Baudot code
The Baudot code (also telex code or telex code ), named after Jean-Maurice Emile Baudot (1845-1903) is a digital , original synchronous 5- Bit - character code and was later known as CCITT -1 standardized. Today, the Baudot code is commonly referred to as the variant adapted by Donald Murray (1865–1945) for use with an alphanumeric keyboard (CCITT-2) . This was used worldwide in telegraph and telex operations and is also calledBaudot-Murray Code or Murray Code .
prehistory
After the development of electrical telegraphy and Morse code in the first half of the 19th century, ways were sought to enable a simpler, direct transmission of text without having to have the operator in the telegraph stations convert the text into a code for transmission. Engineers and inventors endeavored to develop methods so that this code conversion could be carried out automatically by the telegraph devices themselves. The endeavors were aimed at transmitting one letter (or any character) at a time through telegraph lines.
For example, an attempt has been made to transmit the information about the respective character through various voltages on a telegraph line. Other methods have been devised that use 26 lines simultaneously - one for each letter. Codes have also been developed to reduce the number of voltage values and lines required. Here, in particular, 5- and 6-bit codes are a good compromise.
Émile Baudot succeeded - based on the developments already made - in developing a device that, using a 5-bit code, was not only able to print the telegram text directly on a strip of paper on the receiving end, but also to print several telegrams in a multiplex system could transmit simultaneously over a single telegraph line.
The early Baudot code (CCITT-1)
service
The original Baudot code (later International Telegraph Alphabet No. 1 (ITA1), CCITT-1 ) was designed by Émile Baudot in 1870 for a telegraph device he developed. The code was entered directly via a piano-like keyboard with five keys; pressing or not pressing a key corresponded to setting or not setting a bit in the 5-bit word to be sent (the character code ). For this purpose, the keyboard was operated with the index and middle finger of the left hand and with the index, middle and ring finger of the right hand. The keys corresponding to the character to be sent had to be pressed simultaneously and locked in for a moment, until the combination was sent by the device as a series of current pulses and the keyboard was released again for the next character. In this way speeds of 180 characters per minute have been achieved.
Because, unlike with Morse code, all characters were represented by a code of the same length, machine decoding was much easier to implement and feasible with the resources available. In Baudot's telegraph receiver, for example, an electromechanical device brought a type wheel into a corresponding position depending on the code received, in order to print the associated character on a strip of paper.
Coding
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| •• ··· | Delete last character | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Since there are only 32 (= 2 5 ) different key combinations with five keys, each of which can either be pressed or not pressed (31 if you do not include the idle position of the keyboard), not even all 26 letters plus 10 digits would have been coded if Baudot had not introduced a toggle code that allowed almost all combinations to be used twice: it defined two spaces. If one was sent, the following characters should be interpreted according to a table with letters, after the other a table with digits and characters should be used.
The Baudot code was designed from an ergonomic point of view to make it easy to learn. For example, the partial code of the left hand remains the same for the vowels, and successive characters often follow a certain counting pattern. In the table, “ · ” stands for a key that has not been pressed and “ • ” for a pressed key, or a 0-bit and a 1-bit. The numbering of the bits was chosen according to the fingers: 5 4 1 2 3.
Another story
Baudot's telegraph system was adopted by the French administration in 1875 . After the first test operation between Paris and Bordeaux was successfully carried out in November 1877, the system was used throughout Europe in the following years and later also outside of Western and Central Europe in countries such as Russia and Argentina . Since the characters available did not meet the needs of other telecommunications administrations, there were different national character assignments. The later CCITT International Alphabet No. 1 standardized version differs in the assignment of special characters from Baudot's version, but the position of the 26 letters and the digits 0 to 9 has always remained the same.
Baudots Code was the first bit-oriented character code that represented every character with the same number of bits and thus the forerunner of modern computer codes such as EBCDIC , ASCII and Unicode . The unit baud is named after Baudot . The baud rate is a measure of the walking speed.
The Baudot-Murray Code (CCITT-2)
Entering characters
To make it easier to enter characters, Donald Murray developed a keyboard similar to that of a typewriter around 1901 , which punched the corresponding five-bit character string into a punched tape by pressing a single key . This punched tape ran immediately into a transmitter that read the punched code and sent it out. Murray's machines were very fast; in a test run at the British Post Office in 1908, a speed of 1260 letters per minute was achieved. This device became the forerunner of modern teleprinters , which work with an alphanumeric keyboard and an alphanumeric printer.
| code | Letters | Digits / characters |
|---|---|---|
| 00011 | A. | - |
| 11001 | B. | ? |
| 01110 | C. | : |
| 01001 | D. | Who's there? |
| 00001 | E. | 3 |
| 01101 | F. | unused |
| 11010 | G | unused |
| 10100 | H | unused |
| 00110 | I. | 8th |
| 01011 | J | bell |
| 01111 | K | ( |
| 10010 | L. | ) |
| 11100 | M. | . |
| 01100 | N | , |
| 11000 | O | 9 |
| 10110 | P | 0 |
| 10111 | Q | 1 |
| 01010 | R. | 4th |
| 00101 | S. | ' |
| 10,000 | T | 5 |
| 00111 | U | 7th |
| 11110 | V | = |
| 10011 | W. | 2 |
| 11101 | X | / |
| 10101 | Y | 6th |
| 10001 | Z | + |
| 01000 | Carriage return | |
| 00010 | Line feed | |
| 00100 | Space | |
| 11111 | Switching letters | |
| 11011 | Switching digits / characters | |
| 00000 | unused | |
Coding
Murray kept Baudot's level switching, but rearranged the order of the characters in the code so that frequently used letters such as "E" and "T" were assigned code positions, for whose transmission and reception the mechanics in the devices are moved shorter and less often had to. This reduces wear and tear and the need for maintenance. Murray also added a newline character to be able to subdivide telegrams.
In the table on the left the sequence is shown alphabetically according to CCITT-2. In the table on the right, the telex characters are sorted according to their value, i.e. the letter E has the perforation 1-0 -.- 0-0-0 which corresponds to 1 in decimal.
Like Baudots, Murray's code outside of the letters and digits has undergone many changes over time to meet local needs. An important change was that spaces and letters / digits were separated from each other and became separate characters. In order to be able to control sheet writers (devices that write line by line on a sheet instead of endlessly on a strip), control characters for carriage return and line feed have been added. In addition, there was a code to trigger an automatic namesake on the remote device in order to be able to make sure with unmanned counter stations who you were connected to (who there?) , As well as the possibility to control a bell in order to attract the attention of the operating personnel .
This modified code was standardized by the CCITT in 1932 as the International Telegraph Alphabet No. 2 (CCITT-2 or ITA2 for short). Some code positions have been kept free for national expansions. In the table, “0” stands for a pause step, the idle state and “1” for a current step or a mechanical movement.
application
The CCITT-2 code became the standard code in telex networks . The originally used synchronous data transmission , which required that the sending and receiving devices always run in exactly the same cycle, was replaced by an asynchronous one. For this purpose, start and stop bits were inserted before and after each character in order to enable the receiving device to synchronize again with each incoming character. Teletypes in telex networks were operated at fixed, network-dependent speeds of around 400 to 600 characters per minute.
The CCITT-2 code has also been adapted and expanded. In 1963 МТК-2 (including Cyrillic letters) was introduced in the USSR . In the United States in particular , character assignments that differ slightly from the CCITT-2 code have been used in various networks. For example, there were special character sets for a weather network and a stock exchange network. The code has been expanded elsewhere. In order to be able to display more characters, a third level was introduced for the display of Greek or Cyrillic characters, and even more for use in typesetting. The possibility of using the code positions that had been kept free for the German umlauts was not used in the German telex network.
See also
Web links
- International Telegraph Alphabet No. 2. (pdf) International Telecommunication Union , November 1988, accessed on September 1, 2019 (English).
- Biography of Donald Murray ( Memento from June 22, 2008 in the Internet Archive )
- Online conversion from or to Baudot code, ITA 1, ITA 2 and Murray code