redsea is a lightweight command-line FM-RDS decoder that supports many RDS features.

It can decode RDS live into newline-delimited JSON where each line corresponds to one RDS group; or it can print them as raw hex.

Simple usage (note the recommended sampling rate of 171k):

rtl_fm -f 89.3M -s 171k - | redsea -r 171k

Example output:

{"pi":"0xD3C2","group":"0A","ps":"MDR JUMP","di":{"dynamic_pty":true},"is_music":true,"prog_type":"Pop music","ta":false,"tp":false}
{"pi":"0xD3C2","group":"2A","prog_type":"Pop music","tp":false}
{"pi":"0xD3C2","group":"2A","radiotext":"Das Leichteste der Welt von Silbermond JETZT AUF MDR JUMP","prog_type":"Pop music","tp":false}
{"pi":"0xD3C2","group":"12A","prog_type":"Pop music","radiotext_plus":{"item_running":true,"item_toggle":1,"tags":[{"content-type":"item.title","data":"Das Leichteste der Welt"},{"content-type":"item.artist","data":"Silbermond"}]},"tp":false}
{"pi":"0xD3C2","group":"3A","open_data_app":{"app_name":"RadioText+ (RT+)","oda_group":"12A"},"prog_type":"Pop music","tp":false}

• How to install
• Usage
• Requirements
• Troubleshooting
  • Can't find liquid-dsp on macOS
  • Can't find liquid-dsp on Linux
• Contributing
• Licensing

Redsea is written in C++ and needs to be built from source, but this is not very complicated. Commands are provided below.

On Ubuntu:

sudo apt install git build-essential meson libsndfile1-dev libliquid-dev

Or on older Debians:

sudo apt-get install python3-pip ninja-build build-essential libsndfile1-dev libliquid-dev nlohmann-json3-dev
pip3 install --user meson

Or on macOS using Homebrew:

brew install meson libsndfile liquid-dsp nlohmann-json
xcode-select --install

Meson will later download nlohmann-json for you if it can't be found in the package repositories.

It's also possible to build redsea on Windows, either in Cygwin or by building an *.exe with MSYS2/MinGW. This is a bit more involved - instructions are in the wiki.

Downloading a release version is recommended.

Alternatively, if you wish to have the latest snapshot, you can also clone this git repository. The snapshots are work-in-progress, but we attempt to always keep the main branch in a working condition.

git clone https://github.com/windytan/redsea.git
cd redsea

meson setup build && cd build
meson compile

If your build machine has very little RAM (e.g. Raspberry Pi) please refer to our guide on building on a low-end system.

Now the binary executable is compiled and you can run it!

You can install the binary using meson install if you so wish. By default, it will be installed under /usr/local, but this can be changed by providing e.g. meson setup build --prefix /usr/local. See the Meson guide for more.

If you cloned the repository you can later get the latest updates and recompile:

git pull
cd build && meson compile

We have more usage examples in the wiki.

See the full list of command line options in the wiki or type redsea --help.

Redsea reads an MPX signal from stdin by default. It expects the input to be raw 16-bit signed-integer PCM. This means it can easily be used in real-time with rtl_fm.

Here's an example command that listens to 87.9 MHz using rtl_fm and displays the RDS groups:

rtl_fm -M fm -l 0 -A std -p 0 -s 171k -g 20 -F 9 -f 87.9M | redsea -r 171k

*.spy (or *.rds) is the common hexadecimal RDS logging format.

redsea --input hex < sample_hex_file.spy

Use the -f option to read WAV or other audio files that have a header. Redsea will automatically detect the sample rate and encoding. You can even read a WAV from a pipe (stdin) using -f -.

redsea -f mpx_input.wav --output hex

Note that not all rtl_fm versions write a WAV header with parameter -E wav. You might need to do that with sox, see rtl_fm -f 104900k -s 171k - | sox -t raw -r 171k -es -b 16 - -c 1 104900.wav

• Linux/macOS/Windows
• For realtime decoding, a Raspberry Pi 1 or faster (see benchmark results).

• libiconv
• libsndfile
• liquid-dsp
• nlohmann-json

• Linux/macOS/Cygwin/MSYS2+MinGW
• C++17 compiler (tested on GCC 9.5)
• meson + ninja
• enough RAM (see building on a low-end system)

• Catch2
• Perl
• SoX

See CONTRIBUTING.md for how to build and run the tests.

If you've installed liquid-dsp yet meson can't find it, it's possible that XCode command line tools aren't installed. Run this command to fix it:

xcode-select --install

Try running this in the terminal:

sudo ldconfig

We welcome bug reports and documentation contributions. Or take a peek at our open issues to see where we could use a hand. New code is contributed using pull requests; See CONTRIBUTING for more information.

Redsea is released under the MIT license, which means it is copyrighted to Oona Räisänen OH2EIQ yet you're free to use it provided that the copyright information is not removed. (iconvpp has its own license.) See LICENSE.

Note: This software is not safety certified. Do not rely on it for emergency communication, accurate traffic / weather information, or life-critical situations.