Why Audio Drifts Out of Sync on Long Recordings, and How to Fix It
The start lines up and the end doesn't. Separate recorders never run at quite the same speed. How to tell drift from a bad offset, how much to expect and how to correct it.
You lined up the recorder with the camera, the first minute looks perfect, and by the end of the interview the lips are visibly off. That isn't a mistake in your sync. It's clock drift, and a single offset can't fix it.
Why it happens
Every recorder keeps time with its own clock, and no two run at exactly the same speed. One device's "48,000 samples a second" is a little faster or slower than the other's. A difference of 20 to 100 parts per million is common, which adds up to 70 to 360 ms over an hour. Viewers tend to notice from about 45 ms when the sound runs ahead of the picture, and somewhat later when it runs behind.
Drift or a bad offset?
Check the sync in three places: near the start, the middle and the end.
- The same error everywhere: that's a plain offset. Slide the audio.
- Fine at the start and worse as it goes: that's drift.
- Fine, then a sudden jump: that is not drift. Something dropped samples, or the camera split the recording into files. Sync each piece on its own.
Fix it in your editor
You can fix drift by hand. Line up the start, find how far off the end is, and change the speed of the audio by that fraction. If the audio is 200 ms late after 60 minutes, that is 200 ÷ 3,600,000, or about 0.0056%. Premiere's and Resolve's speed controls may not take a change that fine, so another way is to cut the audio into pieces at pauses and slip each one back into place.
Fix it automatically
The audio sync tool compares the sound of the two recordings. Drop both files in and it finds the offset by matching the waveforms, with no clapper needed. With Correct clock drift on, it measures the offset near the start and near the end of each clip of at least two minutes, and uses the difference to stretch the audio so the whole length stays in time. Drift correction is part of Cuescribe Pro; finding the offset is free. Your files stay on your device.
Several clips and one recorder
When the camera stops and starts but the recorder runs the whole time, each clip has its own offset, and later clips sit further along the recorder's drift. This guide covers that setup, and this one covers syncing without a clapper.
Avoiding it next time
- Split long sessions: restart both devices at a natural break, so the drift starts over.
- Record a scratch track on the camera. Without one, there is nothing to match the waveforms against.
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