Prep

Syncing Sound

3 minute read

There are three ways to ensure that the audio and video are matched properly: slating, timecode, and scratch audio.

It’s very common to use two of those methods (or even all three) simultaneously to ensure that, if one of them fails, the sound doesn’t have to be synced by eye, which is a huge headache.

Slating

Slating is the most well-known system, because it goes back to the very beginnings of filmmaking. The iconic film slate is a chalkboard that has the current scene, shot, and take number written on it.

When the actors are ready, someone (often the 1st Assistant Director) will say “roll cameras” and “roll sound.” Once everyone is ready, an assistant will read out the information written on the slate, and then clap down the clapper to make a loud noise.

Later on, when you’re combining the separate audio and video files, you can easily pair the right two files together by matching the scene and take information in the video and audio. The clap of the clapper provides a clear and precise point at which to sync the two files.

As I’m sure you can imagine, syncing sound via a slate is still a somewhat tedious process, but it’s fairly straightforward and reliable.

Timecode

Basically, timecode is a clock that ticks in frames, rather than just seconds. It keeps track of when video and audio is recorded, and then stores that information as metadata within the file. When it comes time to edit the files, your NLE can match up all your audio and video based on this information.

At the beginning of a shoot, timecode is synchronized between devices. By doing this, all the footage and sound run off the same clock, which means that any particular event will have the same timecode across every device.

This system is not perfect, however. Different devices will have slightly different clocks, so they will drift apart over time. That means they need to be resynchronized to each other periodically to keep everything aligned. You can also feed an external timecode source to every device on set, so that no drift occurs.

As long as the timecode sources are synced across all your recording devices, then matching audio and video should be a relatively automated process. It does take a little more work on set than just using clappers, but it saves a lot of time in post.

Contributed by Hilda Saffari, Editor and Post-Production Consultant.

Scratch audio

We mentioned above that almost all digital cameras can record audio, although the quality might not be the best possible. A simple and effective way to sync sound and video is to record the sound both on the camera and on a separate sound recorder.

The waveform of the audio file may be slightly different between the two recordings, but they will be similar enough that software can usually match them up automatically.

If the camera’s internal mics aren’t close enough to the actors to hear them properly, the sound recordist may send a wireless signal out of his equipment into the camera in order to ensure that the two audio signals are similar enough.

Using scratch audio is one of the simplest and easiest ways to sync audio, because it takes almost no extra work on set, and it doesn’t rely on any extra hardware.

It can still be a good idea to have another method as a backup, however. You may forget to turn the camera’s sound on, for instance. Or your sound recordist may capture a ton of ambient noise that makes the recording unusable.

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