Drift and Offset: Two Sync Terms Explained in Plain Words

Why two words for one complaint

When a producer says the voice looks wrong against the picture, they usually mean one of two different faults, and the trade has a word for each. Offset is a fixed gap: the audio sits a set distance early or late and stays there from the first frame to the last. Drift is a gap that changes: the two start together and slowly part company, so by the end of a long explainer the mouth and the words no longer agree.

The distinction matters because the cure is different. Treating drift as offset leaves you with a clip that is right at the start and wrong at the end, and you will spend an evening nudging it back and forth. I teach learners to name the fault before they touch anything, and these two words are the quickest way to do it. This note gives you both in plain terms, with the checks I use on a timeline.

Offset: a fixed gap you can measure

Offset is the easier of the two. Find a clear sound that has a visible partner, such as a clap, a hard consonant on a closed mouth, or a door shutting, and note how far apart the sound and the picture are. If the gap is the same near the end of the programme as near the start, you have offset, and one correction moves the whole clip.

Typical causes are an export that added a little silence at the head, a recording started on a separate device a moment after the camera, or a processing step that delayed the audio. Measure the gap in frames rather than guessing by ear, because the ear forgives more than the eye. Slide the audio by that amount, play it back, and check a second spot further in.

• Pick one clear sound with a visible partner
• Measure the gap in frames, not by feel
• Check a second point later in the clip

Drift: a gap that grows

Drift shows itself when you repeat the offset check at two points and get two different answers. Perhaps the audio is a frame late a minute in and five frames late near the end. The cause is nearly always a mismatch in timing between the devices, such as a sample rate that differs from the project, or a frame rate converted along the way, so the two clocks tick at slightly different speeds.

You cannot fix drift by sliding. The clip has to be brought back to the correct length, either by conforming it to the right sample rate or by making a very small speed adjustment across its whole duration. The change is tiny, a fraction of a percent, and done properly it does not alter the pitch of the voice in any way a listener would notice.

A simple way to tell them apart

Here is the routine I give new learners. Park the playhead early, find a sync point, and note the gap. Move to the middle and do it again, then near the end. Write the three numbers down. If they match, it is offset. If they climb steadily, it is drift. If they wander without pattern, the problem is something else, often an edit that removed a few frames from picture but not from audio.

That last case is worth remembering, because it looks like drift and is not. A cut made on one track and not the other leaves a step in the gap at the point of the cut, rather than a smooth slope. Looking at where the gap changes tells you which fault you have, and saves you from correcting the wrong thing.

• Same number three times: offset
• Number rising steadily: drift
• Sudden step at a cut: a one-sided edit

Sample rate and frame rate, in short

Two settings sit behind most drift, so it is worth knowing what they mean. Sample rate is how many times per second the sound is measured when it is recorded. Frame rate is how many pictures per second the video shows. A project has one of each, and every file brought into it should agree, or be converted on purpose.

When a recording made at one sample rate is placed in a project set to another and is not converted, it plays at the wrong speed and the gap widens over time. A video exported at a slightly different frame rate does the same to picture. Check both settings in the file details before you begin, and you remove the commonest cause of drift before it starts.

What to do on a real explainer

On a typical explainer with a recorded voiceover laid over screen footage, I begin by confirming the project settings against the files, then check sync at the head and the tail before any editing. If both ends agree, I trust the middle and carry on. If they differ, I name the fault, correct it once, and recheck at three points.

Only then do I do the finer work of tightening phrases against the picture. Leaving the large correction until after the small edits means you chase your own tracks around the timeline. Order is most of the craft here: diagnose, fix the whole clip, verify, then refine. It is calm, repeatable work, and it is covered step by step in the sync module of the curriculum.