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FSK 100Bd/620, Polish Intel

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FSK-2 modulation with 620 Hz shift and 100 Baud speed, in use by Polish Intel service.

pic. 1 - baudrate and shift
The signal exhibits clean ACF spikes at 96 bits (~960ms lenght). Looking morecloselyone can distinguish six periods of16bits, most likelythey represent a five digits code and a separator between the groups as also shown in the bitstream (pic. 3).

pic. 2 - 96 bits ACF (6 x 16 bits digits)
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pic. 3
Looking at the whole demodulated bitstream (pic. 4), although inclusive of the overhead bits, we canget an ideaof the signal structure. First of all, note that the ACF is due to the final part "D" and then does not characterizethe entiresignalbut onlya part. While the parts A and E can be assumed as the 'start' and 'end' of transmission, it's difficult to fix the roles of the other groups: message, destination address, coded-instructions and so on. Talking with my friend KarapuZ, he says we need much more recordings for statistic comparisons since a previous recording of 2014 just exhibits that same characteristics. And if he says so...

pic. 4


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