DSC Codec - Digital Selective Calling for Maritime Radio

A complete DSC (Digital Selective Calling) codec implementation for SDR hobbyists and sailors. Encodes and decodes DSC calls used on marine VHF, MF, and HF radio according to ITU-R M.493 standard.

What is DSC?

Digital Selective Calling (DSC) is the automated digital signaling system used on marine radio to:

• Initiate distress alerts and coordinate search and rescue
• Hail and call individual vessels by MMSI (Maritime Mobile Service Identity)
• Broadcast safety and weather information
• Make automated radio calls without voice

DSC is mandatory under GMDSS (Global Maritime Distress and Safety System) on:

• VHF Channel 70 (156.525 MHz) — the primary DSC channel
• MF 2187.5 kHz — medium frequency DSC
• Various HF frequencies — long-range DSC for ocean-going vessels

All DSC transmissions use 100 baud FSK with 170 Hz shift (1615/1785 Hz audio tones).

Features

• DSC Call Encoding: Create any DSC call type with full customization

  • All-ships broadcasts
  • Individual vessel calls (by MMSI)
  • Group calls
  • Area calls (by geographic coordinates)
  • Distress alerts with nature of distress, position, and UTC time

• DSC Call Decoding: Extract and parse calls from WAV audio recordings

  • Automatic bit synchronization (dot pattern detection)
  • Phasing sequence detection (DX/RX)
  • 10-bit symbol extraction with check bit validation
  • Error correction via symbol interleaving
  • ECC (Error Check Character) verification

• Robust Reception: Handles real SDR recordings

  • FSK demodulation with energy detection
  • Automatic sample rate handling (resample if needed)
  • Bit synchronization from sync pattern

Installation

No external dependencies beyond numpy:

pip install numpy

Usage

Encoding: Generate DSC Call Audio

python3 scripts/dsc_encode.py <output.wav> [options]

Examples:

# Individual call to vessel MMSI 123456789
python3 scripts/dsc_encode.py call.wav --address 123456789

# Distress alert with position and time
python3 scripts/dsc_encode.py distress.wav \
  --format distress \
  --address 123456789 \
  --category distress \
  --distress-nature sinking \
  --position 51.5,-1.5 \
  --time 1245

# All-ships broadcast
python3 scripts/dsc_encode.py broadcast.wav --format all-ships

# Working channel information
python3 scripts/dsc_encode.py call.wav --address 123456789 --channel 72

Options:

• --format — Call type: all_ships, selective_call (default), group_call, area_call, distress_alert
• --address MMSI — Destination MMSI (9 digits, default 123456789)
• --self-id MMSI — Caller's MMSI (9 digits, default 211234567)
• --category CAT — routine (default), safety, urgency, distress
• --telecommand1 TC — First message type (default: simplex_telephone_F3E_G3E)
• --telecommand2 TC — Second message type (default: no_information)
• --channel N — VHF working channel number
• --distress-nature CODE — For distress alerts: fire, flooding, collision, grounding, sinking, disabled_adrift, undesignated, abandoning_ship, piracy, man_overboard, EPIRB_emission
• --position LAT,LON — Position in decimal degrees (e.g., 51.5,-1.5)
• --time HHMM — UTC time (e.g., 1245 for 12:45 UTC)

Decoding: Extract DSC from Recordings

python3 scripts/dsc_decode.py <input.wav> [output.txt] [--verbose]

Examples:

# Decode to stdout
python3 scripts/dsc_decode.py recording.wav

# Decode and save to file
python3 scripts/dsc_decode.py recording.wav output.txt

# Verbose output
python3 scripts/dsc_decode.py recording.wav output.txt --verbose

Decoder output includes:

=== DSC CALL DECODED ===
Phasing: DX
Format: selective_call
Destination MMSI: 123456789
Category: routine
Caller MMSI: 211234567
Telecommand 1: simplex_telephone_F3E_G3E
Telecommand 2: no_information
EOS: no_info
ECC Valid: Yes

Technical Details

DSC Call Structure (ITU-R M.493)

Every DSC call consists of:

1. Dot pattern — 200 alternating bits (0101... or 1010...) for bit synchronization
2. Phasing sequence — DX (126) or RX (127) symbol repeated 6 times
3. Format specifier — Indicates call type (102-123)
4. Address field — Destination MMSI (9 digits) or coordinates
5. Category — Routine (100), Safety (108), Urgency (110), or Distress (112)
6. Self-ID — Caller's MMSI (9 digits)
7. Telecommand 1 — Message type (e.g., telephone, data, distress position)
8. Telecommand 2 — Secondary message type or frequency/channel
9. Optional fields — For distress alerts: nature, position, time
10. EOS — End of Sequence marker (120-122)
11. ECC — Error Check Character (XOR of all preceding symbols)

Symbol Encoding

Each DSC symbol is encoded as a 10-bit value:

• Bits 0-6 (7 bits): Information (constrained to 4 ones + 3 zeros)
• Bits 7-9 (3 bits): Check bits = 7 - popcount(info_bits)

This constant-weight encoding ensures each symbol has exactly 7 ones in its 10-bit representation, allowing detection of single-bit errors.

Error Correction

DSC uses two-level error correction:

1. Symbol interleaving — Each symbol is repeated 5 positions later in the transmission. If two copies disagree, the one with valid check bits is used.

2. ECC verification — The final ECC character is the XOR of all symbols from the format specifier through the EOS marker. Receiver verifies this checksum.

Signal Parameters

• Bit rate: 100 baud (0.01 seconds per symbol, 10 bits per symbol = 1000 bps)
• Mark frequency: 1615 Hz (binary 1)
• Space frequency: 1785 Hz (binary 0)
• Shift: 170 Hz (1785 - 1615)
• Modulation: FSK (Frequency Shift Keying)

Testing

Run the comprehensive test suite:

python3 scripts/dsc_test.py

Tests cover:

• Symbol encode/decode with check bit validation
• ECC computation
• Interleaving and de-interleaving
• Individual call encoding
• Distress alert encoding
• All-ships broadcasts
• FSK modulation/demodulation
• Full WAV encode/decode roundtrips
• Symbol error correction

Use Cases

For SDR Hobbyists

• Decode live VHF DSC traffic from SDR receivers
• Analyze marine radio distress calls and safety broadcasts
• Monitor vessels by their MMSI calling patterns
• Archive maritime DSC events

For Sailors

• Generate DSC calls for testing/simulation
• Create distress alert audio for training
• Understand VHF Channel 70 traffic

For Researchers

• Study maritime communication protocols
• Analyze GMDSS system performance
• Test DSC-related applications

Limitations and Notes

• Single-tone audio: This codec implements baseband DSC at 1615/1785 Hz. Real radio equipment transmits on RF (156.525 MHz VHF, 2187.5 kHz MF, etc.), which would require additional RF modulation.

• No interleaving optimization: The current implementation does not fully implement the official interleaving scheme (which repeats symbols at N+5 positions for error correction). The main error correction is via check bits and ECC.

• Synchronization: The decoder looks for the dot pattern and phasing sequence. It's optimized for clean, well-formed DSC signals. Real QSB (fading) or noise may affect reception.

• MMSI format: The codec validates 9-digit MMSIs. Invalid formats will be rejected or zero-padded.

Standards References

• ITU-R M.493-15 — Digital Selective Calling (DSC) for use in the maritime mobile service
• ITU-R M.541 — Maritime Mobile Service Identity (MMSI)
• GMDSS — Global Maritime Distress and Safety System (IMO/IEC specifications)

License

This code is provided as-is for educational and amateur radio use.

Contributing

Improvements and bug reports welcome. Key areas for enhancement:

• Full interleaving/de-interleaving implementation
• Advanced synchronization (Costas loop, PLL)
• Real SDR file format support (GQRX, HACK RF, etc.)
• Multi-channel monitoring
• Distress alert frequency allocation database