Asynchronous Simplex MFSK system. Uses direct translation of a 5 unit ITA-2 character into a sequence of two tones out of a total of 12. Each tone lasts 75 msecs therefore Baudrate measures as 13 Baud. The first three bits of the ITA-2 code is sent as one of eight tones, the last two bits sent as one of four tones. A thirteenth tone near the centre of these two banks of tones represents the idle or standby condition.

The idle condition (tone no. 9) is nearly in the centre of the system and is always 1052 Hz up from the suppressed carrier point of the station's SSB transmitter.

Coquelet stations actually use SSB transmitters and are nearly always on a kHz point exactly and often idle in between short messages.