The
Baudot code, invented by
Émile BaudotJean-Maurice-Émile Baudot , French telegraph engineer and inventor of the first means of digital communication Baudot code, was one of the pioneers of telecommunications...
, is a
character setA character encoding system consists of a code that pairs each character from a given repertoire with something else, such as a sequence of natural numbers, octets or electrical pulses, in order to facilitate the transmission of data through telecommunication networks or storage of text in...
predating
EBCDICExtended Binary Coded Decimal Interchange Code is an 8-bit character encoding used on IBM mainframe operating systems such as z/OS, OS/390, VM and VSE, as well as IBM midrange computer operating systems such as OS/400 and i5/OS...
and
ASCIIThe American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
. It was the predecessor to the International Telegraph Alphabet No 2 (ITA2), the
teleprinterA teleprinter is a now largely obsolete electro-mechanical typewriter which can be used to communicate typed messages from point to point and point to multipoint over a variety of communications channels that range from a simple electrical connection, such as a pair of wires, to the...
code in use until the advent of
ASCIIThe American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
. Each character in the alphabet is represented by a
series of bitsIn telecommunications, Asynchronous communication is transmission of data without the use of an external clock signal. Any timing required to recover data from the communication symbols is encoded within the symbols...
, sent over a communication channel such as a telegraph wire or a radio signal.
Baudot invented his original code during 1870 and patented it during 1874.
The
Baudot code, invented by
Émile BaudotJean-Maurice-Émile Baudot , French telegraph engineer and inventor of the first means of digital communication Baudot code, was one of the pioneers of telecommunications...
, is a
character setA character encoding system consists of a code that pairs each character from a given repertoire with something else, such as a sequence of natural numbers, octets or electrical pulses, in order to facilitate the transmission of data through telecommunication networks or storage of text in...
predating
EBCDICExtended Binary Coded Decimal Interchange Code is an 8-bit character encoding used on IBM mainframe operating systems such as z/OS, OS/390, VM and VSE, as well as IBM midrange computer operating systems such as OS/400 and i5/OS...
and
ASCIIThe American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
. It was the predecessor to the International Telegraph Alphabet No 2 (ITA2), the
teleprinterA teleprinter is a now largely obsolete electro-mechanical typewriter which can be used to communicate typed messages from point to point and point to multipoint over a variety of communications channels that range from a simple electrical connection, such as a pair of wires, to the...
code in use until the advent of
ASCIIThe American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
. Each character in the alphabet is represented by a
series of bitsIn telecommunications, Asynchronous communication is transmission of data without the use of an external clock signal. Any timing required to recover data from the communication symbols is encoded within the symbols...
, sent over a communication channel such as a telegraph wire or a radio signal.
History
Baudot invented his original code during 1870 and patented it during 1874. It was a 5-bit code, with equal on and off intervals, which allowed telegraph transmission of the Roman alphabet and punctuation and control signals. It was based on an earlier code developed by
Carl Friedrich GaussJohann Carl Friedrich Gauss was a German mathematician and scientist who contributed significantly to many fields, including number theory, statistics, analysis, differential geometry, geodesy, geophysics, electrostatics, astronomy and optics...
and
Wilhelm WeberWilhelm Eduard Weber was a German physicist and, together with Carl Friedrich Gauss, inventor of the first electromagnetic telegraph.-Early years:...
in 1834.
The code was entered on a keyboard which had just five piano type keys, operated with 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. Baudot's code became known as
International Telegraph Alphabet No. 1, and is no longer used.
Murray code
During 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 direct correlation between the operator's hand movement and the bits transmitted, there was not any need to worry about arranging the code to minimize operator fatigue and instead Murray designed the code to minimize wear and tear on the machinery, assigning the code combinations with the fewest punched holes to the most
frequently used charactersThe frequency of letters in text has often been studied for use in cryptography, and frequency analysis in particular. No exact letter frequency distribution underlies a given language, since all writers write slightly differently. Linotype machines sorted the letters' frequencies as etaoin shrdlu...
.
The Murray code also introduced what later became known as "format effectors" or "
control characterIn computing and telecommunication, a control character or non-printing character is a code point in a character set, that does not in itself represent a written symbol.It is in-band signaling in the context of character encoding....
s" - the
CROriginally, carriage return was the term for the control character in Baudot code on a teletypewriter for end of line return to beginning of line and did not include line feed...
(Carriage Return) and
LFIn computing, a newline, also known as a line break or end-of-line character, is a special character or sequence of characters signifying the end of a line of text. The name comes from the fact that the next character after the newline will appear on a new line—that is, on the next line below the...
(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.
Early
British CreedCreed & Company was a British telecommunications company founded by Frederick George Creed which was an important pioneer in the field of teleprinter machines...
machines 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
BELA bell code is a device control code originally sent to ring a small electromechanical bell on tickers and other teleprinters and teletypewriters to alert operators at the other end of the line, often of an incoming message...
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
Around 1930, the CCITT introduced the
International Telegraphy 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 (USTTY) which was the basis for 5-bit teletype codes until the debut of 7-bit ASCII in 1963.
ITA2 is still used in TDDs and some
amateur radioAmateur radio, often called ham radio, is both a hobby and a service in which participants, called "hams," use various types of radio communications equipment to communicate with other radio amateurs for public service, recreation and self-training....
applications, such as
radioteletypeRadioteletype is a telecommunications system consisting of two or more teleprinters, or more recently personal computers using software to emulate teleprinters, using radio as the transmission medium....
("RTTY"). Though it is significantly different from Baudot's original code, it is nonetheless often incorrectly referred to as "Baudot code". 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.
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 LeagueThe American Radio Relay League is the largest membership association of amateur radio enthusiasts in the USA. ARRL is a non-profit organization, and was founded in May 1914 by Hiram Percy Maxim of Hartford, Connecticut...
's Amateur Radio Handbook does so, though in more recent editions the tables of codes correctly identifies it as ITA2.
Details
NOTE: This table presumes the space called "1" by Baudot and Murray is rightmost, and least significant. The actual order of transmission varied by manufacturer.
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 code is to be interpreted as being in the FIGS set, until this is reset by the LTRS (11111) character. "ENQuiry" will trigger the other machine's answerback. It means "Who are you?"
CR is
carriage returnOriginally, carriage return was the term for the control character in Baudot code on a teletypewriter for end of line return to beginning of line and did not include line feed...
, LF is line feed, BEL is the
bell characterA bell code is a device control code originally sent to ring a small electromechanical bell on tickers and other teleprinters and teletypewriters to alert operators at the other end of the line, often of an incoming message...
which rang a small
bellA bell is a simple sound-making device. The bell is a percussion instrument and an idiophone. Its form is usually a hollow, cup-shaped object, which resonates upon being struck...
(often used to alert operators to an incoming message), SP is space, and NUL is the
null characterThe null character is a character with the value zero, present in the ASCII and Unicode character sets, and available in nearly all mainstream programming languages. The original meaning of this character was like NOP — when sent to a printer or a terminal, it does nothing...
(blank tape).
Note: the binary conversions of the codepoints are often shown in reverse order, depending on (presumably) from which side one views the papertape. Note further that the
"control" charactersIn computing and telecommunication, a control character or non-printing character is a code point in a character set, that does not in itself represent a written symbol.It is in-band signaling in the context of character encoding....
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 (01000) and LF (00010), generally used as a pair, result in the same output when the tape is reversed. LTRS could also be used to overpunch characters to be deleted on a
paper tapePunched tape or paper tape is a largely obsolete form of data storage, consisting of a long strip of paper in which holes are punched to store data...
(much like DEL in 7-bit
ASCIIThe American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
).
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
FSKFSK can have alternative meanings:*FSK * Frequency-shift keying, a modulation scheme...
modulation, such as is commonly seen in Radio Teletype(RTTY) usage.
US implementations of Baudot code may differ in the addition of a few characters, such as #, & on the FIGS layer. The above table represents the US TTY code.
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 !, &, £, and
BELA bell code is a device control code originally sent to ring a small electromechanical bell on tickers and other teleprinters and teletypewriters to alert operators at the other end of the line, often of an incoming message...
were omitted and replaced by Cyrillics.
See also
- Serial communication
In telecommunication and computer science, serial communication is the process of sending data one bit at one time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent together, on a link with several parallel channels...
- Asynchronous communication
In telecommunications, Asynchronous communication is transmission of data without the use of an external clock signal. Any timing required to recover data from the communication symbols is encoded within the symbols...
- Morse code
Morse code is a type of character encoding that transmits telegraphic information using rhythm. Morse code uses a standardized sequence of short and long elements to represent the letters, numerals, punctuation and special characters of a given message...
- RY (test signal)
RYRYRYRY... is a character string that was widely used to test a five-level teleprinter or RTTY channel. The characters R and Y are "01010" and "10101" in 5-bit ITA2 code, also known as Baudot. Thus they are Boolean complements of each other...