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Fix DSC decoder for ITU-R M.493 compliance

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Correct modulation parameters (1200 bps, 2100/1300 Hz tones), replace
invented format codes with the six ITU-defined specifiers {102, 112,
114, 116, 120, 123}, accept all valid EOS symbols (117, 122, 127),
add parser validation (format, MMSI, raw field, telecommand range),
and fix truthiness bugs that dropped zero-valued fields.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
Smittix
2026-02-25 22:02:08 +00:00
parent 935b7a4d9d
commit 8a46293e5c
4 changed files with 339 additions and 88 deletions
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44
utils/dsc/decoder.py
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@@ -5,9 +5,9 @@ DSC (Digital Selective Calling) decoder.
Decodes VHF DSC signals per ITU-R M.493. Reads 48kHz 16-bit signed
audio from stdin (from rtl_fm) and outputs JSON messages to stdout.
DSC uses 100 baud FSK with:
- Mark (1): 1800 Hz
- Space (0): 1200 Hz
DSC uses 1200 bps FSK on a 1700 Hz subcarrier with:
- Mark (1): 2100 Hz
- Space (0): 1300 Hz
Frame structure:
1. Dot pattern: 200 bits alternating 1/0 for synchronization
@@ -42,6 +42,7 @@ from .constants import (
DSC_AUDIO_SAMPLE_RATE,
FORMAT_CODES,
DISTRESS_NATURE_CODES,
VALID_EOS,
)
# Configure logging
@@ -57,7 +58,7 @@ class DSCDecoder:
"""
DSC FSK decoder.
Demodulates 100 baud FSK audio and decodes DSC protocol.
Demodulates 1200 bps FSK audio and decodes DSC protocol.
"""
def __init__(self, sample_rate: int = DSC_AUDIO_SAMPLE_RATE):
@@ -66,13 +67,13 @@ class DSCDecoder:
self.samples_per_bit = sample_rate // self.baud_rate
# FSK frequencies
self.mark_freq = DSC_MARK_FREQ # 1800 Hz = binary 1
self.space_freq = DSC_SPACE_FREQ # 1200 Hz = binary 0
self.mark_freq = DSC_MARK_FREQ # 2100 Hz = binary 1
self.space_freq = DSC_SPACE_FREQ # 1300 Hz = binary 0
# Bandpass filter for DSC band (1100-1900 Hz)
# Bandpass filter for DSC band (1100-2300 Hz)
nyq = sample_rate / 2
low = 1100 / nyq
high = 1900 / nyq
high = 2300 / nyq
self.bp_b, self.bp_a = scipy_signal.butter(4, [low, high], btype='band')
# Build FSK correlators
@@ -278,11 +279,11 @@ class DSCDecoder:
if len(symbols) < 5:
return None
# Look for EOS (End of Sequence) - symbol 127
# Look for EOS (End of Sequence) - symbols 117, 122, or 127
eos_found = False
eos_index = -1
for i, sym in enumerate(symbols):
if sym == 127: # EOS symbol
if sym in VALID_EOS:
eos_found = True
eos_index = i
break
@@ -337,20 +338,21 @@ class DSCDecoder:
format_code = symbols[0]
format_text = FORMAT_CODES.get(format_code, f'UNKNOWN-{format_code}')
# Determine category from format
category = 'ROUTINE'
if format_code == 100:
# Derive category from format specifier per ITU-R M.493
if format_code == 120:
category = 'DISTRESS'
elif format_code == 106:
category = 'DISTRESS_ACK'
elif format_code == 108:
category = 'DISTRESS_RELAY'
elif format_code == 118:
category = 'SAFETY'
elif format_code == 120:
category = 'URGENCY'
elif format_code == 123:
category = 'ALL_SHIPS_URGENCY_SAFETY'
elif format_code == 102:
category = 'ALL_SHIPS'
elif format_code == 116:
category = 'GROUP'
elif format_code == 112:
category = 'INDIVIDUAL'
elif format_code == 114:
category = 'INDIVIDUAL_ACK'
else:
category = FORMAT_CODES.get(format_code, 'UNKNOWN')
# Decode MMSI from symbols 1-5 (destination/address)
dest_mmsi = self._decode_mmsi(symbols[1:6])