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fix: Improve HF SSTV VIS detection reliability and error correction

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Tolerate intermittent ambiguous windows during leader detection (up to
3 consecutive misses), use energy-based break detection when tone
classification fails at leader-break boundary, and add single-bit VIS
error correction for parity-bit and data-bit corruption on noisy HF.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
Smittix
2026-02-18 21:52:56 +00:00
parent ad4903d4ac
commit 481651c88d
2 changed files with 162 additions and 10 deletions
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55
utils/sstv/vis.py
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@@ -128,6 +128,7 @@ class VISDetector:
self._state = VISState.IDLE
self._buffer = np.array([], dtype=np.float64)
self._tone_counter = 0
self._miss_counter = 0
self._data_bits: list[int] = []
self._parity_bit: int = 0
self._bit_accumulator: list[np.ndarray] = []
@@ -137,6 +138,7 @@ class VISDetector:
self._state = VISState.IDLE
self._buffer = np.array([], dtype=np.float64)
self._tone_counter = 0
self._miss_counter = 0
self._data_bits = []
self._parity_bit = 0
self._bit_accumulator = []
@@ -188,10 +190,19 @@ class VISDetector:
if self._state == VISState.IDLE:
if tone == FREQ_LEADER:
self._tone_counter += 1
self._miss_counter = 0
if self._tone_counter >= self._leader_min_windows:
self._state = VISState.LEADER_1
elif tone is None:
# Ambiguous window (noise/fading) — tolerate up to 3
# consecutive misses before resetting the leader count.
self._miss_counter += 1
if self._miss_counter > 3:
self._tone_counter = 0
self._miss_counter = 0
else:
self._tone_counter = 0
self._miss_counter = 0
elif self._state == VISState.LEADER_1:
if tone == FREQ_LEADER:
@@ -204,7 +215,15 @@ class VISDetector:
self._tone_counter = 1
self._state = VISState.BREAK
elif tone is None:
pass # Ambiguous window at tone boundary — stay in state
# Mixed leader+break window? Check if 1200 Hz energy is
# significant relative to 1900 Hz — indicates the break
# pulse is straddling this analysis window.
leader_e = goertzel(window, FREQ_LEADER, self._sample_rate)
sync_e = goertzel(window, FREQ_SYNC, self._sample_rate)
if sync_e > leader_e * 0.5:
self._tone_counter = 1
self._state = VISState.BREAK
# else: noisy leader window, stay in LEADER_1
else:
self._tone_counter = 0
self._state = VISState.IDLE
@@ -338,24 +357,42 @@ class VISDetector:
def _validate_and_decode(self) -> tuple[int, str] | None:
"""Validate parity and decode the VIS code.
Includes single-bit error correction for HF noise resilience:
if parity fails, tries recovering by assuming either the parity
bit or exactly one data bit was corrupted.
Returns:
(vis_code, mode_name) or None if validation fails.
"""
if len(self._data_bits) != 8:
return None
# VIS uses even parity across 8 data bits + parity bit.
if (sum(self._data_bits) + self._parity_bit) % 2 != 0:
return None
parity_ok = (sum(self._data_bits) + self._parity_bit) % 2 == 0
vis_code = sum(bit << i for i, bit in enumerate(self._data_bits))
# Decode VIS code (LSB first)
vis_code = 0
for i, bit in enumerate(self._data_bits):
vis_code |= bit << i
if parity_ok:
mode_name = VIS_CODES.get(vis_code)
if mode_name is not None:
return vis_code, mode_name
return None # Valid parity but unknown code — not SSTV
# Look up mode
# Parity failed — try error correction
# Case 1: only the parity bit is wrong (data is correct)
mode_name = VIS_CODES.get(vis_code)
if mode_name is not None:
return vis_code, mode_name
# Case 2: one data bit is wrong — try flipping each
for flip in range(8):
corrected = vis_code ^ (1 << flip)
# Flipping one data bit should fix parity too
corrected_parity_ok = (
bin(corrected).count('1') + self._parity_bit
) % 2 == 0
if corrected_parity_ok:
mode_name = VIS_CODES.get(corrected)
if mode_name is not None:
return corrected, mode_name
return None