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

[David McGough]

Kevin,

For years there has been continuing speculation about the necessity of
post-DAC filtering, audio bandwidth and fidelity, etc., related to
AllStar.

I'm in the process of performing *REAL* testing of various radio hardware,
including modern gear and older repeaters, like both a Mastr II phasing
exciter and a PLL type exciter.  The numbers I'm finding are quite
interesting. All this data will be published, when complete.

For now, the significance of the new FIR filter I've implemented is 
hopefully clear from these simulations using the GNU Octave (Matlab)
software.

Here is the original filter in the "stock" simpleusb channel driver. The
graph's horizontal scales are from 0Hz to 24KHz.

https://hamvoip.org/img/octave-old-susb-filter.png

And, next is a simulation of the new filter:

https://hamvoip.org/img/octave-new-susb-filter.png


Note that these filters are used for decimation and interpolation during
the re-sampling process which converts the 48KHz sample rate used by the
CM1xy chips to the 8KHz rate used internally with Asterisk.

What is obvious about the old filter is that is allows significant energy
above the 4KHz Nyquist limit to slip by! That's of particular interest in
the decimation process, when wide-band noise from a discriminator may be
present on weak received signals; thus causing alias artifacts, further
impairing the RX weak signal S/N performance.

Empirically speaking, many of my "local" repeater users immediately
noticed the audio improvement, BTW. I'm running mostly Kenwood TKR
repeaters.


73, David KB4FXC



On Sat, 30 Jun 2018, "Kevin Custer via arm-allstar" wrote:

> On 6/25/2018 1:09 AM, "Doug Crompton via arm-allstar" 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*


Doug,

Are you recommending that people can connect their FOB to a radio or 
repeaters modulator without any hardware low-pass filtering? Obviously, 
to actually benefit from the wider filter you've implemented, you'd need 
to bypass the native filtering in the radios MIC circuitry that normally 
cuts off at 3kHz or less, otherwise, there is no improvement.  I'd 
caution anyone that's considering the modifications required to benefit 
from this filter to also consider a hardware filter to insure nothing 
can modulate the radio outside of 'your' channel.

Kevin W3KKC



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