2 December 2019

Unid FSK 400 (401)Bd/800 bursts
(i56578, cryptomaster, KarapuZ)

Unid FSK 400Bd (401)/800 bursts spotted on 13224.0 Khz (cf + 1700), slight distorsion in the speed. ACF=0, no other results if demodulated using differential mode. Transmissions are not frequent and most often consist of a single burst, sometimes two (maybe two stations).

Fig. 1
Quoting my friend KarapuZ "Despite the good recording quality and SNR ratio, in my opinion, the transmitter modulator malfunctions in the frequency discriminator. As a result, we have a bitstream with many errors at the output".

Fig. 2
Comments by my friend cryptomaster follow.
The signal structure of the first burst is shown in Fig. 3, the transfer is considered without translating it into a relative form:
1. A meander with a frequency of 150 Hz (300bps);
2. The preamble is 31 bits (300bps) of the form 0101110001101000010010001010101;
3. Information transfer with a speed of 401.3 bps;
The preamble is marked in yellow:
Fig. 3

Other than the frequency discriminator, the signal is distorted by the imposition of a stray frequency of 800 Hz, the intensity increases by the end of the transmission (Fig. 4), perhaps this frequency (800 Hz) is somehow connected with the formation of the separation of the frequencies of manipulation.

Fig. 4
On a bitmap, it looks like the pattern in Fig. 5:

Fig. 5