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redsea RDS decoder

redsea is a lightweight command-line FM-RDS decoder for Linux/macOS. It supports a large subset of RDS features.


Decoded RDS groups are printed to the terminal as line-delimited JSON objects or, optionally, undecoded hex blocks (-x). Please refer to the wiki for input data formats.

Redsea can be used with any RTL-SDR USB radio stick with the rtl_fm tool, or any other SDR via csdr, for example. It can also decode raw ASCII bitstream, the hex format used by RDS Spy, and audio files containing multiplex signals (MPX). These use cases are documented in the wiki.



You will need git, the liquid-dsp library, libsndfile, and GNU autotools. On macOS (OSX) you will also need XCode command-line tools (xcode-select --install).

  1. Clone the repository (unless you downloaded a release zip file):

     $ git clone https://github.com/windytan/redsea.git
     $ cd redsea
  2. Compile redsea:

     $ ./autogen.sh && ./configure && make
  3. Install:

     $ make install

It is also simple to later pull the latest updates and recompile:

    $ git pull
    $ ./autogen.sh && ./configure && make clean && make
    $ make install

For a slower machine it can take some time to compile the TMC support. This can be disabled (./configure --disable-tmc).

If you only need to decode hex or binary input and don't need demodulation, you can compile redsea without liquid-dsp (./configure --without-liquid).


By default, a 171 kHz single-channel 16-bit MPX signal is expected via stdin.

The simplest way to view RDS groups using rtl_fm is:

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

Please refer to the wiki for more details and usage examples.

Full usage

radio_command | redsea [OPTIONS]
redsea -f WAVFILE

-b, --input-bits       Input is an unsynchronized ASCII bit stream
                       (011010110...). All characters but '0' and '1'
                       are ignored.

-c, --channels CHANS   Number of channels in the raw input signal. Each
                       channel is demodulated independently.

-e, --feed-through     Echo the input signal to stdout and print
                       decoded groups to stderr.

-E, --bler             Display the average block error rate, or the
                       percentage of blocks that had errors before
                       error correction. Averaged over the last 12
                       groups. For hex input, this is the percentage
                       of missing blocks.

-f, --file FILENAME    Use an audio file as MPX input. All formats
                       readable by libsndfile should work.

-h, --input-hex        The input is in the RDS Spy hex format.

-l, --loctable DIR     Load TMC location table from a directory in TMC
                       Exchange format.

-p, --show-partial     Show some multi-group data even before they've been
                       fully received (PS names, RadioText, alternative
                       frequencies). partial_ will be prepended to their
                       names. This is good for noisy conditions.

-r, --samplerate RATE  Set sample frequency of the raw input signal in Hz.
                       Will resample (slow) if this differs from 171000 Hz.

-t, --timestamp FORMAT Add time of decoding to JSON groups; see
                       man strftime for formatting options (or
                       try "%c").

-u, --rbds             RBDS mode; use North American program type names
                       and "back-calculate" the station's call sign from
                       its PI code. Note that this calculation gives an
                       incorrect call sign for most stations that transmit

-v, --version          Print version string and exit.

-x, --output-hex       Output hex groups in the RDS Spy format,
                       suppressing JSON output.

Formatting and filtering the JSON output

The JSON output can be tidied and/or colored using jq:

$ rtl_fm ... | redsea | jq

It can also be used to extract only certain fields, for instance the program type:

$ rtl_fm ... | redsea | jq '.prog_type'


  • Linux or macOS
  • For realtime decoding, a Raspberry Pi 1 or faster
  • ~8 MB of free memory (~128 MB for RDS-TMC)
  • C++11 compiler
  • GNU autotools
  • libiconv
  • libsndfile
  • liquid-dsp
  • rtl_fm (from rtl-sdr) or any other source that can output demodulated FM multiplex signals


Can't find liquid-dsp on macOS

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

xcode-select --install

Can't find liquid-dsp on Linux

Try running this in the terminal:

sudo ldconfig


Bug reports are welcome. Be prepared to check back with GitHub occasionally for clarifying questions.

Also, if a station in your area is transmitting an interesting RDS feature that should be implemented in redsea, I would be happy to see a minute or two's worth of hex data using the -x switch.


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. (jsoncpp and iconvpp have their own licenses.) See LICENSE.