Audio drift
Also known as: sync drift, audio desync
Sound and picture gradually falling out of step across a long clip, so a recording that starts perfectly in sync ends up visibly late.
Updated
What it means
Audio drift is a slow accumulating error rather than a fixed offset. The start of a clip is correct, the end is not, and the gap grows steadily in between, which is what distinguishes it from a simple sync problem.
The cause is almost always a mismatch in clocks rather than anything in the Timecode. A camera recording at a nominal frame rate that is slightly off, or a separate recorder running on its own clock, will diverge from the other by a predictable amount per minute.
It is most visible on Dialogue, where a lip and a syllable disagree. On an action clip with no dialogue it can pass unnoticed until something lands at a different moment than it sounds.
How it works in practice
A clap or a slate at the start of a long take is still the most reliable remedy. It gives a point to align, and comparing that point with another near the end reveals the rate of drift.
Correcting it means stretching rather than shifting. Moving the whole track fixes the start and leaves the end wrong, so the audio has to be resampled to the length the picture actually occupies.
Variable frame rate recordings make this worse and need converting first, because Variable frame rate (VFR) footage has no single rate to resample against in the first place.
What to watch out for
Checking sync only at the start is how drift ships. A clip that looks correct in its first seconds can be a frame or two out by the end of a long take, and the error is cumulative rather than random.
Phone recordings drift unpredictably because they adjust their own rate while recording, so the offset is not even linear and a single stretch factor does not fix it.
Trimming before fixing sync also destroys the evidence. Once the reference point is cut away there is nothing left to align to, which is why Audio sync work comes before any cutting.
How RawClip handles it
Clips are checked against their own metadata before anything is cut, and sync is handled per clip rather than per order. Where a long recording has drifted, the audio is resampled to the length of the picture rather than merely nudged.