. With each click, a rotating part that Baudot called the distributor would read the position of each key in order and, if the key was pressed, send a corresponding pulse down the telegraph wire. At the other end, a remote printer would translate the codes back into a printed character on a piece of paper tape.

Baudot was one of the first people to combine key inventions by others into one working system. He patented his invention in 1874, started selling devices to the French Telegraph Administration in 1875, and was awarded the gold medal at the Paris Exposition Universelle in 1878. Baudot’s code was adopted as the International Telegraph Alphabet No. 1 (ITA1), one of the original international telecommunications standards. In recognition of his contribution, the baud, a unit of data transmission speed equal to the number of signal changes per second, is named after him.

In 1897, the Baudot system expanded to incorporate punched paper tape. The keyboard was disconnected from the telegraph line and connected to a new device that could punch holes across a strip of paper tape, with one hole corresponding to each key. Once punched, the tape could be loaded into a reader and the message sent down the telegraph wire faster than a human could type. In 1901, the inventor Donald Murray developed an easier-to-use punch that was based on a typewriter keyboard. Murray also made changes to Baudot’s code; the resulting code was known as the Baudot-Murray code (ITA2) and remained in use for more than 50 years.

Paper tape punched with the five-level Baudot code. The large holes correspond to the 5 bits of the code, while a rotating toothed tractor wheel fit into the small holes and used them to pull the tape through the machine.