gr-selcal is an Out-Of-Tree (OOT) module for GNU Radio dedicated to the encoding and decoding of analog selective calls (SelCall). The module supports the most widely used standard protocols in PMR/LMR (ZVEI, CCIR) and offers advanced features such as code-based squelch control (Audio Gate) and automatic PTT signalling.

• Multi-Protocol Support: ZVEI-1, ZVEI-2, CCIR-1, CCIR-2, PCCIR.

• Decoder:

  • Real-time tone recognition.
  • Filtering based on ‘Target Code’ (opens the audio only if it receives the correct selective).
  • Asynchronous message output (PMT) with decoding metadata.
  • Ringer output: sends a notification message when the target code is detected.

• Encoder:

  • Generation of the complete sequence: Destination + Pause +Source.
  • Automatic management of repeat tones (e.g. “E”).
  • PTT (ptt_out) signalling via asynchronous message to activate transmitters or GUI.

• Ringer Block:

  • Audible alert generator with a two-tone siren 🚒.
  • Visual indicator output via asynchronous message.

• GNU Radio 3.8 or higher (tested on 3.10).
• Python 3.
• Standard libraries: numpy.
• CMake 3.5 or higher.

Compilation can be performed manually or using the install.sh script.

mkdir build
cd build
cmake ..
make
sudo make install
sudo ldconfig

Alternatively, you can use the installation script provided:

chmod +x install.sh
./install.sh

After installation, you will find the blocks in GNU Radio Companion under the category [SelCall].

This block listens to an audio stream, decodes the tones, and only lets the audio through when a specific sequence is detected.

Input:

• audio_in (Float): Demodulated audio stream (e.g., output of an NBFM Receive).

Output:

• audioout (Float): ‘Gated’ audio stream. Contains the original audio only when the gate is open, otherwise silence.
• selcall_out (Message): PMT message containing the decoded code, timestamp and match status.
• ringer (Message): PMT message that sends a notification when the target code is detected.

Parameters:

• Protocol: Standard to be used (e.g. ZVEI-1).
• Target Code: The selective code that opens the squelch (e.g. ‘12345’).
• Code Length: Expected length of the selective code (usually 5).
• Tone Duration: Duration of each tone in milliseconds (e.g. 100 ms).
• Sample Rate: Sampling frequency (e.g. 48000).
• Debug Mode: Enables the output of detailed debug messages.

This block generates the audio tone sequence to make a call. It automatically combines the personal ID (Source) with the recipient ID received via message.

Sending logic: When it receives a message ‘12345’ on msg_in, and the Personal Code is ‘99999’, it will generate tones for: 12345 (Destination) → PAUSE → 99999 (Source) .

Input:

• msg_in (Message): The Destination code (string).

Output:

• audio_out (Float): The generated audio tones (to be sent to an Audio Sink or NBFM Transmit).
• ptt_out (Message): Boolean signal. Sends True (PTT ON) at the start of transmission and False (PTT OFF) at the end.

Parameters:

• Protocol: Standard to use.
• Personal Code (Source): Your unique ID that will be prefixed to each transmission.
• Amplitude: Tone volume (0.0 - 1.0).
• Sample Rate: Sampling frequency (e.g. 48000).

This block functions as an audible alert generator, typically triggered when a specific selective call is successfully decoded. It produces a distinctive two-tone alarm (European Siren style: 800Hz / 1010Hz) and provides a status signal for visual indicators.

Input:

• trigger (Message): Any asynchronous message received on this port starts (or restarts) the alarm timer.

Output:

• audio_out (Float): The generated alarm audio stream. Contains the two-tone siren signal when active, otherwise silence.
• led (Message): Boolean signal. Sends True when the alarm starts and False when the duration expires. Designed to drive a QT GUI LED Indicator.

Parameters:

• Duration: Length of the alarm in seconds (e.g. 5.0).
• Amplitude: Audio volume level (0.0 - 1.0).
• Sample Rate: Sampling frequency (must match the audio sink).

Inside the grc-flowgraph folder, there are grc files for testing GNURadio blocks once they have been installed.

SelCallDecoderUSRP.grc

graph LR
    A[Audio Source / SDR] --> B[NBFM Receive]
    B --> C[Selcall Decoder]
    C -- audio_out --> D[Audio Sink]
    C -- selcall_out --> E[Message Debug]
    C -- ringer --> F(Selcall Ringer)

SelCallEncoderUSRP.grc

graph LR
    A[QT GUI Message Edit] -- msg_in --> B[Selcall Encoder]
    B -- audio_out --> C[NBFM Transmit / Audio Sink]
    B -- ptt_out --> D[Control Logic / LED]

graph LR
    A[Selcall Decoder] -- ringer --> B[Selcall Ringer]
    B -- audio_out --> C[Audio Sink]
    B -- led --> D[QT GUI LED Indicator]

ℹ️ If you don't have a USRP or SDR device available, you can find the test flowgraph in the selcall_test file, where you can test both encoding and decoding with ringing.

• Rocchio Pietro
• De Cao Andrea
• Marku Juri

This project is released under the MIT licence. See the LICENCE file for details.