Achieving a good synchronization of audio and video is one of the most challenging aspect of movie repairs.
The problems can be classified in 4 grand categories:
- Audio and Video are stored in separate blocks
- Audio blocks are difficult to identify. There is a small percentage of errors.
- Video frames do not all have the same duration
- Contiguous Audio and Video blocks can correspond to very different times. The difference varies randomly.
In the best case, for example a DV stream, none of those problems is present, since audio and video are synchronized right into the stream: each DV frame contains both audio and video.
When repairing through the Movie Repair Service, we try to achieve audio/video sync through automated methods, as described below. We can find many combinations of problems, from easy to very difficult:
- A - Most professional formats fall into this category: ProRes422, XDCAM, DVCPro HD, ...
- If the video speed is correctly set, sync is achieved by adjusting the A/V offset.
- If we have clip cuts, like when repairing after a storage failure, we can use a strict GOP policy to avoid drifting.
- A B - This happens with AAC, ima4, or PCM audio blocks in the middle of padding.
- A C - Since video frame do not indicate anything about time, we must use audio as a time base.
- We rely on audio, whose duration is easily measured, to predict video speed. This is based on the assumption that contiguous audio and video blocks correspond to the same time.
- Cycorder iPhone repairs, among many others, use this technique.
- A D - We repair audio and video separately. Theoretically, times should perfectly match...
- A C D - Impossible to sync automatically. Need to be done manually.
- A B C - We use a complex mix of A B and A C techniques.
- MPEG4 / AAC combo falls in this category.
- Automated method gives a rough sync, usually it needs to be improved manually.
Sometimes, some manual correction must be applied: See how to correct audio sync.