29 January 2016

BELL 103 compatible FSK modem 300Bd/200 (Algerian AF)


heard on 11446.2 KHz (cf) at 0756 UTC. FSK bursts preceeded by three unmodulated tones 150 Hz spaced, manipulation speed is 300 symbols/s and shift is 200 Hz. A short 250ms preamble is embedded in the 3 pre-tones (pic.3) , data are encrypted. Since the baudrate (300Bd) is higher than the shift (200Hz), modulation could be a so-called semi-mode such as MSK or GMSK.
The waveform is compatible with AT&T Bell 103 modem and suggested to be used by Algerian Air Force. A short recording is available here
 
pic. 1 baudrate 300Bd
pic. 2 shift 200 Hz
pic. 3 FSK preamble

1 comment:

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