[arm-allstar] notch

[Patrick Perdue]

Somewhat related to this:

I remember, some time ago, you mentioned that simpleusb has filters to 
prevent anti-aliasing. Does this only apply to TX audio? I ask, because 
I just set up a radio-less node using an Icom HM-207 microphone (DTMF 
doesn't work for obvious reasons), which has a bit of a peak around 3.8 
kHz, which still passes through. I notched that frequency out using 
three cascaded filters in rpt.conf, and it did flatten out the sound a bit.

The TX audio on HamVoIP's simpleusb channel definitely sounds better 
than the ASL version, but I didn't really notice any difference in RX 
audio, other than I got a lot more jitter on the same hardware using ASL.


On 3/18/2021 11:31 PM, "David McGough via ARM-allstar" wrote:
> 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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>>
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>>
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>
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>
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