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

[Elliott L]

Hi David,

That's awesome, I did not realize the software samples at 48k. I
suppose this means the only hardware(analog) filtration necessary
would be a gentle RC at around 20 kHz?

I look forward to trying out the new filters. Out of curiosity, what
was the previous filter like?

--E
On Mon, Jun 25, 2018 at 2:07 PM David McGough <kb4fxc at inttek.net> wrote:
>
>
> 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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> > > arm-allstar at hamvoip.org
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> > >
> > > Visit the BBB and RPi2/3 web page - http://hamvoip.org
> > _______________________________________________
> >
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> >
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> >
>