30 June 2020

CIS-75 FSK 75Bd/250 (3): 126-bit LFSR sequence

(for background, read all the posts of this topic)
Russ-Mil CIS-75 75Bd/250 FSK system spotted this morning on 15832.0 KHz (cf). After differential decoding, the bitstream shows a clear 385-bit period and a 126-bit pseudo random sequence, generated by the polynomial x^7+x^6+1, which is inserted in the data stream probably to re-sync the receive modem among the messages:

111110111110011110101110000110111010011000101011000001011110001
110110110010010100100001001110010110100010001100110101010000000

Fig. 1 - 126-bit pseudorandom sequences
Fig.2 - the synched stream
It's interesting to note that in previous CIS-75 recordings, we saw the use of a 128-bit (!) length pseudo-random sequence transmitted in positive and negative polarity: those sequences are easily identifiable by inspecting the stream with a window, coincidentally, 385-bit wide.
By the way, the same polynomial x^7+x^6+1 is also used by the French-Ny in their 50Bd/850 FSK fleet broadcast as one of the two stream LFSR delimiters [1].  


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