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 50 Baud 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 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.
No comments:
Post a Comment