30 March 2017

Baudot FSK 100Bd/500 (prob. M42b)

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 analyzed signals have a shift of 500 Hz, a manipulation speed of 100 Baud (48-bit ACF) and a 5-bit size data field. 
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 groups of 5 LTs (or 5 FGs ?). The used procedure, and the results, are shown below.
Fig.1
Fig.2
Groups of 5 LTs/FGs 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/Russian users, most likely Enigma M42b.



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