[arm-allstar] Low-pass filter used ---was: notch

[David McGough]

Patrick,

For various HamVoIP channel drivers (including chan_simpleusb), HamVoIP
uses a custom, high performance FIR low-pass filter that I designed. This
filter is optimized for the best practical performance within the
constraints of the 8KHz sampling rate used by AllStar and is symmetrically
used in both the decimation (down sampling) and interpolation (up
sampling) signal chains.

More discussion can be found here:
 
http://lists.hamvoip.org/pipermail/arm-allstar/2018-June/009091.html


73, David KB4FXC



On Fri, 19 Mar 2021, "Patrick Perdue via ARM-allstar" wrote:

> 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.
> 
>