Baudot code

Did You Know?

• Nearly all 20th-century teleprinter equipment used Western Union's code, ITA2, or variants thereof.
• Radio amateurs casually call ITA2 and variants "Baudot" incorrectly, and even the American Radio Relay League's Amateur Radio Handbook does so.

History

Émile Baudot patented his first multiplexed telegraph in 1874, having developed it two years earlier. In 1876, he switched from a six-bit code to a five-bit code, an idea suggested by Carl Friedrich Gauss and Wilhelm Weber back in 1834. This new code used equal on and off intervals, could transmit the Roman alphabet, and included punctuation and control signals. Baudot did not patent the code itself—only the machine—because French patent law does not protect concepts. His five-bit code was designed for a manual keyboard and was never used in its original form with teleprinter equipment. The keyboard had five piano-like keys: the first three fingers of the right hand operated keys 1, 2, and 3, while the first two fingers of the left hand worked keys 4 and 5. When an operator pressed the keys, they locked until mechanical contacts in a distributor unit passed over the keyboard’s sector, at which point the keys unlocked for the next character. An audible click, called the “cadence signal,” warned the operator. Steady rhythm was essential, and the usual speed was 30 words per minute. The table in the original source shows how the British Post Office allocated the Baudot code for continental and inland services. In the inland code, some continental characters were replaced with fractionals. Baudot’s code later 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, then a transmitter sent the message from that tape. At the receiving end, a printing mechanism printed on paper tape, and a reperforator could also create a perforated copy of the message.

Details

In ITA2, each character is represented by five bits. The system relies on two distinct character subsets: the letter shift, known as LTRS, and the figure shift, called FIGS. The FIGS code, 11011, tells the receiving machine that the next characters belong to the figures set, and this mode continues until it is cancelled by the LTRS code, 11111. To use these shifts, an operator presses and releases the LTRS or FIGS key, which sends the shift character to the other machine, and then types the desired letters or figures. Unlike a typewriter or a modern computer keyboard, the shift key is not held down while typing the shifted characters. The ENQ character, short for ENQuiry, triggers the other machine’s answerback, essentially asking “Who are you?” Other control characters include CR for carriage return, LF for line feed, BEL to ring a small bell (often used to alert operators to an incoming message), SP for space, and NUL for the null character, which represents blank tape.

Binary representations of these codepoints are sometimes shown reversed, depending on which side of the paper tape one is viewing. The control characters were deliberately designed to be either symmetric or to form useful pairs, so that if a tape was inserted upside down, the equipment would still function and the printout could be understood. As a result, FIGS (11011), LTRS (11111), and space (00100) remain the same when inverted, while CR (00010) and LF (01000), typically used together, are treated identically by page printers regardless of their order. LTRS could also be used to overpunch characters on paper tape for deletion, similar to the DEL character in 7-bit ASCII.

The repeating sequence RYRYRY was commonly used in test messages and at the start of every transmission. Because R is 01010 and Y is 10101, this pattern exercises many of a teleprinter’s mechanical parts at maximum stress. Additionally, when using two-tone FSK modulation, as in radioteletype (RTTY) applications, the sequence produces a 0101010101... pattern. This allowed operators to fine-tune the receiver by adjusting until two colored lights, one for each tone, glowed with equal brightness. This tuning method only works with that specific modulation scheme.