Baudot code

The Baudot code , invented by Émile Baudot, is a character set predating EBCDIC and ASCII. It was the predecessor to the International Telegraph Alphabet No. 2 (ITA2), the teleprinter code in use until the advent of ASCII. Each character in the alphabet is represented by a series of five bits, sent over a communication channel such as a telegraph wire or a radio signal. The symbol rate measurement is known as baud, and is derived from the same name. Technically, five-bit codes began in the 17th century, when Francis Bacon developed the cipher now called Bacon's cipher. The cipher was not designed for machine telecommunications (it was instead a method of encrypting a hidden message into another) and, although in theory it could be adapted to that purpose, it only covered 24 of the 26 letters of the English alphabet (two sets of letters, I/J and U/V, were expressed with the same code) and contained no punctuation, spaces, numbers or control characters, rendering it of little use. Baudot invented his original code in 1870 and patented it in 1874. It was a 5-bit code, with equal on and off intervals, which allowed for transmission of the Roman alphabet, and included punctuation and control signals. It was based on an earlier code developed by Carl Friedrich Gauss and Wilhelm Weber in 1834. It was a Gray code (when vowels and consonants are sorted in their alphabetical order), nonetheless, the code by itself was not patented (only the machine) because French patent law does not allow concepts to be patented. Baudot's original code was adapted to be sent from a manual keyboard, and no teleprinter equipment was ever constructed that used it in its original form. The code was entered on a keyboard which had just five piano-type keys and was operated using two fingers of the left hand and three fingers of the right hand. Once the keys had been pressed, they were locked down until mechanical contacts in a distributor unit passed over the sector connected to that particular keyboard, when the keyboard was unlocked ready for the next character to be entered, with an audible click (known as the 'cadence signal') to warn the operator. Operators had to maintain a steady rhythm, and the usual speed of operation was 30 words per minute. The table 'shows the allocation of the Baudot code which was employed in the British Post Office for continental and inland services. A number of characters in the continental code are replaced by fractionals in the inland code. Code elements 1, 2 and 3 are transmitted by keys 1, 2 and 3, and these are operated by the first three fingers of the right hand. Code elements 4 and 5 are transmitted by keys 4 and 5, and these are operated by the first two fingers of the left hand.' Baudot's code became known as the International Telegraph Alphabet No. 1 (ITA1). It is no longer used. In 1901, Baudot's code was modified by Donald Murray (1865–1945), prompted by his development of a typewriter-like keyboard. The Murray system employed an intermediate step; a keyboard perforator, which allowed an operator to punch a paper tape, and a tape transmitter for sending the message from the punched tape. At the receiving end of the line, a printing mechanism would print on a paper tape, and/or a reperforator could be used to make a perforated copy of the message. As there was no longer a connection between the operator's hand movement and the bits transmitted, there was no concern about arranging the code to minimize operator fatigue, and instead Murray designed the code to minimize wear on the machinery, assigning the code combinations with the fewest punched holes to the most frequently used characters. For example, the one-hole letters are E and T. The ten two-hole letters are AOINSHRDLZ, very similar to the 'Etaoin shrdlu' order used in Linotype machines. Ten more letters have three holes, and the four-hole letters are VXKQ.