FT8/FT4 Weak Signal Digital Mode Codec

FT4:

• Modulation: 4-GFSK (4-tone Gaussian FSK)
• Symbol rate: 20.8333 baud
• Bandwidth: ~80 Hz
• Duration: 4.48 seconds per transmission
• Tone spacing: 20.8333 Hz

Message Types

FT8/FT4 supports several message types encoded in a 77-bit payload:

1. Standard messages: Two callsigns (28 bits each) + grid locator (15 bits) + type (3 bits)

   • Example: K1JT W9XYZ EN37 (contact with grid)
   • Example: K1JT W9XYZ -15 (signal report in dB)

2. CQ calls: CQ K1JT FN20 (callsign + own grid)

3. Free text: Up to 13 ASCII characters from the FT8 alphabet

4. Telemetry: 18 hex digits

5. Signal reports: R-01 to R+30 or -01 to +30 dB

Encoding/Decoding Pipeline

1. Message packing → 77-bit payload (callsign encoding, grid/signal-report encoding)
2. CRC-14 → Add 14-bit CRC for error detection (91 bits total)
3. LDPC(174,91) → Forward error correction encodes to 174 bits
4. Symbol mapping → Convert bits to symbols (3 bits/symbol for FT8, 2 bits/symbol for FT4)
5. Sync insertion → Add Costas array sync patterns
6. GFSK modulation → Modulate symbols to audio using Gaussian-smoothed FSK

For decoding, reverse the pipeline:

1. GFSK demodulation → Extract symbols from audio
2. Sync detection → Verify Costas arrays and extract data symbols
3. Symbol demapping → Convert symbols back to 174-bit codeword
4. LDPC decoding → Belief propagation decoder with error correction
5. CRC verification → Check integrity
6. Message unpacking → Decode callsigns, grid locators, and text

Usage

Encoding

python3 scripts/ft8_encode.py output.wav --message "CQ K1JT FN20" --freq 1000 --mode ft8

Decoding

python3 scripts/ft8_decode.py input.wav --mode ft8 --freq 1000

To auto-search for signals across the audio spectrum:

python3 scripts/ft8_decode.py input.wav --mode ft8 --all

Testing

Run the comprehensive test suite:

python3 scripts/ft8_test.py

Tests cover:

• Callsign encoding/decoding roundtrips
• Grid locator and signal report encoding
• Free text encoding/decoding
• CRC-14 calculation
• LDPC encode/decode with error correction
• GFSK modulation/demodulation
• Complete FT8/FT4 WAV encode/decode roundtrips

Requirements

• Python 3.7+
• numpy

References

• WSJT-X: https://physics.princeton.edu/pulsar/K1JT/wsjtx.html
• FT8 specification by Joe Taylor K1JT and Steve Franke K9AN
• Amateur Radio Weak Signal Digital Modes