darc2json

darc2json is a proof-of-concept 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

darc2json can 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

Not implemented

A list of things to fix for your own implementation:

• No PLL and symbol synchronization (amazingly, it kind of works)
• No error correction beyond single bit flips
• No Fragmented L5
• No Short message channel
• No Synchronous Frame Messages
• No COT, SCOT, AFT, SAFT
• No Conditional Access at L4
• Drops block sync at first error
• Needs more allocation-efficient handling of bitstrings
• Repeats unchanged service messages

Installation

You will need git, a C++17 compiler, the liquid-dsp library, libsndfile, and meson. 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/darc2json.git
    $ cd darc2json
   

2. Compile darc2json:

    $ meson setup build
    $ cd build
    $ meson compile
   

3. Install:

    $ meson install
   

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

    $ git pull
    $ cd build
    $ meson compile
    $ meson install

Usage

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
stdin.

-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.

Troubleshooting

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

Licensing

See LICENSE.