[arm-allstar] notch

[David McGough]

Ken,

Yes, there is notch filter included in the HamVoIP software, based on the
notch filter code from Tony Fisher. This code allows up to 10 cascaded
2-pole IIR bandstop resonators, each of variable center frequency and
width.  You can enable by adding the notch=<freq>,<width>,...  parameter
to your rpt.conf node stanza. Note that for valid syntax, each cascaded
instance must have a <freq>,<width> pair.

Here is an example for a 3 cascaded filters, center frequency 2880Hz and 
width 100Hz. This was used to filter some power supply noise.

rxnotch=2880,100,2880,100,2880,100


The per-instance notch depth depends on the frequency and width. I forget
the actual depth curve, I've got it simulated in MatLab, somewhere. It's
the standard curve for a 2-pole IIR bandstop resonator. Cascading makes 
deeper notches, of course, if they're on the same frequency.

Note that with various defined parameters, the filters can ring, which is
typical of this type DSP code.  Overall, I've found the notch function
works well.  The original code is here:

https://github.com/university-of-york/cs-www-users-fisher



73, David KB4FXC



On Thu, 18 Mar 2021, "Ken via ARM-allstar" wrote:

> Does the current version of HamVOIP support the receive notch filter?  
> 
> How "deep" is one notch? 
> 
> I assume it gets deeper when you cascade a couple of them.
> 
>  
> 
> Thanks
> 
> Ken
> 
> KE2N
> 
>  
> 
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