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fix: Expand Scottie sync deviation search window to fix under-correction

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The slant correction was severely under-correcting because bwd=50 caused
the sync deviation measurements to saturate after only ~25 lines (for a
2-sample/line SDR clock drift). Lines 25-256 all reported deviation=-50,
pulling the linear regression slope toward zero.

Increase bwd and fwd to 800 samples each — sufficient to track cumulative
drift from up to ~±200 ppm SDR clock offset across the full 256-line image.

Also use a full-sync-length (432-sample) Goertzel window instead of 1/3
length, giving ~111 Hz frequency resolution to cleanly separate the 1200 Hz
sync tone from 1500 Hz pixel data. Search is stepped at 5 samples (~0.1 ms)
for efficiency, keeping the goertzel_batch batch size at ~320 windows/line.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
This commit is contained in:
Smittix
2026-02-19 11:04:06 +00:00
parent a397271553
commit f7fad076c2
1 changed files with 26 additions and 13 deletions
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39
utils/sstv/image_decoder.py
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@@ -246,21 +246,33 @@ class SSTVImageDecoder:
# slant correction without touching pos or consumed —
# so a noisy/false measurement never corrupts the decode.
r_samples = self._channel_samples[-1]
bwd = min(50, pos)
fwd = max(0, len(self._buffer) - pos
- self._sync_samples - self._porch_samples
- r_samples)
fwd = min(fwd, self._sync_samples)
# Large window to cover cumulative SDR clock drift over
# the full image. bwd=50 saturates after ~25 lines for
# a 2-sample/line drift; 800 samples covers ±200 ppm.
bwd = min(800, pos)
remaining = len(self._buffer) - pos
fwd = min(800, max(
0,
remaining - self._sync_samples
- self._porch_samples - r_samples))
deviation: float | None = None
if bwd + fwd > 0:
region_start = pos - bwd
sync_region = self._buffer[
region_start: pos + self._sync_samples + fwd]
win = max(20, self._sync_samples // 3)
n_win = len(sync_region) - win + 1
if n_win > 0:
windows = np.lib.stride_tricks.sliding_window_view(
sync_region, win)
# Full-sync-length window gives best freq resolution
# (~111 Hz at 48 kHz) to cleanly separate 1200 Hz
# sync from 1500 Hz pixel data.
win = self._sync_samples
n_raw = len(sync_region) - win + 1
if n_raw > 0:
# Step 5 samples (~0.1 ms) — enough resolution
# for pixel-level drift, keeps batch size small.
step = 5
all_windows = (
np.lib.stride_tricks.sliding_window_view(
sync_region, win))
windows = all_windows[::step]
energies = goertzel_batch(
windows,
np.array([FREQ_SYNC, FREQ_BLACK]),
@@ -269,9 +281,10 @@ class SSTVImageDecoder:
black_e = energies[:, 1]
valid_mask = sync_e > black_e * 2
if valid_mask.any():
fine_best = int(
np.argmax(np.where(valid_mask, sync_e, 0.0)))
deviation = float(fine_best - bwd)
fine_idx = int(
np.argmax(
np.where(valid_mask, sync_e, 0.0)))
deviation = float(fine_idx * step - bwd)
self._sync_deviations.append(deviation)
pos += self._sync_samples + self._porch_samples
elif self._separator_samples > 0: