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Standard Telex Crypto MTK-2 → ITA-5 →
Baudot-Murray code
In digital telegraphy (teleprinter, telex)
a 5-bit encoding standard is commonly
used to represent a character (letter, number or punctuation mark).
Although this code is generally known as the Baudot code,
this name is actually wrong.
The official name for the latest telegraphy standard
is ITA-2 (International Telegraph Alphabet No 2).
It was superceeded by ITA-5 (ASCII) in 1963,
but is still used by amateurs today.
The most common 'Baudot' code is also known as Murray code, or as
Baudot-Murray code. The ITA-2 standard is used widely with
historical cipher machines.
Normal text consists of over 50 different characters (26 letters,
10 numbers, 10 punctuation marks and some control codes).
In the ITA-2 standard, 5 bits are used to represent a character, which
means that only 32 different codes can be created (25).
As this would not be sufficient for normal text, most codes
are used twice (i.e. have two different meanings) and special
reserved codes
are used to shift between the two code sets,
known as Letters (LTRS) and Figures (FIGS).
The table below shows the interpretation of the commonly used ITA-2
standard. Although this code has officially been superceeded by
ITA-5,
it is still used today on some old telex networks and by Radio Amateurs.
Some of the cipher machines described on this website,
use 5-bit digital encoding and many of them
(if they support the Latin character set) follow the ITA-2 standard.
LTRS-shift is represented by 111·11 (5 holes), so that it can be
used to wipe part of a paper tape, without affecting the rest.
Normally, a paper tape would start with two LTRS characters, to ensure
that the teletype is in Letters-mode.
By convention, the holes (bits) in the tape are called channels
or tracks and the tape is shown as it runs through the tape reader
away from you. In that case, channel 1 (also known as bit 0 or b0)
will be at the right. This is known as the least significant bit.
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Cipher machines that use ITA-2
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ITA2 is a serial protocol, which means that each data word is sent one bit
at a time, starting with the least significant bit (b0). In a wired network
this is usually done with a constant current circuit. In rest, there is
a constant current of, say, 20mA. This situation is known as a 'Mark'. It
represents the logic state '1'. A logic '0' can be sent by interrupting the
current. This situation is known as a 'Space'. Each character consists of
5 data bits, preceeded by one start bit (0) and succeeded by two stop bits (11).
A complete character therefore consists of 8 bits as shown in this diagram:
In the above example, the bit pattern for the character 'D' is shown.
In Mark/Space notation it can be written as SMSSMSMM. In binary notation
this would be 01001011. As the start and stop bits are usually left out,
the bit pattern can be written as 10010. When specifying the
binary value of a character, we do this in reverse order – 01001 –
as by convention the least significant bit is always written at the right.
In the above table, we've also inserted the sprocket hole for clarity:
010·01.
The data format — one start bit, five data bits and two stop bits — is sometimes
written as 5N2, which means 5 data bits, no partity and 2 stop bits.
Although this is the most common format for teletype (TTY), there are
variations that use 1 stop bit or sometimes even 1.5 stop bit.
When sending ITA2 data over radio (RTTY) the Mark and Space states
can be represented by two audio tones of which the frequencies do not
share a comman factor. This method is known as Audio Frequency Shift
Keying, or AFSK (A2B, F2B). The data can also be sent by
varying the carrier frequency with a fixed shift. This method is
known as Frequency Shift Keying, or FSK (F1B).
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Standard
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Mark '1'
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Space '0'
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Shift
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Remark
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United States (US)
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2295 Hz
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2125 Hz
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170 Hz
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Commercial and amateur use
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Europe (EU)
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2125 Hz
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1955 Hz
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170 Hz
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Commercial and amateur use
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Modern equipment
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1500 Hz
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1670 Hz
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170 Hz
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Used in modern equipment
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The speed at which the ITA2 data is sent, is usually called the baudrate.
It is equivalent to bits-per-second (bps) when each data element (including
start and stop bits) is regarded as a bit. The most common baud rates are
45.45, 50, 75 and 100 baud, although higher speeds like 300, 600 and 1200 baud
are sometimes used on VHF and UHF frequencies.
Below are some examples of popular baudrates and frequency shifts.
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Baudrate (bps)
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Shift (Hz)
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User
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45.45, 50, 75
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170
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Amateur radio 1
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75, 100
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850
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NATO
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50
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425, 450
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Commercial, diplomatic, weather service
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50
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170
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USSR, Russia
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50
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85
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RTTY on LF
and VLF frequencies
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With Amateur Radio, the most common baudrate is 45.45 baud, although
50 and 75 baud are occasionally used.
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© Crypto Museum. Created: Tuesday 20 January 2015. Last changed: Friday, 12 July 2024 - 09:28 CET.
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