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The Baudot code (French pronunciation: [bodo]) is an early character encoding for telegraphy invented by Émile Baudot in the 1870s. It was the predecessor to the International Telegraph Alphabet No. 2 (ITA2), the most common teleprinter code in use before 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 by asynchronous serial communication. The symbol rate measurement is known as baud, and is derived from the same name.

History

Baudot code (ITA1)

Baudot code (ITA1)

An early version from Baudot's 1888 US patent, listing A through Z, t and ∗ (Erasure)
Alias(es)	International Telegraph Alphabet 1
Current status	Replaced by ITA2 (not mutually compatible).
Classification	5-bit stateful basic Latin encoding
Preceded by	Morse code
Succeeded by	ITA2
v t e

In the below table, Columns I, II, III, IV, and V show the code; the Let. and Fig. columns show the letters and numbers for the Continental and UK versions; and the sort keys present the table in the order: alphabetical, Gray and UK

Baudot developed his first multiplexed telegraph in 1872 and patented it in 1874. In 1876, he changed from a six-bit code to a five-bit code, as suggested by Carl Friedrich Gauss and Wilhelm Weber in 1834, with equal on and off intervals, which allowed for transmission of the Roman alphabet, and included punctuation and control signals. The code itself was not patented (only the machine) because French patent law does not allow concepts to be patented.

Baudot's 5-bit 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, at which time 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.

Murray code

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: an operator used a keyboard perforator to punch a paper tape and then a transmitter to send 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 would make a perforated copy of the message.

Because 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. Instead, Murray designed the code to minimize wear on the machinery by 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, BCGFJMPUWY, have three holes each, and the four-hole letters are VXKQ.

The Murray code also introduced what became known as "format affectors" or "control characters" – the CR (Carriage Return) and LF (Line Feed) codes. A few of Baudot's codes moved to the positions where they have stayed ever since: the NULL or BLANK and the DEL code. NULL/BLANK was used as an idle code for when no messages were being sent, but the same code was used to encode the space separation between words. Sequences of DEL codes (fully punched columns) were used at start or end of messages or between them which made it easier to separate distinct messages. (BELL codes could be inserted in those sequences to signal to the remote operator that a new message was coming or that transmission of a message was terminated).

Early British Creed machines also used the Murray system.

Western Union

Murray's code was adopted by Western Union which used it until the 1950s, with a few changes that consisted of omitting some characters and adding more control codes. An explicit SPC (space) character was introduced, in place of the BLANK/NULL, and a new BEL code rang a bell or otherwise produced an audible signal at the receiver. Additionally, the WRU or "Who aRe yoU?" code was introduced, which caused a receiving machine to send an identification stream back to the sender.

ITA2

ITA2 Baudot–Murray code

British variant of ITA2
Alias(es)	International Telegraph Alphabet 2
Classification	5-bit stateful basic Latin encoding
Preceded by	ITA1
Succeeded by	FIELDATA, ITA 3 (van Duuren code), ITA 5 (ISO 646, ASCII)
v t e

In 1932, the CCITT introduced the International Telegraph Alphabet No. 2 (ITA2) code as an international standard, which was based on the Western Union code with some minor changes. The US standardized on a version of ITA2 called the American Teletypewriter code (US TTY) which was the basis for 5-bit teletypewriter codes until the debut of 7-bit ASCII in 1963.

Some code points (marked blue in the table) were reserved for national-specific usage.

The code position assigned to Null was in fact used only for the idle state of teleprinters. During long periods of idle time, the impulse rate was not synchronized between both devices (which could even be powered off or not permanently interconnected on commuted phone lines). To start a message it was first necessary to calibrate the impulse rate, a sequence of regularly timed "mark" pulses (1), by a group of five pulses, which could also be detected by simple passive electronic devices to turn on the teleprinter. This sequence of pulses generated a series of Erasure/Delete characters while also initializing the state of the receiver to the Letters shift mode. However, the first pulse could be lost, so this power on procedure could then be terminated by a single Null immediately followed by an Erasure/Delete character. To preserve the synchronization between devices, the Null code could not be used arbitrarily in the middle of messages (this was an improvement to the initial Baudot system where spaces were not explicitly differentiated, so it was difficult to maintain the pulse counters for repeating spaces on teleprinters). But it was then possible to resynchronize devices at any time by sending a Null in the middle of a message (immediately followed by an Erasure/Delete/LS control if followed by a letter, or by a FS control if followed by a figure). Sending Null controls also did not cause the paper band to advance to the next row (as nothing was punched), so this saved precious lengths of punchable paper band. On the other hand, the Erasure/Delete/LS control code was always punched and always shifted to the (initial) letters mode. According to some sources, the Null code point was reserved for country-internal usage only.

The Shift to Letters code (LS) is also usable as a way to cancel/delete text from a punched tape after it has been read, allowing the safe destruction of a message before discarding the punched band. Functionally, it can also play the same filler role as the Delete code in ASCII (or other 7-bit and 8-bit encodings, including EBCDIC for punched cards). After codes in a fragment of text have been replaced by an arbitrary number of LS codes, what follows is still preserved and decodable. It can also be used as an initiator to make sure that the decoding of the first code will not give a digit or another symbol from the figures page (because the Null code can be arbitrarily inserted near the end or beginning of a punch band, and has to be ignored, whereas the Space code is significant in text).

The cells marked as reserved for extensions (which use the LS code again a second time—just after the first LS code—to shift from the figures page to the letters shift page) has been defined to shift into a new mode. In this new mode, the letters page contains only lowercase letters, but retains access to a third code page for uppercase letters, either by encoding for a single letter (by sending LS before that letter), or locking (with FS+LS) for an unlimited number of capital letters or digits before then unlocking (with a single LS) to return to lowercase mode. The cell marked as "Reserved" is also usable (using the FS code from the figures shift page) to switch the page of figures (which normally contains digits and national lowercase letters or symbols) to a fourth page (where national letters are uppercase and other symbols may be encoded).

ITA2 is still used in telecommunications devices for the deaf (TDD), Telex, and some amateur radio applications, such as radioteletype ("RTTY"). ITA2 is also used in Enhanced Broadcast Solution, an early 21st-century financial protocol specified by Deutsche Börse, to reduce the character encoding footprint.

Nomenclature

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, though in more recent editions the tables of codes correctly identifies it as ITA2.

Character set

The values shown in each cell are the Unicode codepoints, given for comparison.

Original Baudot variants

Original Baudot, domestic UK

Original Baudot, Continental European

Original Baudot, ITA 1

Baudot–Murray variants

Murray Code

ITA 2 and US-TTY

Weather code

Meteorologists used a variant of ITA2 with the figures-case symbols, except for the ten digits, BEL and a few other characters, replaced by weather symbols:

Details

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Note: This table presumes the space called "1" by Baudot and Murray is rightmost, and least significant. The way the transmitted bits were packed into larger codes varied by manufacturer. The most common solution allocates the bits from the least significant bit towards the most significant bit (leaving the three most significant bits of a byte unused).

In ITA2, characters are expressed using five bits. ITA2 uses two code sub-sets, the "letter shift" (LTRS), and the "figure shift" (FIGS). The FIGS character (11011) signals that the following characters are to be interpreted as being in the FIGS set, until this is reset by the LTRS (11111) character. In use, the LTRS or FIGS shift key is pressed and released, transmitting the corresponding shift character to the other machine. The desired letters or figures characters are then typed. Unlike a typewriter or modern computer keyboard, the shift key isn't kept depressed whilst the corresponding characters are typed. "ENQuiry" will trigger the other machine's answerback. It means "Who are you?"

CR is carriage return, LF is line feed, BEL is the bell character which rang a small bell (often used to alert operators to an incoming message), SP is space, and NUL is the null character (blank tape).

Note: the binary conversions of the codepoints are often shown in reverse order, depending on (presumably) from which side one views the paper tape. Note further that the "control" characters were chosen so that they were either symmetric or in useful pairs so that inserting a tape "upside down" did not result in problems for the equipment and the resulting printout could be deciphered. Thus FIGS (11011), LTRS (11111) and space (00100) are invariant, while CR (00010) and LF (01000), generally used as a pair, are treated the same regardless of order by page printers. LTRS could also be used to overpunch characters to be deleted on a paper tape (much like DEL in 7-bit ASCII).

The sequence RYRYRY... is often used in test messages, and at the start of every transmission. Since R is 01010 and Y is 10101, the sequence exercises much of a teleprinter's mechanical components at maximum stress. Also, at one time, fine-tuning of the receiver was done using two coloured lights (one for each tone). 'RYRYRY...' produced 0101010101..., which made the lights glow with equal brightness when the tuning was correct. This tuning sequence is only useful when ITA2 is used with two-tone FSK modulation, such as is commonly seen in radioteletype (RTTY) usage.

US implementations of Baudot code may differ in the addition of a few characters, such as #, & on the FIGS layer.

The Russian version of Baudot code (MTK-2) used three shift modes; the Cyrillic letter mode was activated by the character (00000). Because of the larger number of characters in the Cyrillic alphabet, the characters !, &, £ were omitted and replaced by Cyrillics, and BEL has the same code as Cyrillic letter Ю. The Cyrillic letters Ъ and Ё are omitted, and Ч is merged with the numeral 4.

See also

• Bacon's cipher – A 5-bit binary encoding of the English alphabet devised by Francis Bacon in 1605.
• List of information system character sets
• CCIR 476

Explanatory notes

1. ^ "At the disposal of each administration for its internal service"
2. ^ "ives invisible correction on page printers & * on slip printers."

References

1. Ralston, Anthony; Reilly, Edwin D., eds. (1993), "Baudot Code", Encyclopedia of Computer Science (Third ed.), New York: IEEE Press/Van Nostrand Reinhold, ISBN 0-442-27679-6
2. ^ H. A. Emmons (1 May 1916). "Printer Systems". Wire & Radio Communications. 34: 209.
3. ^ Fischer, Eric N. (20 June 2000). "The Evolution of Character Codes, 1874–1968". ark:/13960/t07x23w8s. Retrieved 20 December 2020. In 1872, started research toward a telegraph system that would allow multiple operators to transmit simultaneously over a single wire and, as the transmissions were received, would print them in ordinary alphabetic characters on a strip of paper. He received a patent for such a system on June 17, 1874. Instead of a variable delay followed by a single-unit pulse, Baudot's system used a uniform six time units to transmit each character. his early telegraph probably used the six-unit code that he attributes to Davy in an 1877 article. in 1876 Baudot redesigned his equipment to use a five-unit code. Punctuation and digits were still sometimes needed, though, so he adopted from Hughes the use of two special letter space and figure space characters that would cause the printer to shift between cases at the same time as it advanced the paper without printing. The five-unit code he began using at this time was structured to suit his keyboard , which controlled two units of each character with switches operated by the left hand and the other three units with the right hand.
4. Baudot, Jean-Maurice-Émile (June 1874). "Système de télégraphie rapide" (in French). Archives Institut National de la Propriété Industrielle (INPI). Patent Brevet 103,898. Archived from the original on 16 December 2017.
5. William V. Vansize (25 January 1901). "A New Page-Printing Telegraph". Transactions. 18. American Institute of Electrical Engineers: 22.
6. Procès d'Amiens Baudot vs Mimault
7. ^ Jennings, Tom (2020). "An annotated history of some character codes: Baudot's code".
8. Beauchamp, K.G. (2001). History of Telegraphy: Its Technology and Application. Institution of Engineering and Technology. pp. 394–395. ISBN 0-85296-792-6.
9. Alan G. Hobbs, 5 Unit Codes, section Baudot Multiplex System
10. Gleick, James (2011). The Information: A History, a Theory, a Flood. London: Fourth Estate. p. 203. ISBN 978-0-00-742311-8.
11. Foster, Maximilian (August 1901). "A Successful Printing Telegraph". The World's Work: A History of Our Time. II: 1195–1199. Retrieved 9 July 2009.
12. Copeland 2006, p. 38
13. Telegraph and Telephone Age. 1921. I allocated the most frequently used letters in English language to the signals represented by the fewest holes in the perforated tape, and so on in proportion.
14. "Telegraph Regulations and Final Protocol (Madrid, 1932)" (PDF). Archived from the original on 21 August 2023. Retrieved 10 May 2024.
15. Smith, Gil (2001). "Teletype Communication Codes" (PDF). Baudot.net. Archived (PDF) from the original on 20 August 2008. Retrieved 11 July 2008.
16. ^ Steinbuch, Karl W.; Weber, Wolfgang, eds. (1974) . Taschenbuch der Informatik - Band III - Anwendungen und spezielle Systeme der Nachrichtenverarbeitung (in German). Vol. 3 (3 ed.). Berlin, Germany: Springer Verlag. pp. 328–329. ISBN 3-540-06242-4. LCCN 73-80607. {{cite book}}: |work= ignored (help)
17. dataIP Limited. "The "Baudot" Code". Archived from the original on 23 December 2017. Retrieved 16 July 2017.
18. ITU-T Recommendation S.2 / 11/1988, published in Fascicle VII.1 of the Blue Book
19. "Enhanced Broadcast Solution – Interface Specification Final Version" (PDF). Deutsche Börse. 17 May 2010. Archived from the original (PDF) on 8 February 2012. Retrieved 10 August 2011.
20. Gillam, Richard (2002). Unicode Demystified. Addison-Wesley. p. 30. ISBN 0-201-70052-2.
21. ^ "Five-unit codes". NADCOMM museum. Archived from the original on 4 November 1999. Retrieved 5 December 2001.
22. This article is based on material taken from Baudot+code at the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.
23. Jennings, Tom (5 February 2020). "An annotated history of some character codes: ITA2". Retrieved 1 June 2022. the characters that are 'transmission control' related are bit-wise symmetrical – the codes for FIGS, LTRS, space and BLANK – are the same reversed left to right! Further, the codes for CR and LF, equal each other when reversed left to right!
24. Bacon, Francis (1605). The Proficience and Advancement of Learning Divine and Humane.

Further reading

• Copeland, B. Jack, ed. (2006). Colossus: The Secrets of Bletchley Park's Codebreaking Computers. Oxford: Oxford University Press. ISBN 978-0-19-284055-4.
• Hobbs, Alan G. "NADCOMM Papers and Writings: Five-unit codes". Retrieved 10 February 2017.
• MTK-2 code table
• Baudot, Murray, ITA2, ITA5, etc.
• "Jean-Maurice-Émile Baudot". Archived from the original on 13 September 2009.

External links

• Media related to Baudot code at Wikimedia Commons

v t e Character encodings
Early telecommunications	Telegraph code Needle Morse Non-Latin Wabun/Kana Chinese Cyrillic Baudot and Murray Fieldata ASCII ISO/IEC 646 BCDIC Teletex and Videotex/Teletext T.51/ISO/IEC 6937 ITU T.61 ITU T.101 World System Teletext background sets Transcode
ISO/IEC 8859	Approved parts -1 (Western Europe) -2 (Central Europe) -3 (Maltese/Esperanto) -4 (North Europe) -5 (Cyrillic) -6 (Arabic) -7 (Greek) -8 (Hebrew) -9 (Turkish) -10 (Nordic) -11 (Thai) -13 (Baltic) -14 (Celtic) -15 (New Western Europe) -16 (Romanian) Abandoned parts -12 (Devanagari) Proposed but not approved KOI-8 Cyrillic Sámi Adaptations Welsh Barents Cyrillic Estonian Ukrainian Cyrillic
Bibliographic use	MARC-8 ANSEL CCCII/EACC ISO 5426 5426-2 5427 5428 6438 6862
National standards	ArmSCII Big5 BraSCII CNS 11643 DIN 66003 ELOT 927 GOST 10859 GB 2312 GB 12345 GB 12052 GB 18030 HKSCS ISCII JIS X 0201 JIS X 0208 JIS X 0212 JIS X 0213 KOI-7 KPS 9566 KS X 1001 KS X 1002 LST 1564 LST 1590-4 PASCII Shift JIS SI 960 TIS-620 TSCII VISCII VSCII YUSCII
ISO/IEC 2022	ISO/IEC 8859 ISO/IEC 10367 Extended Unix Code / EUC
Mac OS Code pages ("scripts")	Armenian Arabic Barents Cyrillic Celtic Central European Croatian Cyrillic Devanagari Farsi (Persian) Font X (Kermit) Gaelic Georgian Greek Gujarati Gurmukhi Hebrew Iceland Inuit Keyboard Latin (Kermit) Maltese/Esperanto Ogham Roman Romanian Sámi Turkish Turkic Cyrillic Ukrainian VT100
DOS code pages	437 668 708 720 737 770 773 775 776 777 778 850 851 852 853 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 897 899 903 904 932 936 942 949 950 951 1040 1042 1043 1046 1098 1115 1116 1117 1118 1127 3846 ABICOMP CS Indic CSX Indic CSX+ Indic CWI-2 Iran System Kamenický Mazovia MIK
IBM AIX code pages	895 896 912 915 921 922 1006 1008 1009 1010 1012 1013 1014 1015 1016 1017 1018 1019 1046 1124 1133
Windows code pages	CER-GS 932 936 (GBK) 950 1169 Extended Latin-8 1250 1251 1252 1253 1254 1255 1256 1257 1258 1270 Cyrillic + Finnish Cyrillic + French Cyrillic + German Polytonic Greek
EBCDIC code pages	Japanese language in EBCDIC DKOI
DEC terminals (VTx)	Multinational (MCS) National Replacement (NRCS) French Canadian Swiss Spanish United Kingdom Dutch Finnish French Norwegian and Danish Swedish Norwegian and Danish (alternative) 8-bit Greek 8-bit Turkish SI 960 Hebrew Special Graphics Technical (TCS)
Platform specific	1052 1053 1054 1055 1056 1057 1058 Acorn RISC OS Amstrad CPC Apple II ATASCII Atari ST BICS Casio calculators CDC Compucolor 8001 Compucolor II CP/M+ DEC RADIX 50 DEC MCS/NRCS DG International Galaksija GEM GSM 03.38 HP Roman HP FOCAL HP RPL SQUOZE LICS LMBCS MSX NEC APC NeXT PETSCII PostScript Standard PostScript Latin 1 SAM Coupé Sega SC-3000 Sharp calculators Sharp MZ Sinclair QL Teletext TI calculators TRS-80 Ventura International WISCII XCCS ZX80 ZX81 ZX Spectrum
Unicode / ISO/IEC 10646	UTF-1 UTF-7 UTF-8 UTF-16 UTF-32 UTF-EBCDIC GB 18030 DIN 91379 BOCU-1 CESU-8 SCSU TACE16 Comparison of Unicode encodings
TeX typesetting system	Cork LY1 OML OMS OT1
Miscellaneous code pages	ABICOMP ASMO 449 Digital encoding of APL symbols ISO-IR-68 ARIB STD-B24 Fieldata HZ IEC-P27-1 INIS 7-bit 8-bit ISO-IR-169 ISO 2033 KOI KOI8-R KOI8-RU KOI8-U Mojikyō SEASCII Stanford/ITS Symbol TRON Unified Hangul Code
Control character	Morse prosigns C0 and C1 control codes ISO/IEC 6429 JIS X 0211 Unicode control, format and separator characters Whitespace characters
Related topics	CCSID Character encodings in HTML Charset detection Han unification Hardware code page MICR code Mojibake Variable-length encoding
Character sets

• Amateur radio
• Character encoding
• Character sets
• Telegraphy
• 1870s introductions
• French inventions