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

[David McGough]

David,

Yes, the FIR filter is used directly as part of the decimator and 
interpolator functions, limiting the bandwidth to 4KHz during the 
resampling process.

73, David KB4FXC






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

> 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
> >
> >
> >
> > _______________________________________________
> >
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> >
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> >
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> >
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> >
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> 
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