Getting started with Digital Modes in Linux

Brandon Rozek

September 4, 2019

This blog post is going to describe what steps I took to be able to decode signals using digital modes. Hardware wise, you will either need a RTL-SDR receiver or a transceiver radio with a cable plugging into the computer’s soundcard.

Software Required

As you can see, if you are only using a transceiver then you can skip the parts mentioning Gqrx and Pavucontrol

Process

Gqrx

When you first open up Gqrx, it’ll ask you to configure the device. I have the RTL2832U, so if you have the same tool…

Select the profile partially labeled RTL2832U. If you want to look at frequencies between 0 and 28MHz then add to the device string “direct_samp=3”. Otherwise don’t add that string.

You can keep the rest of the settings the same.

Fldigi

When you first open up Fldigi, it’ll ask you for our operator info.

Field description
Station Callsign Your callsign
Station QTH City, State
Station Locator Your grid code. (Can look this up on QRZ)
Operator Callsign Your callsign
Operator Name Your name
Antenna Your antenna type

In Audio->Devices, make sure you select the physical device if you’re using a transceiver, otherwise select PulseAudio.

You can keep the rest of the settings as the default.

Pavucontrol

Finally to link the two, run the following lines in bash to create a sink in order to push the audio from Gqrx to Fldigi.

pacmd load-module module-null-sink sink_name=MyLoopback
pacmd update-sink-proplist MyLoopback device.description=MyLoopback
pacmd load-module module-loopback sink=MyLoopback

Now open up pavucontrol. On the Playback tab, change the output device of GQRX to be “MyLoopback”

On the Recording tab, change the input of fldigi to “Monitor of Null Output”

Conclusion

Now the software is sufficiently setup to be able to receive and decode signals. Best of luck finding things out there!

Beginner Pitfalls

Since I’m also a beginner, I will share what I got stuck on and how you can avoid it.

Make sure that you know the appropriate mode to demodulate the signal. I’ve had luck getting information from https://www.sigidwiki.com/wiki/Signal_Identification_Guide

-> This needs to be configured on both Gqrx and Fldigi