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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.

Cipher machines that use ITA-2
The Siemens T-52 'Geheimschreiber'
Lorenz SZ-40/42 cipher machine
The Siemens T-43 mixer machine
British/Canadion one-time tape cipher machine used during and after WWII
British 5-UCO (BID/30) OTT cipher machine
SIGTOT one-time tape cipher machine (US)
ATCRRM mixer machine used on the Washington-Moscow hotline
British one-time tape cipher machine compatible with Rockex
PTT Colex (predecessor of Ecolex)
Ecolex I, developed by PTT, manufactured by Philips Usfa
Ecolex II, developed by PTT, manufactured by Philips Usfa
Philips Ecolex IV
Siemens Schlüsselgerät D
Siemens M-190 OTT cipher machine, used on the Washington-Moscow hotline
Hagelin TC-52
Hagelin C-446-RT, the OTP (OTT) version of the C-446
OTP/OTT version of the Hagelin CX-52
DUDEK StG-1 (T-352 / T-353) one-time tape cipher machine developed in Poland
OTT cipher machine (mixer) for teletype networks (telex)
Mils Elektronik one-time tape cipher machine, developed in the mid-1970s.
Mils Elektronik one-time tape cipher machine, with key generator
Lorenz Mixer (Mi-544)
SELMA OKA-150, telegraphy cipher machine
TAROLEX key stream generator
Ecolex X (Ecolex 10) online/offline cipher machine
Aroflex UA-8116
RACE (KL-51)
HC-570 CRYPTOMATIC desktop electronic cipher machine
HC-550 CRYPTOMATIC desktop electronic cipher machine
The Siemens T-1285CA (Aroflex) cipher machine
 Teletypewriters (telex machines)

ITA-2 encoding
Dec Ltr Fig Hex Bin Remark
0 NUL 00 000·00 NULL, Nothing (blank tape)
1 E 3 01 000·01  
2 LF 02 000·10 Line Feed (new line)
3 A - 03 000·11  
4 SP 04 001·00 Space
5 S ' 05 001·01  
6 I 8 06 001·10  
7 U 7 07 001·11  
8 CR 08 010·00 Carriage Return
9 D ENC 09 010·01 Enquiry (Who are you?, WRU)
10 R 4 0A 010·10  
11 J BEL 0B 010·11 BELL (ring bell at the other end)
12 N , 0C 011·00  
13 F ! 0D 011·01 Can also be %
14 C : 0E 011·10  
15 K ( 0F 011·11  
16 T 5 10 100·00  
17 Z + 11 100·01  
18 L ) 12 100·10  
19 W 2 13 100·11  
20 H $ 14 101·00 Currency symbol — Can also be £
21 Y 6 15 101·01  
22 P 0 16 101·10  
23 Q 1 17 101·11  
24 O 9 18 110·00  
25 B ? 19 110·01  
26 G & 1A 110·10 Can also be @ 
27 FIGS 1B 110·11 Figures (Shift on)
28 M . 1C 111·00  
29 X / 1D 111·01  
30 V ; 1E 111·10  
31 LTRS 1F 111·11 Letters (Shift off)
Control character

Data format
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).

Standard Mark '1' Space '0' Shift Remark
United States (US) 2295 Hz 2125 Hz 170 Hz Commercial and amateur use
Europe (EU) 2125 Hz 1955 Hz 170 Hz Commercial and amateur use
Modern equipment 1500 Hz 1670 Hz 170 Hz Used in modern equipment
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.

Baudrate (bps) Shift (Hz) User
45.45, 50, 75 170 Amateur radio 1
75, 100 850 NATO
50 425, 450 Commercial, diplomatic, weather service
50 170 USSR, Russia
50 85 RTTY on LF and VLF frequencies
  1. With Amateur Radio, the most common baudrate is 45.45 baud, although 50 and 75 baud are occasionally used.

Other standards
  1. Sam Hallas, RTTY - The Easy Way
    British Amateur Radio Teleprinter Group, Undated.
  1. ECMA, Standard ECMA-10 for Data Interchange on Punched Tape
    2nd Edition. July 1970.

  2. Wikipedia, Punched tape
    Retrieved January 2015.

  3. R&S Manual of Transmission Methods
    4070.0711.02-03. Rohde & Schwarz, München (Germany) 2014.

  4. Wikipedia, Radioteletype
    Visited 12 July 2024.
Further information
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© Crypto Museum. Created: Tuesday 20 January 2015. Last changed: Friday, 12 July 2024 - 09:28 CET.
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