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

[Kevin Custer]

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