fix: Improve HF SSTV VIS detection reliability and error correction

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>

tests/test_sstv_decoder.py

@@ -395,6 +395,121 @@ class TestVISDetector:
         assert vis_code == 8
         assert mode_name == 'Robot36'
 
+    def test_noisy_leader_detection(self):
+        """Should detect VIS despite intermittent None windows in leader.
+
+        Simulates HF fading by inserting short silence gaps (which produce
+        ambiguous tone classification) into the leader tone.
+        """
+        detector = VISDetector()
+        parts = []
+
+        # Build leader1 with gaps: 50ms tone, 10ms silence, repeated
+        # Total ~300ms of leader with interruptions
+        for _ in range(6):
+            parts.append(generate_tone(FREQ_LEADER, 0.050))
+            parts.append(np.zeros(int(SAMPLE_RATE * 0.010)))  # 10ms gap
+
+        # Break (1200 Hz, 10ms)
+        parts.append(generate_tone(FREQ_SYNC, 0.010))
+
+        # Leader 2 (clean)
+        parts.append(generate_tone(FREQ_LEADER, 0.300))
+
+        # Start bit + data bits + parity + stop (standard for Robot36 = VIS 8)
+        parts.append(generate_tone(FREQ_SYNC, 0.030))  # start bit
+        vis_code = 8
+        ones_count = 0
+        for i in range(8):
+            bit = (vis_code >> i) & 1
+            if bit:
+                ones_count += 1
+                parts.append(generate_tone(FREQ_VIS_BIT_1, 0.030))
+            else:
+                parts.append(generate_tone(FREQ_VIS_BIT_0, 0.030))
+        parity = ones_count % 2
+        parts.append(generate_tone(
+            FREQ_VIS_BIT_1 if parity else FREQ_VIS_BIT_0, 0.030))
+        parts.append(generate_tone(FREQ_SYNC, 0.030))  # stop bit
+
+        audio = np.concatenate([np.zeros(2400)] + parts + [np.zeros(2400)])
+        result = detector.feed(audio)
+        assert result is not None
+        assert result[0] == 8
+        assert result[1] == 'Robot36'
+
+    def test_vis_error_correction_parity_bit(self):
+        """Should recover when only the parity bit is corrupted."""
+        detector = VISDetector()
+        # Generate Martin1 header (VIS 44) but flip the parity bit
+        parts = []
+        parts.append(generate_tone(FREQ_LEADER, 0.300))
+        parts.append(generate_tone(FREQ_SYNC, 0.010))
+        parts.append(generate_tone(FREQ_LEADER, 0.300))
+        parts.append(generate_tone(FREQ_SYNC, 0.030))  # start bit
+
+        vis_code = 44  # Martin1
+        ones_count = 0
+        for i in range(8):
+            bit = (vis_code >> i) & 1
+            if bit:
+                ones_count += 1
+                parts.append(generate_tone(FREQ_VIS_BIT_1, 0.030))
+            else:
+                parts.append(generate_tone(FREQ_VIS_BIT_0, 0.030))
+
+        # Wrong parity (flip it)
+        correct_parity = ones_count % 2
+        wrong_parity = 1 - correct_parity
+        parts.append(generate_tone(
+            FREQ_VIS_BIT_1 if wrong_parity else FREQ_VIS_BIT_0, 0.030))
+        parts.append(generate_tone(FREQ_SYNC, 0.030))  # stop bit
+
+        audio = np.concatenate([np.zeros(2400)] + parts + [np.zeros(2400)])
+        result = detector.feed(audio)
+        assert result is not None
+        assert result[0] == 44
+        assert result[1] == 'Martin1'
+
+    def test_vis_error_correction_data_bit(self):
+        """Should recover Martin1 when one data bit is flipped by HF noise.
+
+        Simulates: Martin1 (VIS 44) transmitted correctly, but bit 0 is
+        corrupted during reception. The parity bit is received correctly
+        (computed for the original code 44), so parity check fails → error
+        correction tries flipping each data bit and finds VIS 44.
+        """
+        detector = VISDetector()
+        original_code = 44   # Martin1
+        corrupted_code = 44 ^ 1  # flip bit 0 → 45
+
+        parts = []
+        parts.append(generate_tone(FREQ_LEADER, 0.300))
+        parts.append(generate_tone(FREQ_SYNC, 0.010))
+        parts.append(generate_tone(FREQ_LEADER, 0.300))
+        parts.append(generate_tone(FREQ_SYNC, 0.030))  # start bit
+
+        # Transmit corrupted data bits
+        for i in range(8):
+            bit = (corrupted_code >> i) & 1
+            if bit:
+                parts.append(generate_tone(FREQ_VIS_BIT_1, 0.030))
+            else:
+                parts.append(generate_tone(FREQ_VIS_BIT_0, 0.030))
+
+        # Parity bit computed for the ORIGINAL code (received correctly)
+        original_ones = bin(original_code).count('1')
+        parity = original_ones % 2
+        parts.append(generate_tone(
+            FREQ_VIS_BIT_1 if parity else FREQ_VIS_BIT_0, 0.030))
+        parts.append(generate_tone(FREQ_SYNC, 0.030))  # stop bit
+
+        audio = np.concatenate([np.zeros(2400)] + parts + [np.zeros(2400)])
+        result = detector.feed(audio)
+        assert result is not None
+        assert result[0] == 44
+        assert result[1] == 'Martin1'
+
 
 # ---------------------------------------------------------------------------
 # Mode spec tests
@@ -405,7 +520,7 @@ class TestModes:
 
     def test_all_vis_codes_have_modes(self):
         """All defined VIS codes should have matching mode specs."""
-        for vis_code in [8, 12, 44, 40, 60, 56, 93, 95]:
+        for vis_code in [8, 12, 44, 40, 60, 56, 93, 95, 96, 98, 113, 55]:
             mode = get_mode(vis_code)
             assert mode is not None, f"No mode for VIS code {vis_code}"
 

utils/sstv/vis.py

@@ -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