fix: morse decoder splitting dahs into dits due to mid-element signal dropout

Add dropout tolerance (2 blocks ~40ms) to bridge brief signal gaps that
caused the state machine to chop dahs into multiple dits. Also fix scope
SNR display to use actual noise_ref instead of noise_floor.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_morse.py

@@ -218,6 +218,46 @@ class TestEnvelopeMorseDecoder:
         assert metrics['detect_mode'] == 'goertzel'
 
 
+class TestDropoutTolerance:
+    def test_dah_with_mid_gap_produces_single_dah(self):
+        """A dah-length tone with a 1-block silence gap in the middle should
+        still be decoded as a single dah, not two dits."""
+        sample_rate = 8000
+        wpm = 15
+        tone_freq = 700.0
+        dit_dur = 1.2 / wpm
+        dah_dur = 3 * dit_dur
+
+        decoder = MorseDecoder(
+            sample_rate=sample_rate,
+            tone_freq=tone_freq,
+            wpm=wpm,
+        )
+        block_size = decoder._block_size
+        block_dur = block_size / sample_rate
+
+        # Number of blocks for a full dah.
+        dah_blocks = int(dah_dur / block_dur)
+        half = dah_blocks // 2
+
+        # Build audio: silence warmup, first half of dah, 1-block silence gap,
+        # second half of dah, trailing silence.
+        warmup = generate_silence(0.4, sample_rate)
+        first_half = generate_tone(tone_freq, half * block_dur, sample_rate)
+        gap = generate_silence(block_dur, sample_rate)
+        second_half = generate_tone(tone_freq, (dah_blocks - half) * block_dur, sample_rate)
+        tail = generate_silence(0.5, sample_rate)
+
+        audio = warmup + first_half + gap + second_half + tail
+
+        events = decoder.process_block(audio)
+        events.extend(decoder.flush())
+        elements = [e['element'] for e in events if e.get('type') == 'morse_element']
+
+        # Should produce a single dah, not two dits.
+        assert elements == ['-'], f'Expected single dah but got {elements}'
+
+
 class TestTimingAndWpmEstimator:
     def test_timing_classifier_distinguishes_dit_and_dah(self):
         decoder = MorseDecoder(sample_rate=8000, tone_freq=700.0, wpm=15)

utils/morse.py

@@ -254,6 +254,10 @@ class MorseDecoder:
         self._current_symbol = ''
         self._pending_buffer: list[int] = []
 
+        # Dropout tolerance: bridge brief signal dropouts mid-element (~40ms).
+        self._dropout_blocks: float = 0.0
+        self._dropout_tolerance: float = 2.0
+
         # Output / diagnostics.
         self._last_level = 0.0
         self._last_noise_ref = 0.0
@@ -581,6 +585,7 @@ class MorseDecoder:
             if tone_detected and not self._tone_on:
                 # Tone edge up.
                 self._tone_on = True
+                self._dropout_blocks = 0.0
                 silence_count = self._silence_blocks
                 self._silence_blocks = 0.0
                 self._tone_blocks = 0.0
@@ -615,11 +620,17 @@ class MorseDecoder:
                         self._record_dit_candidate(silence_count)
 
             elif (not tone_detected) and self._tone_on:
-                # Tone edge down.
+                # Possible tone dropout — tolerate brief gaps before confirming edge-down.
+                self._dropout_blocks += 1.0
+                if self._dropout_blocks <= self._dropout_tolerance:
+                    continue
+
+                # Confirmed tone edge down — dropout was genuine silence, not a glitch.
                 self._tone_on = False
                 tone_count = max(1.0, self._tone_blocks)
+                self._silence_blocks = self._dropout_blocks
                 self._tone_blocks = 0.0
-                self._silence_blocks = 0.0
+                self._dropout_blocks = 0.0
 
                 element = ''
                 if tone_count >= self._dah_threshold:
@@ -641,7 +652,9 @@ class MorseDecoder:
                         self._record_dit_candidate(tone_count / 3.0)
 
             elif tone_detected and self._tone_on:
-                self._tone_blocks += 1.0
+                # Recover any dropout blocks — tone resumed, so they were part of the element.
+                self._tone_blocks += self._dropout_blocks + 1.0
+                self._dropout_blocks = 0.0
 
             elif (not tone_detected) and (not self._tone_on):
                 self._silence_blocks += 1.0
@@ -668,8 +681,8 @@ class MorseDecoder:
                 snr_mult = max(1.15, self.threshold_multiplier * 0.5)
                 snr_on = snr_mult * (1.0 + self._hysteresis)
                 snr_off = snr_mult * (1.0 - self._hysteresis)
-                scope_event['snr'] = round(self._last_level / max(self._noise_floor, 1e-6), 2)
-                scope_event['noise_ref'] = round(self._noise_floor, 4)
+                scope_event['snr'] = round(self._last_level / max(self._last_noise_ref, 1e-6), 2)
+                scope_event['noise_ref'] = round(self._last_noise_ref, 4)
                 scope_event['snr_on'] = round(snr_on, 2)
                 scope_event['snr_off'] = round(snr_off, 2)
             events.append(scope_event)
@@ -680,8 +693,8 @@ class MorseDecoder:
         """Flush pending symbols at end-of-stream."""
         events: list[dict[str, Any]] = []
 
-        if self._tone_on and self._tone_blocks >= self._dit_min:
-            tone_count = self._tone_blocks
+        if self._tone_on and (self._tone_blocks + self._dropout_blocks) >= self._dit_min:
+            tone_count = self._tone_blocks + self._dropout_blocks
             element = '-' if tone_count >= self._dah_threshold else '.'
             self._current_symbol += element
             events.append({
@@ -699,6 +712,7 @@ class MorseDecoder:
         self._tone_on = False
         self._tone_blocks = 0.0
         self._silence_blocks = 0.0
+        self._dropout_blocks = 0.0
         return events