[arm-allstar] notch filter

[David McGough]

Hi Ken,

The notch filter functionality is part of the app_rpt code, so that is all
4KHz audio (8K sampling rate). Each defined filter implements a 2-pole IIR
bandstop resonator. You can cascade up to 10 filter definitions.

In your example: "rxnotch=3500, 400" ...this would calculate the
coefficients for a filter centered at 3500Hz, with a 400Hz bandwidth (so:
3300Hz to 3700Hz). In your example, the notch depth will be shallow and
you'll probably need to cascade several filters.

All this code was originally added to app_rpt.c over 10 years ago, I don't
know by whom, Jim I guess? I've optimized it in the hamVoIP release.

This code is based on the DSP work by Tony Fisher. More info about that
can be found here:

https://www-users.cs.york.ac.uk/~fisher/mkfilter/
https://www-users.cs.york.ac.uk/~fisher/software/mkfilter/

BTW, I've got all the original mkfilter, genplot and related utilities
compiled and functional. I've also got a .pdf of the documentation for the
various utilities.

73, David KB4FXC



On Sat, 18 Feb 2017, Ken wrote:

> Another question:  how is the bandwidth of the notch filter defined?  (in
> rpt.conf)
> 
> In other words if I set "rxnotch=3500, 400" is that taken to mean 3100-3900
> or 3300-3700?
> 
> Is there an upper limit of 4000 Hz for these parameters due to the 8k data
> rate used internally in Asterisk? 
> Or is this filtering applied right at the input stream which I think is at
> 48k?
> 
> Thanks
> Ken
> KE2N
> 
> 
>