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Fix Morse decoder silent on real HF signals via AGC and warm-up

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Add automatic gain control (AGC) before Goertzel processing to normalize
quiet audio from direct sampling mode where the -g gain flag has no effect.
Fix broken adaptive threshold bootstrap by adding a 50-block warm-up phase
that collects magnitude statistics before seeding noise floor and signal peak.
Lower threshold ratio from 50% to 30% for better weak-CW sensitivity.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
Smittix
2026-02-26 09:10:37 +00:00
parent df84c42b8b
commit 2eea28da05
2 changed files with 91 additions and 22 deletions
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utils/morse.py
+44 -7
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@@ -95,11 +95,21 @@ class MorseDecoder:
self._char_gap = 3.0 * dit_sec / self._block_duration # blocks
self._word_gap = 7.0 * dit_sec / self._block_duration # blocks
# AGC (automatic gain control) for direct sampling / weak signals
self._agc_target = 0.3 # target RMS amplitude (0-1 range)
self._agc_gain = 1.0 # current AGC multiplier
self._agc_alpha = 0.05 # EMA smoothing for gain changes
# Warm-up phase constants
self._WARMUP_BLOCKS = 50 # ~1 second at 50 blocks/sec
self._SETTLE_BLOCKS = 200 # blocks for fast→slow EMA transition
self._mag_min = float('inf')
self._mag_max = 0.0
# Adaptive threshold via EMA
self._noise_floor = 0.0
self._signal_peak = 0.0
self._threshold = 0.0
self._ema_alpha = 0.1 # smoothing factor
# State machine (counts in blocks, not wall-clock time)
self._tone_on = False
@@ -136,20 +146,47 @@ class MorseDecoder:
# Normalize to [-1, 1]
normalized = [s / 32768.0 for s in block]
# AGC: boost quiet signals (e.g. direct sampling mode)
rms = math.sqrt(sum(s * s for s in normalized) / len(normalized))
if rms > 1e-6:
desired_gain = self._agc_target / rms
self._agc_gain += self._agc_alpha * (desired_gain - self._agc_gain)
self._agc_gain = min(self._agc_gain, 500.0) # cap to prevent runaway
normalized = [s * self._agc_gain for s in normalized]
mag = self._filter.magnitude(normalized)
amplitudes.append(mag)
self._blocks_processed += 1
# Update adaptive threshold
if mag < self._threshold or self._threshold == 0:
self._noise_floor += self._ema_alpha * (mag - self._noise_floor)
# Warm-up phase: collect statistics, suppress detection
if self._blocks_processed <= self._WARMUP_BLOCKS:
self._mag_min = min(self._mag_min, mag)
self._mag_max = max(self._mag_max, mag)
if self._blocks_processed == self._WARMUP_BLOCKS:
# Seed thresholds from observed range
self._noise_floor = self._mag_min
self._signal_peak = max(self._mag_max, self._mag_min * 2)
self._threshold = self._noise_floor + 0.3 * (
self._signal_peak - self._noise_floor
)
tone_detected = False
else:
self._signal_peak += self._ema_alpha * (mag - self._signal_peak)
# Adaptive EMA: fast initially, slow in steady state
alpha = 0.3 if self._blocks_processed < self._WARMUP_BLOCKS + self._SETTLE_BLOCKS else 0.05
self._threshold = (self._noise_floor + self._signal_peak) / 2.0
if mag < self._threshold:
self._noise_floor += alpha * (mag - self._noise_floor)
else:
self._signal_peak += alpha * (mag - self._signal_peak)
tone_detected = mag > self._threshold and self._threshold > 0
# Threshold at 30% between noise and signal (sensitive to weak CW)
self._threshold = self._noise_floor + 0.3 * (
self._signal_peak - self._noise_floor
)
tone_detected = mag > self._threshold and self._threshold > 0
if tone_detected and not self._tone_on:
# Tone just started - check silence duration for gaps