I have a need for a very low speed < 20 bits/second transmitter and receiver pair. I've worked something based off the packet tx and packet rx examples, added in a modified DVB - Reed solomon instance that operates on nibbles at a time instead of bytes, and got it mostly working. My main difficulty is getting it working through an audio input for RX and audio output for RX. It has trouble getting a timing sync with the example parameters for constellation estimator, polyphase clock sync and costas loops from the packet RX example. Sometime it locks solid enough to get a few packets through, then it just keep rotating.
I'm thinking I"m missing something, but the only things that come to mind are the sample rates and filter sizes for the TX RRC, RX RRC filter and polyphase sync loop gain. Ideally, I'd like be at just a few baud, so I'm interpolating the heck out of the input before sending on the audio sink. Is there a relationship I'm missing for baud rate vs some of these parameters? Is the minimum buffer sizes to large for such low data rates?
Any thought would be helpful. I can't share the flowgraph, but basically using the packet TX and RX example, with lots interpolation for TX through audio sink and matching decimation on RX from audio source.
Thanks,
Alex.