This is a very interesting transmission heard on 14736.5 KHz/USB which consists of repetitive bursts lasting about 12.6 s and separated by an interval of 1300 ms. The most interesting aspect is that a "combined" waveform is used, that is, a first segment with PSK2 modulation followed by a second segment with FSK2 modulation: both the segments are modulated at the same rate of 31 bps (Figure 1).
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| Fig. 1 - PSK2/FSK2 "combined" waveform |
An interesting feature that my friend cryptomaster pointed out is the PSK2 mode, where the bitstream is transmitted in single bits: this feature can be seen in Figure 2 using both the "oscilloscope" function and the AM demodulator of SA. The demodulation of this bit-keying mode is complicated, apparently due to the division of the carrier into separate bits.
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| Fig. 2 - PSK2 segment |
The FSK2 segment modulation uses a shift of about 370 Hz (measured 373) and the resulting bitstream after demodulation has an 8N1 framing which appears to be transmitted in reverse polarity (Figure 3).
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| Fig. 3 - FSK2 segment |
The interesting things about this curious transmission don't end here: going back to the previous posts I discovered that the FSK2 segment carries exactly the same content as the 50Bd/612 FSK2 bursts analyzed some months ago [1] and compared in Figure 4.
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| Fig. 4 |
So, it's likely the same (unid) user, maybe experimenting with different waveforms... who knows.
The PSK2 segment too probably carries the same content of the FSK2 segment, unfortunately demodulation with SA was not successful due to the particular mode which is here used.
https://disk.yandex.com/d/HV3j9zdSSRqLww
[1] http://i56578-swl.blogspot.com/2024/11/unid-fsk-50bd612-bursts.html
