Unid FSK 300Bd/300 bursts

i56578-swl.blogspot.com 3 miesięcy temu

Interesting and unidentified FSK 300Bd/300 bursts heard on ~14490 KHz and sent to me by my friends ANgazu and cryptomaster (Figure 1).

Fig. 1 - FSK 300Bd/300 bursts

The signals recorded by the second (6 bursts) have a better SNR and so more suitable to be analyzed. As you can see in Figure 2, the demodulated bitstreams (d1-d6) can be divided into the 4 groups G1, G2, G3, and G4:

Fig. 2 - couples of demodulated bitstreams d1-d2, d3-d4, d5-d6

G1: (40 bits) most likely a header/SOM sequence, this group is common to all the bitstreams;

G2: (40 bits) this group is different in all bitstream and possibly consists of something related to the message. In a 10-bit format it's possible to see repeated "fields", erstwhile reshaped to a 20-bit format the groups may consist of a 11-bit "field" followed by a common 9-bit pattern (Figure 3);

Fig. 3 - G2 groups

G3: (variable length) I think this group is the data part of the message, this is sent twice into 2 different bursts (sometimes 3 times in 3 bursts). These groups have a period of 50 bits in dimension that appears to have any form of structure (Figure 4);


Fig. 4 - G3 groups

G4: (20 bits) most likely the EOM sequence, this group is common to all the bitstreams.

During the formation of this FSK signal the phases of the 2 frequencies are preserved after each "shift" (Figure 5), i.e. the frequency shift is generated by a single generator and its clock frequency changes, so the manipulation is achieved without disrupting the phase of the signal. If 2 frequency generators are utilized then we should see changes of phase in both f1 and f2, unless the 2 generators are in any way phase synchronized.

f2 ~ 1976,98 HZ (2:0,001011640)
f1 ~ 1676,95 Hz (2:0,001192640)

as expecetd, 300 Hz shift (f2 - f1).