CCIR 242
(aka TDM 242 / ARQM2-242, ARQM4-242)

Synchronous Duplex ARQ system using the 7 bit error correcting ITA 3 alphabet consisting of one, two or four channels in time division multiplex to the CCIR 242 Recommendation. Although now superseded by more recent and improved CCIR 342-2 it is still in use by many stations. The two stations use different frequencies for full duplex operation.

The 1 channel system does not use any interleaving at all. All characters are sent erect.

The 2 channel system is simply made from interleaving erect characters for channel A and inverted characters

for channel B, i.e. A,-B,A,-B,A,-B,A,-B etc.

The 4 channel system has channels A and C character interleaved and with the addition of channel B being bit interleaved with channel A and channel C bit interleaved with D. Channel A is erect, B/C/D inverted.

Due to the transmission system not containing any reliable information to enable correct synchronisation, both the genuine users of this mode (and you!) will have to manually resynchronise occasionally.

Possible character repetition cycles are 4, 5 or 8 characters.