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.