24 October 2016

Unid BPSK 1500Bd (prob. Chinese modem)

Unid modem (prob. Chinese origin) using BPSK modulation at 1500 Baud and 1500 Hz sub-carrier. Since the alternation of frames with different strength, this may be a duplex channel.

fig. 1
fig. 2
fig. 3
Once demodulated, the analysis of the bitstream reaveals an interesting 3 bit structure (fig. 4): my friend Karapuz suggested to try a differential (relative) decoding of the signal.

fig. 4
Differential decoding can be obtained directly by running the proper tool of  the bit-editor or by demodulating the signal using the SA demodulator with option "Diff 1" as in fig. 5

fig. 5
Results are similar and in the output bistream, visually more logical, is visible the sync bit and the two data bits (fig. 6).

fig. 6

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