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

[David]

David,

I presume your new filter is applied before the down sample to 8KHz and 
after the up sample to 48KHz.  is that correct?  Thanks for sharing the 
simulation plots.

- Dave k6dvd



On 6/30/2018 2:37 PM, "David McGough via arm-allstar" wrote:
> 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
>
>
>
> _______________________________________________
>
> arm-allstar mailing list
> arm-allstar at hamvoip.org
> http://lists.hamvoip.org/cgi-bin/mailman/listinfo/arm-allstar
>
> Visit the BBB and RPi2/3 web page - http://hamvoip.org
>
> _______________________________________________
>
> arm-allstar mailing list
> arm-allstar at hamvoip.org
> http://lists.hamvoip.org/cgi-bin/mailman/listinfo/arm-allstar
>
> Visit the BBB and RPi2/3 web page - http://hamvoip.org