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Decoder for DARC subcarriers on FM radio
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darc2json is a DARC demodulator for Linux/macOS. It takes an FM multiplex signal as input, in raw PCM format, and outputs line-delimited JSON messages. It was designed to be used with the RTL-SDR tool rtl_fm, but it can read any FM multiplex signals.

The default input sample rate is 228 kHz.

Supported features

Currently, darc2json will decode:

  • L5 Group Data (type 12)
  • Raw Layer 4 LMCh data if type is something else
  • Block application channel: Layer 3 data
  • TDT: Country code, network name, date and time


  • Fragmented L5
  • Short message channel
  • Synchronous Frame Messages
  • PLL and symbol synchronization
  • Error correction beyond single bit flips
  • Don't drop block sync at first error
  • More allocation-efficient handling of bitstrings
  • Conditional Access at L4
  • Don't repeat unchanged service messages


You will need git, the liquid-dsp library, and GNU autotools. Audio files can be decoded if libsndfile is installed. 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
     $ cd darc2json
  2. Compile darc2json:

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

     $ make install

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

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


The simplest way to view DARC messages using rtl_fm is:

rtl_fm -M fm -l 0 -A std -p 0 -s 228k -g 20 -F 9 -f 87.9M | darc2json

Full usage

radio_command | darc2json [OPTIONS]

By default, a 228 kHz single-channel 16-bit MPX signal is expected via

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

-r, --samplerate RATE  Set stdin sample frequency in Hz. Will resample
                       (slow) if this differs from 228000 Hz.

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

-v, --version          Print version string.


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


darc2json 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 has its own license.) See LICENSE.

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