How to Sync Separately Recorded Audio Without a Clapper
Recorded sound on a Zoom, Tascam, lav or phone? How waveform sync works, how accurate it is, and how to line up camera and audio in minutes without a slate.
Recording sound separately (a Zoom or Tascam recorder, a lavalier, a phone in a pocket) gives you far better audio than a camera microphone. The price is syncing it afterwards. A clapperboard makes that easy, but plenty of shoots don't have one: run-and-gun documentaries, interviews, events, a second camera somebody forgot to slate.
How waveform sync works
The camera's own microphone still records the scene, just badly. That scratch track and the good recording both contain the same sounds: speech, footsteps, a door closing. Software slides one recording against the other and looks for the position where the sounds line up best. That position is the offset.
Matching the rises in loudness (onsets) works much better than matching raw waveforms, because two different microphones colour the sound differently and may even record it with inverted polarity. That's how the Cuescribe audio sync tool does it; it then refines the match on the full-resolution audio.
How accurate is it?
On overlapping speech it lands within about a millisecond. One video frame is 33 to 42 ms, so that's far finer than anyone can see. In our own test, a recorder track that started 1.250 s after the camera was found at 1249.9 ms, through a duller, quieter, echoey copy of the sound.
Step by step
- Drop the camera file (the one with the picture) and the separate audio file.
- Press Find the offset. You get the offset in milliseconds and in frames for common frame rates.
- Either place the audio at that offset in your editor, or download the aligned WAV and drop it at 0:00 under the video.
- Mute the camera's scratch track.
When it can struggle
- The camera microphone recorded almost nothing (muted, or far away in wind).
- The recordings don't overlap at all.
- Very long takes from devices with cheap clocks, which drift apart by a frame or two per hour. Check the end of long takes by ear.
The tool tells you how confident the match is. On a weak match, listen at a hard sound (a clap, a "p" or "t") for any echo before you trust it.