There are three ways to sync separate sound and picture. Timecode sync matches the clock recorded in both files automatically. Clap sync lines up the frame where the clapperboard closes with the spike of the clap in the audio. Waveform sync lets software match the camera's scratch audio to the recorder's audio.
Cameras can record sound, but a dedicated recorder with good microphones and a sound recordist gives far better results. The cost is that picture and sound end up in separate files, and someone has to put them together. How quickly that happens depends almost entirely on what was done on set.
If the camera and sound recorder were jammed to the same timecode, every file carries the same clock. Edit software can then sync a whole day's footage in one step by matching the numbers. It is the fastest and most reliable method, especially for long takes, multi-camera shoots and documentaries where there is no clap. It depends on everything running the same frame rate and drop-frame setting, and on devices being re-jammed regularly so they don't drift.
The clapperboard gives a precise visual and audio event. In the edit, find the first frame where the sticks are fully closed, find the sharp peak of the clap in the audio, and line them up. It takes a little time per clip, but it always works, which is why productions keep slating even when they have timecode. A clean, well-lit, in-focus clap makes this quick; a blurred or half-visible one makes it guesswork.
Most edit systems can compare the camera's own scratch audio with the recorder's audio and line them up automatically. It works well for dialogue and for shoots with no timecode, but it needs the camera to record usable scratch audio, and it can fail in noisy locations, very quiet scenes or when the camera is far from the action.
If sound starts in sync and slowly slides out over a long take, the usual cause is a frame-rate mismatch, such as 23.976 picture against 24 fps sound timecode, or a camera whose internal clock has drifted since it was jammed. Check the project and file frame rates first. For long takes, re-jamming during the day and syncing to the clap at the start of each take keeps errors small.
A timecode slate combines the first two methods: True-Slate can jam to the recorder's LTC and display the running timecode on screen, so every clap also shows the exact time in frame.
Gradual drift usually means a frame-rate mismatch between picture and sound, or a camera clock that has drifted since it was jammed. Check that every device used the same rate.
It is strongly recommended. The clap gives a backup sync point and labels each take, which timecode alone does not do.
Yes. Most edit systems can sync by matching audio waveforms, provided the camera recorded usable scratch audio.
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