30 March 2017

async FSK-2 50/100 Bd 500 Hz, "5-38" code (prob. CIS source)

Some day ago, me and my friend KarapuZ were talking about some asynchronous 500 Hz shift FSK-2 signals and the way to get a meaningful decoding from the demodulator of SA. The first step is the removal of the START/STOP bits according to the size of the data field  (ie 5, 6, 7 or 8 bits) and then try different alphabet codes in turn, until you find one that produces an output that makes a sense. 
The two analyzed signals have a shift of 500 Hz, a different manipulation speed (50 and 100 Baud) and different ACF (45 and 48 bits) but both exhibit a 5-bit size data field. 
 
Fig. 1 - the FSK 50Bd/500 signal
Once removed the START/STOP bits, we tried the 5-38 code (1 start bit, 1.5 stop bits, the good old Baudot), as KarapuZ advised, and we got interesting outputs: groups of 5 figures  in the 50 Baud signals, and more clear groups of 5 letters in the 100 Baud signal. The used procedure, and the results, are shown below for the 100Bd signal.
Fig. 2
Fig. 3
Groups of 5 letters/figures is a "format" that is frequently used by several Military and Governative Agencies so in the absence of other evidence it's a bit difficult to identify the source, although there are good chances in favour of  CIS/Russia:  I'm not experienced in "numbers" stations, comments are welcome.



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