Hello all, I'm using the FSK demodulator from codec2 in a side project of mine. I have a 9600 bps stream running at 4.8KHz deviation. Packets have 32 bits of preamble, followed by a 32 bit sync word and then the actual data.
I believe the FSK demodulator is having trouble locking on, and it's causing some of the initial bits to be flipped. My reasoning is as follows: - I captured a sample of about a hundred packets, with 10 seconds of nothing between each packet, and a few ms of transmission time per packet - If I allow, say, 10 flipped bits in the sync word, every packet decodes properly. - If I allow only 5 flipped bits in the sync word, the decode rate drops, and continues to drop as I lower the allowed number of flipped bits. - When I only allow 5 flipped bits, suddenly the modem is super sensitive to parameters such as n_sym and the upper/lower frequency estimation limits. For example, changing n_sym from 38 to 39 may cause the decode rate to fall by 10% or more. However, different n_sym values seem to work better for different packets. If I capture two samples of a hundred packets each, each sample may have a different n_sym where the maximum number of packets are decoded (at 5 allowed bit flips) - 5/32 bit flips would be enough to mangle many packets, if that error rate was maintained for the entire duration of the packet. Thus, the error rate must be dropping shortly after the sync word Initially I thought it might be an issue with a slow AGC on the RTL-SDR. After turning off AGC, I'm still getting the same behaviour. This makes me think it's a problem with the FSK demodulator itself. As far as I can tell, the FSK demodulator is primarily used in projects that send many packets in quick succession. I am wondering if it's poorly optimized for my use-case of having long periods between packets. Thanks, Stephen _______________________________________________ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
