Audio Sync Assembly

Audio Sync Assembly

Use this skill when video timing must follow narration or when multiple rendered scenes need to become one final timeline.

Source Of Truth

• Use the latest approved narration audio as timing source.
• Use scene renders as visual source.
• Use templates/timeline-manifest.example.json to record target timing.
• Do not infer timings from old final MP4s if source renders and audio clips exist.

Probe First

For every clip:

• Duration.
• Frame rate.
• Resolution.
• Audio stream presence.
• Sample rate and channels.

Use ffprobe or this repo's media probe script once available.

Sync Strategy

Choose the smallest timing intervention:

• Simple scene: retime whole scene to narration length with setpts.
• Scene with cue points: split into segments and retime each segment.
• Narration pause: insert silence and hold a frame.
• Replacement audio slightly longer: use small audio-only atempo only if preserving downstream sync matters more than raw duration.
• Product moment late/early: retime the segment before the product moment.

Read references/ffmpeg-retiming.md for command patterns.

Phrase Alignment

Map spoken phrase starts to global timestamps when visuals depend on narration:

• Question cards.
• Product states.
• Chart points.
• Scene title reveals.
• CTA/logo arrival.

Caption timing and visual timing should share the same narration timeline.

Assembly Order

1. Build synced scene segments.
2. Concatenate synced segments into a narration-only master.
3. Generate captions from narration-only master.
4. Burn captions.
5. Build music bed.
6. Mix narration and ducked music.
7. Produce final MP4 and preview clips.

QA

• Probe final duration, frame rate, resolution, and audio.
• Watch boundary previews.
• Check exact cue timestamps with proof frames.
• Listen for clipped words after trims.
• Confirm no accidental blank final frame appears.

Output

Return:

• Timeline manifest or script updates.
• Final path.
• Scene start map.
• Preview clips.
• Proof frames and probe results.

Timeline Manifest Fields

Track:

• Scene ID.
• Source render path.
• Source duration.
• Target duration.
• Audio path.
• Audio trim start/end.
• Inserted silence.
• Hold frames.
• Segment retime ratios.
• Global scene start.
• Proof timestamps.

This prevents later caption and music work from guessing.

Audio Replacement Rules

When narration changes:

• Probe old and new durations.
• Decide whether visuals move or audio fits the existing slot.
• If preserving downstream sync, use small audio-only tempo correction when acceptable.
• If phrase timing changes materially, rebuild the segment map.
• Regenerate captions from the new narration master.

Segment Mapping Heuristics

Split on:

• Scene transitions.
• Product moments.
• Question reveals.
• Chart points.
• Title reveal.
• Final hold.
• Narration pauses.

Avoid splitting on arbitrary equal chunks.

Pause Handling

For intentional silence:

• Insert silent audio.
• Hold or subtly animate visuals.
• Do not let music rise unless requested.
• Preserve global downstream scene starts by accounting for pause duration.

Preview Clip Strategy

Generate preview clips for:

• Updated narration insertion.
• Product demo transition.
• Question timing.
• Caption style section.
• Final 10 seconds.
• Any scene boundary changed by the edit.

Common Mistakes

• Retiming the full video when only one scene changed.
• Letting replacement audio shift all downstream scenes.
• Forgetting caption timings after narration changes.
• Copying stream segments with non-keyframe starts and causing preview oddities.
• Ending music before final video.

Handoff Format

Report:

• Final scene start map.
• Audio files used.
• Any tempo changes.
• Any inserted pauses.
• Preview paths.
• Final duration.