13 January 2018

(differential) FSK 75Bd/200Hz

Unid and quite rare FSK system running at 75Bd and using a 200Hz shift, heard on 4540.0 Khz (central frequency) around 1755z. The measured ACF is = 0 but I went on this transmission after its start, ie when in traffic mode, so I do not know if the initial header part has any pattern that could help in signal identification.

Fig. 1
As said, the ACF is =0 but using the differential decoding you may get a strong 385-bit period (Fig. 2).

Fig. 2
I posted my analysis in radioscanner (my nickname there is "ansanto") asking for comments:
and my friend cryptomaster in his reply suggested a "relative" FSK (DFSK2) modulation and pointed that the 385-bit period is due to the length of the scrambler generated by the polynomial x^7 + x^6 + 1. After descrambled the bistream exhibits a 398-bit period (Fig. 3), thanks to cryptomaster.

Fig. 3

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