I have played around by using a smaller number of coefficients. Currently the filter uses 160 and I created a filter that uses 63 instead. The current filter is very sophisticated (designed by James Ahlstrom), as it uses complex coefficients (essentially doubling the numbers I listed above, one for the I and one for the Q sides. This filter can be tuned to other center frequencies.
It's kind of neat how it works, but you need about 2 GHz CPU. It provides 100 dB attenuation for a @1100 Hz BW and looks great on the SDR spectrum view :-) My change to 63 filters cut the time in half, but I only used a 40 dB attenuation. This left some higher amplitudes on the ends, but probably acceptable. I didn't transmit the signal, just looked at it from a saved file in Audacity and using FreeDV spectrum view. I also tried an 81 coefficient filter with 55 dB attenuation, and to tell the truth I didn't see much change. Be a good area to experiment on the air. I believe the filter was implemented early on because there was kind of a step in the spectrum, so it cleaned that up, but I don't think the step would be a killer, and leaving the filter off if you are out of CPU ought to be fair game. 73, Steve
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