Why does my audio drift out of sync over ten minutes?

The recorder and camera are running at slightly different rates, either a frame rate mismatch or independent sample clocks.

Does timecode prevent drift?

It reveals it and helps you correct it, but two devices with independent clocks can still drift between jams.

How do I check for drift quickly?

Compare the exact duration of the camera audio and the recorder file for the same long take using the audio inspector.

6 min read

Audio drift explained: why sound slips over a long take and how to stop it

Perfect sync at the clap and half a frame out ten minutes later. That is drift, and it has causes you can eliminate.

Drift is not a sync error, it is a rate error. The clap lines up and the take slowly falls apart because the recorder and the camera are not counting time at exactly the same speed.

Where drift comes from

Independent sample clocks: two devices running at nominally 48 kHz that differ by a few parts per million.
Frame rate mismatch: 23.976 against 24, or 29.97 against 30, which is a 0.1 percent error.
Variable frame rate recording from phones and some mirrorless cameras.
Sample rate conversion done badly somewhere in the chain.

How much matters

A one frame slip becomes visible on dialogue at around 40 ms. A 0.1 percent frame rate error reaches that in roughly 40 seconds. A clock drift of 10 parts per million takes over an hour. That difference tells you which cause you are dealing with: if drift appears fast, suspect frame rates, not clocks.

Preventing it on set

Record timecode from a shared source, or jam sync devices at the start of every block.
Set camera and recorder to the same sample rate and the same frame rate family before day one.
Avoid recording long interviews on a phone as the master audio.
Slate every take even with timecode, because a visual reference costs nothing and saves everything.

Detecting drift before you edit

Take the longest shot of the day. Sync the head, then check the tail. If the tail needs a nudge, you have drift and you need to resolve it once, globally, rather than clip by clip.

Extract the camera scratch audio from the video file and inspect both files: sample rate, duration and channel count. Two files of the same take with durations differing by a fraction of a percent tell you the whole story before you open the timeline.

Fixing it

Once identified, drift is fixed by resampling the audio by the exact ratio, or by applying a pull up or pull down in your NLE. Do it at the top of the pipeline, on the master files, not with time stretch on individual clips.