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I’m attempting to develop a GNU Radio FSK decoder for Digital Selective Calling on marine VHF but I seem to be stuck. I got my hands on a Standard Horizon handheld marine VHF unit with DSC capability and have recorded a call from/to myself using Universal Radio Hacker and my HackRF. I also created a recorder and a playback flow graph in GRC and have successfully recorded and played back calls with them.

I know they are good recordings because when I play them back, the handheld radio alerts me to an incoming call. However, I cannot manage to demodulate them correctly using URH or GNU Radio.

Potential problems I see:

  • Samples/Symbol in URH: The protocol is 1200 baud and the recording was taken at 2.0Msps. Shouldn’t samples/symbol be 1666.66? Very few bits come out if I do that, though.
  • Clock Recovery in GNU Radio: I’ve looked at every FSK receiver example I could find online and still don’t really understand the parameters for this block, or whether I should use Clock Recovery MM or Symbol Sync.

If anybody could give me a push in the right direction or some additional resources to review it would be greatly appreciated.

Thanks!

My draft GNU Radio flow graph

URH settings

Recorder flow graph

Playback flow graph

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  • $\begingroup$ A single "O" in the console means signal was lost. Your console tells me that what your flowgraphs sees is such a tiny fraction of the actual signal that it's completely broken. $\endgroup$ Sep 21 at 22:59
  • $\begingroup$ @MarcusMüller That's good to know, any thoughts on what might be causing that? I figured that since I can play the file back and the radio will identify it then the recording must be okay, so I'm guessing the problem is somewhere in my flow graph. $\endgroup$ Sep 22 at 18:24

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