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Decoding radio signal with dl-fldigi

24 05 2014   //   Written by Yannick   //   Classified in : HAB  //  Tags : dl-fldigi, gqrx


dl-fldigi is a modified version of Fldigi to add some features related to HAB.

dl-fldigi use sounds created by Gqrx (see previous post ) to decode informations sent from the balloon, then is able to send these informations on a server on Internet.

If a request has been made this display the balloon on the map.

What is great, is the decoded packets can been sent by anyone, not only the owner of the balloon. This makes possible to send a balloon from UK, then track it for exemple to Ukraine with listeners in this area.


Informations on installation (compilation) of dl-fldigi are available on UKHAS wiki and are working fine on Debian too.

You need to enable deb-src in source.list then:

sudo apt-get update
sudo apt-get build-dep fldigi
sudo apt-get install git-core libcurl4-openssl-dev libjpeg62-dev autoconf
git clone git://github.com/jamescoxon/dl-fldigi.git
cd dl-fldigi
git submodule init
git submodule update
autoreconf -vfi
./configure --disable-flarq

Test 1 - Decoding without the sound card

This UKHAS page provide the link of a record that can be used to test the configuration of dl-fldigi.

Menu Op Mode, RTTY, and for this sound, use the folowing settings

To load the downloaded wav file, select File/Audio/Playback to play it without using the sound card for first test.

This sound record of the 2 frequencies can be decoded and flight informations displayed.

If the decoding is fine, you can move to test #2

Test 2 - Decoding with the sound card

This test is close to a real flight where sound comes from Gqrx output.

Use pavucontrol (Pulse Audio Volume Control) to adjust the recording settings:

Bellow, dl-fldigi is listening (choose Monitor of… ) et select analog input.

Bellow, the output volume on the speakers:

Bellow, the mic disabled

Bellow,the volume of totem playing the wav file.

On dl-fldigi side:

It seems PortAudio is using ALSA.. which is using PulseAudio…
It’s also possible to directly use PulseAudio.

HAB mode

To start dl-fldigi in HAB mode which contain the list of registered flight, with all settings ready, use the folowing command:

./src/dl-fldigi --hab

Result of playing the wav file with totem with dl-fldigi in HAB mode.

In case of problem decoding with the sound card

/!\ During my fists tests, with several computers, several distros, the decoding with the sound card didn’t work.
After few days of tests, I solved the problem by changing the default-sample-rate to 48000:

sudo vi /etc/pulse/daemon.conf
; default-sample-rate = 44100
default-sample-rate = 48000
pulseaudio --kill

SDR - Software Defined Radio on GnuLinux with Gqrx

24 05 2014   //   Written by Yannick   //   Classified in : HAB  //  Tags : gqrx, sdr


SDR (Software Defined Radio) is a radio communication system where components that have been typically implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, etc.) are instead implemented by means of software on a personal computer.

The main advantage of SDR is the reduced cost.

In France (and many countries in Europe), we are free to use the 433MHz frequency (433,050 à 434,790 MHz).

This way we could be able to receive signal sent from the balloon, then decode the signal to get the coordinates etc...


I ordered a USB dongle DVB-T RTL2832U+R820T:

  • 7 € on Ebay (shipping included)
  • Include antenna (and unused remote)
  • Able to receive from 24 MHz to 1766 MHz.
Product: RTL2838UHIDIR
Manufacturer: Realtek
Realtek Semiconductor Corp. RTL2838 DVB-T

The dongle is supposed to be RTL2832 but report on lsusb and dmesg RTL2838


I use Gqrx (GPLv3) to visualize and hear radio signal.

To install all dependencies I used this script.
I modified it a bit for my LinuxMint Debian Edition distro.
On my computer I take 1 hour and half to compile.

We can test with:

$ rtl_test
Found 1 device(s):
0: ezcap USB 2.0 DVB-T/DAB/FM dongle
Using device 0: ezcap USB 2.0 DVB-T/DAB/FM dongle
Found Rafael Micro R820T tuner
Supported gain values (29):

To install Gqrx:

$ git clone https://github.com/csete/gqrx.git
$ cd gqrx
$ qmake
$ make
$ sudo cp gqrx /usr/local/bin/

Starting Gqrx:

$ gqrx
linux; GNU C++ version 4.7.2; Boost_104900; UHD_003.005.002-56-g34052015
gr-osmosdr 871f0cc2 (0.0.1git) gnuradio
built-in device types: file fcd rtl rtl_tcp uhd

Then we can test by listering a “regular” radio:

We can see the space of few hundreds of KHz between radio channels.

We can change the volume of Gqrx with pavucontrol