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

[David McGough]

Hi Elliot,

There is re-sampling being done. All CM1xy audio is sampled at 48KHz. One 
function is called for decimation (down-sampling from 48KHz ---> 8KHz) and 
another for interpolation (up-sampling from 8KHz ---> 48KHz). The FIR 
filter used is symmetrical.  Also, ultimately, the user will be able to 
select an arbitrary number of filter curves,  loading the filter 
coefficients at runtime from a database. I back-ported portions of this 
code from the new chan_iopipe driver in development.

The CM1xy chips have an integrated LDO voltage regulator for conversion
from the 5VDC supplied by USB to the 3.3V used internally. The chip
supplies different power filter paths for the analog and digital sections,
if this is desired, so that the chip power is very clean.

73, David KB4FXC


On Mon, 25 Jun 2018, "Elliott L via arm-allstar" wrote:

> 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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> >
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