[arm-allstar] Interesting info on the new hamvoip lowpass filter

[Elliott L]

Hi Doug,

Wouldn't an out-of-band signal (for example, a 10KHz artifact from a
switching PSU) cause an alias at 2 kHz when sampled at 8 kHz? I think
hardware anti-alias filters are still needed at the input to the ADCs.
Perhaps these fob ICs already have that as an internal component.
Correct me if I'm wrong, but I think software filtering should really
only be needed if there is any resampling happening, for example,
playing back a higher-sample rate file or running an output at a
higher sample rate.

Very nice filter though! The passband is extremely smooth.

73,

--Elliott
W6EL
On Sun, Jun 24, 2018 at 10:09 PM "Doug Crompton via arm-allstar"
<arm-allstar at hamvoip.org> wrote:
>
> This was mentioned before but not to any great detail. If you are using the
> hamvoip V1.5 image v1.5.3-32 5/28/2018 or later your image has the new
> brick-wall low-pass filter. The low-pass filter has a corner frequency at
> 3.5KHz, with all components at least -50dB down at 4KHz and above. This
> filter is always on and adds the most fidelity that you can get at an 8Khz
> sample rate.
>
> Here is an image of the new Low Pass filter:
> https://hamvoip.org/docs/FIR-LP-simpleusb-2018-05-27-1.png
>
> This filter eliminates the need for any hardware filtering in FOBS so the
> "fancier" FOB's with hardware low-pass filtering like the DMK-URI and
> others are not needed to achieve this well filtered and excellent audio.
> Using this software and the filter means you no longer have to worry about
> out of band audio components when using the cheaper FOB's without any
> hardware low-pass filtering.
>
>
> *73 Doug*
>
> *WA3DSP*
>
> *http://www.crompton.com/hamradio <http://www.crompton.com/hamradio>*
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