31 August 2021

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)


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

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