[arm-allstar] notch filter

[Patrick Perdue]

I have access to a repeater's Allstar node. This repeater has a 162.2 hz 
PL tone, and it's rather loud, especially the third harmonic, not so 
much the fundamental frequency. Aside from cleaning things up at the 
source, which is the best solution, I've tried all sorts of things to 
minimize that tone. That frequency is high enough that even using the 
rather sharp PL filter, it still leaks through. I've tried various 
stacks of rxnotch with different bandwidth, located at the center 
frequency, and also half an octave to either side. This gets rid of that 
harmonic, but it rings out where the notch center frequency lives. I can 
adjust how much it resonates by changing the width or adding more 
filters. Ultimately, I'd love to use something else to process the 
audio, like a parametric EQ rather than a hard notch. A long time ago, I 
read about a future iopipe driver for HamVoIP that might allow for such 
things to be used. Just wondering what the progress on that is.