Audio sync

What it means

Audio sync means lining up sound and picture so a clap is heard at the exact frame where hands meet. It becomes necessary when audio is recorded on a separate device, such as a wireless lavalier with internal recording, a field recorder or a phone, or when several cameras film the same moment and need to line up for multicam editing.

Even a small offset is noticeable. Viewers spot lips moving out of time with speech when the offset reaches about two frames, and music cuts that drift against the beat feel sloppy. Accurate sync is the foundation of any edit that mixes sources.

How it works in practice

The classic method is a clap or slate at the start of recording: a sharp sound and a visible action on every camera. In the edit, line up the spike in each waveform and the sources are in sync.

Most editors can sync automatically by comparing waveforms. Premiere Pro, Final Cut Pro and DaVinci Resolve all offer audio-based sync, which works as long as each camera recorded some scratch audio.

Some setups use timecode shared between devices, which removes the guesswork entirely on long shoots. Action cameras rarely support it, so for them a loud clap at the start of each recording remains the simplest and most reliable habit.

What to watch out for

Drift is the silent problem. Two devices recording for an hour can slowly separate as their clocks differ. Resync long recordings at several points, or keep takes short.

Clips with variable frame rate, common on phones, can drift against audio inside an editor. Convert them to a constant frame rate before syncing.

Waveform matching fails when one source has only wind noise and the other clean speech. Clap loudly, near both mics, at the start of each take. Cameras that split recordings into chapters also need each part joined before syncing, or offsets appear at each break.