[arm-allstar] arm-allstar Digest, Vol 49, Issue 28

[Ken]

I have to comment that the audio filtering is great for audio sources that
have a lot of high frequency content (>3 kHz).  Fusion and DMR have a lot of
that, as do the recordings of some bulletins if it comes from an external
source.  Audio generated internally in Allstar will not have any high
frequency components since, it uses an 8 kHz clock rate that cannot generate
anything above 4 kHz.

But this filtering does not fix one potential problem on the output side.
An unfiltered fob will have a strong 48 kHz component in its output.  If you
feed this into a microphone input or 1200 bps packet input the filtering in
the radio will take out the 48 kHz.  But if you feed it in some other way -
where it goes direct to the modulator stage without any other filtering -
then you will create birdies at offsets of +/- 48 kHz  and more.  Software
filtering does not fix that. Just a cautionary warning.

Ken
KE2N



Message: 2
Date: Mon, 25 Jun 2018 01:09:33 -0400
From: Doug Crompton <wa3dsp at gmail.com>
To: ARM Allstar <arm-allstar at hamvoip.org>
Subject: [arm-allstar] Interesting info on the new hamvoip lowpass
	filter
Message-ID:
	<CAMp6vsv4b8b-LGD1GMm0YkjhVYFiVuYnqpAeFbjQ_Sc3+b2XFg at mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"

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*

*http://www.crompton.com/hamradio <http://www.crompton.com/hamradio>*