The Tool Desk
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Find where the sync problem begins
Check each source clip and the assembled output at three points: near the beginning, around every join, and near the end. Compare what you hear with the corresponding visible action, and note whether the offset stays roughly constant or changes.
- The same offset is inside a source clip: Correct that clip’s timing before rebuilding the playlist. A join method cannot repair a mismatch that is already present within a segment.
- The problem starts at a join: Inspect that segment’s duration information and whether its streams match the other entries. The concat demuxer places each file after the previous one using durations, so inaccurate duration information can create artifacts.
- The offset grows over time: Investigate duration, frame-rate, and timestamp behavior across the affected streams. A single fixed timestamp shift will not correct drift that changes over time.
This diagnosis follows FFmpeg’s documented concat and timestamp behavior; without the files and their metadata, no one command can be assumed to fit every playlist.
Choose the right FFmpeg concat method
| Method | Use it when | Important conditions |
|---|---|---|
| Concat demuxer | You want to avoid re-encoding and stream-copying is suitable. | Inputs must have the same streams, codecs, and time bases. Duration inaccuracies can cause artifacts; override a duration only when you know its correct value. |
| Concat filter | Inputs need re-encoding or stream normalization. | Prepare the streams consistently for the filter workflow, including handling segments with missing audio. |
FFmpeg’s FAQ distinguishes the approaches: the concat filter is for concatenation with re-encoding, while the demuxer is useful when stream-copying is appropriate. The demuxer documentation states that files must have the same streams, including the same codecs and time base. See the FFmpeg FAQ and concat demuxer documentation.
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If stream-copying appears suitable
Use the concat demuxer only after confirming that the files’ stream layouts and relevant codec and time-base properties match. Check that each listed segment has a reliable duration. If a stored or estimated duration is wrong, the demuxer can place the next segment incorrectly; its duration directive can override the duration when the correct value is known. Do not guess a replacement duration.
If segments need normalization
Use the concat filter workflow when stream-copy compatibility is absent or re-encoding is required. Make the relevant streams consistent before concatenation. In particular, a playlist entry without audio does not match entries that have audio; handle that difference as part of the filter workflow rather than assuming the demuxer will repair it. Adding silence is one possible normalization approach, but the appropriate treatment depends on the clips.
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Use timestamp options only for a fixed offset
-itsoffset adds an offset to an input’s timestamps. It can address a fixed start-time difference when the evidence points to that cause; it does not fix unequal track duration, changing drift, or incorrect timing within a clip.
-isync aligns input start-time offsets using the difference between their start times. FFmpeg expects the inputs to come from the same clock source for this to produce the expected result. These options solve specific timestamp relationships, not general synchronization problems. Consult the FFmpeg documentation for input options before using them.
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Validate the finished file before uploading
- Compare track durations. YouTube’s upload troubleshooting advises: “Make sure the durations of your audio and video tracks are the same.” Equal duration is an important check, though it does not by itself prove that every point is synchronized. See YouTube’s upload audio and video troubleshooting.
- Inspect multiple positions. Check the start, each join, and the end. A clip can begin in sync and drift later, or a playlist can be correct except for one discontinuity at a join.
- Keep the recorded frame rate. YouTube recommends maintaining the frame rate of the original recording. Its general upload encoding guidance lists MP4, H.264 video, AAC-LC or Opus audio, and a 48 kHz audio sample rate. These are delivery recommendations, not a guarantee that concatenation is synchronized. See YouTube’s recommended upload encoding settings.
Common causes and fixes
- Audio is missing from one playlist entry: The stream layouts do not match for a demuxer workflow. Normalize the inputs in a filter workflow, handling missing audio consistently.
- Sync changes at one segment boundary: Check that segment’s duration and stream properties. Correct duration information only when its accurate value is known.
- Sync is wrong throughout one source clip: Repair the clip’s own timing before concatenating it with the rest.
- Sync gradually worsens: Do not apply a blanket offset. Examine frame rate, duration, and timestamp behavior across the streams to identify the source of drift.
- A fixed offset remains from the start: Consider whether the input timestamps show a fixed start-time difference. Use
-itsoffsetor, where the inputs share a clock source,-isynconly if that relationship fits the problem. - The export is still out of sync: Recheck durations and inspect at the start, joins, and end before uploading; matching total duration alone is not a full sync test.
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