02 What is the CCITT ITA-1 Code?

Baudot code – Wikipedia

The original Baudot code (later International Telegraph Alphabet No. 1 (ITA1), CCITT-1) was designed by Émile Baudot in 1870 for a telegraph apparatus he developed.

Baudot applied for a patent for his invention

  • in France: January 06, 1882, No. 146,716
  • in Belgium: January 25, 1882, No. 56,883
  • in England: January 28, 1882, No. 436
  • in Germany: February 04, 1882, No. 20,826
    DEPATISnet | document DE000000020826A (dpma.de)
  • in the USA: August 21, 1888, No. 388244

The code was entered directly via a five-key keypad. Pressing or not pressing a key transmitted either a working current or a rest current.

In this respect, this was equivalent to setting or not setting a bit in a 5-bit word (the character code).

Since with five keys, each of which could be either pressed or not pressed, there were only 32 (= 25) different key combinations (31 if you don’t count the idle position of the keyboard), not even all 26 letters plus 10 digits could have been encoded if Baudot hadn’t introduced a switch code that allowed double assignment of almost all combinations: it defined two characters. When one was sent, subsequent characters were to be interpreted according to a table of letters; when the other was sent, a table of digits and characters was to be used.

The challenge was the different parallel national developments, which made international communication difficult.

In 1926, at the first plenary meeting of CCITT in Berlin, an overview of the telegraph alphabets in use at that time was presented, among other things, and standardization was advocated.

CCIF, CCIT, CCITT, and World Telecommunication Standardization Assemblies (itu.int)


International Telegraph Alphabet Nr. 1 – CCITT-1

On the basis of Baudot’s code, standardization took place under the name International Telegraph Alphabet No. 1.

Due to the international application, individual characters were left free for national adaptations.

In 1929, at the second plenary meeting in Berlin, the International Telegraph Alphabet No. 1 was adapted:

the character É (No. 6) was deleted
The special character Ṯ (No. 28) was deleted.
The following were introduced
a pause character
line change
carriage return
Changed the characters for change between letters and digits