Ital-Mil 1200Bd/800 FSK

Short Ital-Mil 1200Bd/800 FSK used before STANAG-4285/HDLC QUEDRE [1]. The bitstream after demodulation exhibits a 232-bit period length and a bit more strutured patterns after differential decoding (figure 1).

Figure 1 - demodulated bitstream before (upper) and after differential decoding

Reshaping the differential decoded stream it's interesting notice the 8-bit framing in figure 2 with one bit always set to "1" (or "0" according the polarity). The same 8-bit framing can be seen in a 46.6 ms window of the SA raster: 46.6ms @1200Bd -> 56 bit -> 7 x 8-bit frames (figure 3). As a guess, the 232-bit period could correspond to a 29-byte protocol data unit carryng almost the same contents.

Figure 2 - 8-bit framing

 

Figure 3 (notice the change of polarity each 8 bit)

https://disk.yandex.com/d/xj5xo3vrZB00Jg

[1] https://i56578-swl.blogspot.com/search/label/QUEDRE

Ital-Mil FSK, FSK 1200Bd/800

FSK-2 1200Bd/800Hz waveform, approximately 1500 Hz bandwidth, used by Italian Military; in this record it's used by Italian Navy HQ (IDR Rome) on 12200.6 KHz/USB as unusual intro before STANAG-4285 1200bps/L transmission, receive peer had the tactical callsign AZWL.

fig. 1 ~1500 Hz bandwidth

fig. 2 - baudrate line

fig. 3 - involution module and shift between carriers

 

The main features of this signal are what seems a polarity change that occurs each 8 bits (fig. 4), as reported by radioscanner.ru and confirmed in this analysis, and, at least in this sample, its quite pronounced 232-bit period shown in figure 5.

fig. 4 - polarity change

fig. 5 - 232-bit period (fisrt part looks like a sort of preamble)