Use Your RasperryPi to Decode Sound on FM Frequencies

rtl_fm is one of the utilities from the rtl_sdr package for using a TV dongle as an SDR. Head over here if you need more info on this.

Direwolf is a soundcard based packet modem.

According to the Direwolf docs, it supports using rtl_fm as an input, so I thought I would take a look at look at getting these running together.

I’ve used rtl_sdr and rtl_tcp on my Pi before, but not rtl_fm, so first to get this working.

To playback the stream from rtl_fm you need to pipe into into some audio app. This is the same way that direwolf is going to read the stream too. Following the suggestion here on the rtl_sdr page, this command works fine for a local broadcast radio station on 96.9MHz:

rtl_fm -f 96.9M -M wbfm -s 200000 -r 48000 | play -r 48000 -t s16 -L -c 1  -

I’m not sure what all these options are, but the key options seem to be -s for the sample rate, and -r for the resolution. The -r value needs to match on the rtl_fm side and on the play side.

Now to get direwolf installed on Raspbian:

– per the userguide, first install libasound-dev:

sudo apt-get install libasound-dev

– download the source zip from: https://home.comcast.net/~wb2osz/site/?/page/Download/

– unzip and cd into the direwolf folder

– make with:

make -f Makefile.linux

make install-conf

make install_rpi

At this point I have rtl_fm on the Pi working as it should, and direwolf working great when decoding audio input from a 2m radio input via a USB soundcard. Combining the two though is giving me issues.

I don’t thing I’m able to get a strong enough received signal on 2m on the RTL stick even with an external 1/4wave 2m antenna.

This is the combination of commands, rtl_fm, piping into Direwolf:

rtl_fm -f 145048467 -M fm -s 200000 -r 32000 -g 35 | direwolf -n 1 -r 32000 -b 16 -t 0 -

I’ve allowed for the tuning offset ppm on this RTL card, but maybe I’ve calculated this wrong?

Anyway, since Direwolf by itself on the Pi is working well and what I needed for a small packet project was to be able to decode packet on the Pi, I’ll be putting rtl_fm on hold for the time being and playing with Direwolf by itself.