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Coquelet was a Multi Frequency-Shift Keying (MFSK) telegraphy standard, used for transmitting textual messages over narrow-band short wave radio channels. It uses more than two different frequencies (tones), as opposed to the more standard Frequency Shift Keying (FSK) that uses only two frequencies. Coquelet was developed by ACEC in Belgium in the 1950s in order to avoid the common problems of FSK over radio, such as selective fading and multi-path timing distortion. It was used by the Armies and Police Services of Belgium, France, Algeria and other countries [2].

There are three variants:

  1. Coquelet 13, or Mk 1, using 2 tones from a set of 13 (13, 20 baud)
  2. Coquelet 8, or Mk 2, Synchronous MFSK, using 2 tones from a set of 8 (20, 26 baud)
  3. Coquelet 80, like Coquelet 8 but with Forward Error Correction (FEC) (20, 26 baud)
The name COQUELET is the French word for GRID. The system works by sending two different tones in sequence (one after the other) for each character from the ITA-2 character table.

Coquelet 13
With Coquelet 13, the first tone was taken from a set of 8 different tones known as Group I, whilst the second tone was selected from 4 different tones, known as Group II. Each tone lasts 75 ms, resulting in baud rates of 13.3 and 20 Bd, less than half the speed of Baudot (50 Baud) [3].

The upper table above show the organisation of the characters in the Coquelet table when the machine is in letters-mode (LTR). The lower table shows the characters that are used in figures-mode (FIG). When ACEC introduced the first teleprinters and cipher machines that used Coquelet 13, they also developed interfaces that translated Coquelet into Baudot (ITA-2) and vice versa. In fact, Coquelet uses the ITA-2 table as its basis, and the machines were constructed in such a way that messages could be stored on standard 5-level paper tape, using the Baudot-Murray code.

Within the ITA-2 standard, each character is represented by 5 bits. The first three bits are represented by one of eight tones from Group I (23=8), whilst the last two bits are sent as one of four tones from Group II (22=4). When there is no activity (idle), tone number 0 is sent. The table above shows how the Coquelet 13 tones are translated to the ITA-2 bits.

Tone frequencies
Idle tone
  1. 1052 Hz
Group I
  1. 812 Hz
  2. 842 Hz
  3. 872 Hz
  4. 902 Hz
  5. 932 Hz
  6. 962 Hz
  7. 992 Hz
  8. 1022 Hz
Group II
  1. 1082 Hz
  2. 1112 Hz
  3. 1142 Hz
  4. 1172 Hz

The Coquelet standard was used in the following countries:

  • Algeria
  • Belgium
  • France
  • Rhodesia 1
  • Zimbabwe 1
  1. Confirmed by [5].

Other MFSK systems
  • MFSK8
  • MFSK16
  • Olivia MFSK
  • Piccolo
  • ALE
  • DominoF
  • DominoEX
  • CIS-36 MFSK, a.k.a. CROWD-36
  • XPA and XPA2
  1. Wikipedia, Multiple frequency-shift keying
    Retrieved January 2013.

  2. Opentopia, What is COQUELET (Signal)?
    Retrieved January 2013. 1

  3. Roland Proesch, Technical Handbook for Radio Monitoring
    Edition 2008. ISBN 9783837045734.

  4. Wikipedia, Radioteletype
    Retrieved january 2014.

  5. Anonymous contributor, The use of Coquelet in Rhodesia and Zimbabwe
    Radio technician in the late 1970s. Personal correspondence, August 2015.
  1. Website and/or page no longer available in January 2014.

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Crypto Museum. Created: Wednesday 19 November 2014. Last changed: Saturday, 06 March 2021 - 13:26 CET.
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