Using SDR dongles with Linux – Part 2

Using SDR dongles with Linux – Part 2, First Steps with the RTL-SDR.
Part 2: First steps with the RTL-SDR dongle
Topic for January 7, 2019 (KC7MM, NCS)

I have been experimenting with an RTL-SDR dongle, using both the gqrx graphical application and terminal-based utilities contained in the rtl-sdr package (from the Mint repository) as the interface to it. Tonight’s Net topic is about my experience so far.
rtl-sdr Utilites

This package contains a set of command line utilities:

rtl_adsb: a simple ADS-B decoder for RTL2832 based DVB-T receivers
rtl_eeprom: an EEPROM programming tool for RTL2832 based DVB-T receivers
rtl_fm: a narrow band FM demodulator for RTL2832 based DVB-T receivers
rtl_sdr: an I/Q recorder for RTL2832 based DVB-T receivers
rtl_tcp: an I/Q spectrum server for RTL2832 based DVB-T receivers
rtl_test: a benchmark tool for RTL2832 based DVB-T receivers


After some manipulation of the TV module (detailed on the Web site), I was able to use rtl_fm to listen to the FM broadcast band with the command:

rtl_fm -f 89.9e6 -s 240000 -r 96000 – | aplay -r 96000 -f S16_LE

Changing the frequency of 89.9 to 147.04 got me the ARRG repeater at Sylvan.

gqrx is a graphical user interface for running SDR devices on Linux. I installed it from the Linux Mint repository. When I started up the program, it detected the RTL-SDR dongle, and I had no trouble selecting it and starting it going. There are many YouTube videos on using this application; search for gqrx.

gqrx Web site

For Ham Radio purposes, just listening to the signal might not be sufficient. For example one might want to send the received signal to fldigi to decode digital modes. That’s not hard to do. I was able to get it working using the Pulse Audio Volume Control to link the input of fldigi to the output of gqrx. I learned how by watching YouTube videos by Kevin Loughlin, KB9RLW, which were most helpful:

Ham Radio and Linux – The easy way to patch audio between applications
Decoding signals from your SDR using FLDigi and Pulse Audio

I also succeeded in getting the signal into Audacity.

What’s Next

The rtl_tcp command-line utility will stream the SDR signal to a TCP port. I’d like to see about using that in place of gqrx as the signal source for fldigi. Pulse Audio won’t work for that, so I’ll try to use multimon-ng, as described in another KB9RLW video: Decoding POCSAG pager transmissions with GQRX and multimon-ng. As a practical application, I’m thinking that streaming an RTL-SDR might be a simple way for the base unit of a weather station I’m building to receive telemetry from remote sensors and route it to processing code I’ll write in Python.


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