wjs-syncing-multicam

Compute a single time offset for each multi-source recording of the same event using audio cross-correlation, and emit a .sync.json sidecar next to each original. Originals are never modified, copied, or re-encoded. Downstream tools use -itsoffset to apply the offset at consume time.

Design principle — sidecar over re-encode

Earlier versions of this skill produced *_synced.MOV files by trimming + re-encoding to bake the offset into the file. We removed that:

• Disk — a 75-min 4K shoot from 3 cameras is 60+ GB. Re-encoded synced copies double that for no information gain.
• Quality — every re-encode is lossy. The originals are the source of truth; sidecars are reversible metadata.
• Speed — _synced.MOV generation took 10+ min per file on Apple Silicon; sidecar emission takes seconds.
• Composability — any downstream tool (autoedit.py, NLE import, ffmpeg one-liners) reads the sidecar and applies the offset itself. No tool-specific file format lock-in.

When NOT to use

• Single-camera footage — nothing to sync to. For splitting one source into clips, use video-segmentation.
• Sources already aligned in an NLE timeline — don't fight the editor.
• For the auto-edit / cut / PiP rendering step that comes AFTER sync, use wjs-editing-multicam (consumes these sidecars).

Why envelope-based, not raw waveform

Raw PCM cross-correlation gives weak peaks and false matches when the two mics have different gain / room response — i.e., almost always with a secondary cam. The log-energy envelope captures dialogue and music dynamics, which both mics hear regardless of frequency response. Don't skip the envelope step — it's the entire reason this skill is robust at low SNR.

Algorithm

1. Extract mono PCM at 8 kHz, 16-bit from each input.
2. Log-energy envelope at 100 Hz (10 ms hop, 50 ms window). High-pass with a 2nd-order Butterworth, 0.05 Hz cutoff, filtfilt — removes slow drift and gain offsets.
3. FFT cross-correlate envelopes end-to-end → coarse offset (~10 ms).
4. Refine at sample level with a 60 s probe from B near the coarse-aligned position in A, ±2 s search window, parabolic peak interpolation.
5. Multi-probe drift check — repeat step 4 every ~3 min. Linear fit delta(t) = slope·t + intercept reveals real clock drift (5–50 ppm typical). Use the midpoint-canonical offset (slope · midpoint + intercept) so residual error is symmetric around zero.
6. Compute overlap window in the reference timeline: overlap = [max(0, delta), min(ref_dur, delta + src_dur)].
7. Emit .sync.json sidecar next to each non-reference input. No file is copied, trimmed, or re-encoded. The reference input gets a sidecar too (with delta_seconds: 0) so downstream code can treat all inputs uniformly.

scripts/sync.py is the implementation. Note: the current script still emits _synced.MOV files alongside the sidecar — that path is deprecated; the sidecar is the only authoritative output.

Sidecar schema (\x3Cinput>.sync.json)

One sidecar per original input, written next to it. Pure JSON, no comments in-file — the field reference below is canonical.

{
  "_about": "Sync metadata for cam_b.MOV. Apply via ffmpeg -itsoffset. See wjs-syncing-multicam SKILL.md for full schema.",
  "schema_version": 1,
  "source": "cam_b.MOV",
  "reference": "cam_a.MOV",
  "delta_seconds": 12.345,
  "drift_slope": 1.8e-5,
  "overlap_in_reference": [12.345, 4512.180],
  "overlap_in_source":    [0.000,   4499.835],
  "verification": {
    "median_residual_ms": 4.2,
    "residual_spread_ms": 11.8,
    "probe_count": 24
  }
}

Field reference

How downstream consumes the sidecar

-itsoffset is per-input in ffmpeg and applies BEFORE -i. Always read the source's delta_seconds from the sidecar:

# Play cam_b aligned to cam_a's timeline
ffmpeg -itsoffset $(jq -r .delta_seconds cam_b.MOV.sync.json) -i cam_b.MOV \
       -i cam_a.MOV \
       -filter_complex "[0:v][1:v]hstack" out.mp4

# Trim to mutual overlap window (read from cam_b.MOV.sync.json)
ffmpeg -ss \x3Coverlap_in_source[0]> -i cam_b.MOV -t \x3Coverlap_dur> ...

For wjs-editing-multicam, the EDL builder in autoedit.py ingests every \x3Cinput>.sync.json automatically; you don't compose these flags by hand.

Partial-coverage clips

Common case — main cams cover 75 min, a Riverside / phone / lavalier recorder only covers the middle 30 min. scripts/sync_partial.py REF.MOV NEW.mp4:

1. Cross-correlates the new input against the reference.
2. Finds where the new clip's t=0 sits in the reference timeline (delta_seconds may be large, e.g. 1842.5).
3. Writes the sidecar — that's it. No black padding, no audio padding, no re-encode. overlap_in_reference tells consumers exactly when this input has coverage; outside that window, fall back to the main cams.

--audio-only flag is meaningful only for hinting downstream that this source has no video stream — there's no encoding step to skip anymore.

When to skip drift correction

For camera-cut editing (the common case), ±25 ms residual across an hour is below human perception — pass drift_slope: 0.0 and use only the midpoint delta_seconds.

For sync-sound / lip-sync at long durations (>30 min and verification.residual_spread_ms > 40), downstream applies atempo = 1 + drift_slope to the source. Source files are still not modified — the atempo filter runs at consume time.

Verification (always run)

scripts/verify.py REF.MOV SRC.MOV SRC.sync.json re-extracts audio from BOTH originals (with -itsoffset applied to the source per the sidecar) and runs multi-probe correlation again. Writes results back into the sidecar's verification field.

Pass criteria — median_residual_ms \x3C 15 and residual_spread_ms \x3C 1 frame at delivery fps. Fail = retry with drift correction enabled.

Common pitfalls

• Raw waveform cross-correlation gives false peaks under low SNR. Always envelope first — this is not a tunable, it's the entire premise.
• -itsoffset semantics differ for audio vs video — for sync-correctness it must be the FIRST flag for that input. ffmpeg -i src -itsoffset X is wrong; ffmpeg -itsoffset X -i src is right.
• Sidecar paths must be relative to the sidecar file's directory, not the working directory of the consuming process. Resolve source / reference against Path(sidecar).parent.
• Don't bake drift_slope into the sidecar's delta_seconds. They're separate fields for a reason — naive consumers can ignore drift, sync-sound consumers can apply it. Mixing them loses information.