The questions related to the "nature" of the 1022-bit period of the QPSK 1200Bd signal (see the previous post) has been solved thanks to the help of my friend *cryptomaster*: I was waiting for traffic ("I tuned it waiting for traffic, but luckless until it went off") but actually it was already there!

He had the great intuition to reshape the bitstream into di-bit symbols, ie 2 bits per row given the 4ary modulation, then we gone on analyze the two columns separately and found the m-sequence * x^9+x^5+1* (or the equivalent x^9+x^4+1) in the second column: therefore, each bit of data is followed by one bit of the m-sequence, ie the 1022-bit period consists of 511 bit of data interleaved with the 511 bit of the sequence generated by the LFSR x^9+x^5+1 (Figure 1).

Fig. 1 - x^9+x^5+1 sequence in the 2nd column |

*H(m,k)*code is a Hamming code that encodes

*k*bits of data into

*m*bits (the codeword), adding

*m-k*parity bits (CRC).

Fig. 2 - Hamming parity check (511,502) matrix |

*demod-bit1.txt*) has been reshaped to form a 511-column matrix of codewords. Manually checking the about two hundred 511-bit length codewords would have been a nightmare, so I wrote a short Octave script that would do the job for me and calculate the 9-bit CRC of each row along with a simulation of the final PSK4 modulation - the result in Figure 3.

Fig. 3 - 9 bit CRC rows (left); PSK4 modulation of data +CRC (right) |

**each row of the bitstream is nothing more than a "codeword" consisting of 502 bits for data plus 9 bits for Hamming CRC**, ie the 511-bit period that we saw.

Fig. 4 - the bistream of column #1 |

*cryptomaster*tested also a 2016 recording and found it matches with the above conclusions (Figure 5). For what concerns the nature of the 502-bit strings of data they are probably telecontrols, further recordings are need. A possible (!) functional block diagram of the modulator is shown in Figure 6.

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