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

[Kevin Custer]

On 6/30/2018 5:37 PM, David McGough 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

David,

I don't disagree that a filter change like this is necessary for the 
application.  I think you told me before that you use the usbradio 
channel driver in most of your installations.  I assume that this filter 
is implemented in either simpleusb or usbradio selections. Which audio 
input are you using on the TKR's?

Kevin W3KKC