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Re: [Discuss-gnuradio] Resampling audio using rational resampler doesn't

From: Albin Stigö
Subject: Re: [Discuss-gnuradio] Resampling audio using rational resampler doesn't work for multimon-ng
Date: Sat, 18 May 2019 09:04:46 +0200

You need to use a low pass filter (taps) in you resampler. Look at the spectrum after the resampler and this will be obvious to you. A lowpass filter with a cutt off at 3-4kHz or so should do it.

A quadrature rate of 250ksps is unnecessary for NBFM, 25ksps is more than enough.

On Sat, May 18, 2019, 08:29 Ignatius Rivaldi <address@hidden> wrote:
I'm not sure whether I should put this or in multimon-ng github page. multimon-ng wants 22050 Hz sample rate, and to feed this I use this flow:

To generate the APRS RF signal I used USRP B210 and another gnuradio flowgraph.

If I use sox to resample the 48 kHz audio into 22050 Hz, using this command:  netcat -ulk ::1 8001 | sox -t raw -esigned-integer -b 16 -r 48000 - -esigned-integer -b 16 -r 22050 -t raw - | multimon-ng -Aq - , multimon-ng decodes the APRS signal normally. If I use rational resampler to resample it inside Gnuradio from 48k to 22050, and run multimon-ng with this command:  netcat -ulk ::1 8000 |  multimon-ng -A  - it doesn't decode the APRS signal at all.

The thing is that if I generate the signal from inside the flowgraph, by using NBFM transmit, and resample it then connect it into frequency xlating filter instead of the RTL-SDR source, both sox resampler and rational resampler works, same if I use audio source instead of RTL-SDR.

I have the flowchart attached
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