Thanks Glen, yea, the 7500 is pretty much hardwired. It comes from the 75 baud, and the number of symbol rows which just comes up to that odd rate. Since the idea is to take the cheap Pi and make it useful, the obvious thing would be to just implement the OFDM and be done with it. But I thought I'd see if there was an easy way to throw in COFDM and 4FSK which are the other 700 bit/sec modes.
I'm using a cheapo Adafruit sound card ($9) which is just a USB connector with two audio plugs. These usually don't work well at 8 KHz native, so I thought with all the spare CPU I could just play with a FIR. Thanks for the ideas, I'll play around with these. I'm sort of interested in using the OFDM for the basis of a data mode protocol, so maybe the COFDM will give way to a different interest.
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