Multi-camera timecode sync
Also known as: timecode sync, camera sync
Getting several cameras to agree about what time it is, so angles of the same moment line up without anybody matching them by eye.
Updated
What it means
Sync is about a shared reference rather than about the footage. Two cameras that agree on the time stamp every frame comparably, and an application can align them without looking at the pictures at all.
Professional rigs use Timecode generators for this. Action cameras do not have the hardware, so the practical equivalent is setting every camera clock from one source before the session starts.
The alternative is aligning on sound. A clap, a shout or an engine note appears in every recording, and Audio sync finds the offset from the waveform rather than from any clock.
How it works in practice
Setting the clocks from one phone takes two minutes. Doing it before a session with three cameras saves considerably longer than that during the edit, and nothing else achieves the same thing afterwards.
A clap at the start of each run is the fallback that always works, and Markers on it make the alignment obvious. One loud sound visible in every angle gives an application a point to align on, and it works where clocks were never set.
GoPro bodies can copy the setting between themselves. Connecting them to one phone application pushes the same time to each, which is the closest thing to a real sync these cameras have.
What to watch out for
Clocks drift apart over days. Two cameras set together in the morning can be a second apart by evening, and a second is thirty frames of misalignment at a normal rate.
Different frame rates complicate the alignment. Two angles at thirty and sixty frames need resampling before they line up properly, which is where Constant frame rate matters more than it seems.
A battery change can reset a clock. A body that lost its setting records a date somewhere in the past, and that one camera lands in entirely the wrong place on a timeline.
How RawClip handles it
Clips covering the same moment are recognised from their content rather than from a clock, so angles that were never synchronised still get grouped. Setting the camera clocks beforehand still helps, because it makes the ordering within a session unambiguous.