Synchronized playback of separate picture and sound rolls — film and mag locked together frame-accurate. Pre-digital post-production standard.
In the classic editing workflow of the analog era, interlock operation was the central method for keeping picture and sound material synchronized. You had two separate machines—a 35mm film projector and a magnetic tape recorder—connected by an electronic control loop. A sync signal (pilot tone) ran on the film strip or was generated by a third control unit, forcing both machines to run at precisely the same speed. This was crucial: a drift of just a few frames led to lip-sync problems that were later prohibitively expensive to fix.
The practical application in editing worked like this: the editor would spool up the rough cut and the sound tape on the interlock machines, start synchronization, and watch live as picture and sound ran together. If a cut needed to be made, they would first mark both media with ink or pencil—exactly on the same frame point. Then, both were physically cut. This required craftsmanship and patience. For longer sequences—especially in music or dialogue editing—interlock was indispensable because it was the only way to ensure that the original sound stayed with the picture.
A common problem: if the drive belt of the magnetic tape recorder was worn or the electronic synchronization wasn't perfect, the lock would gradually be lost. At the end of a 20-minute reel, several frames of offset could occur—leading to trouble in the next editing iteration. Therefore, regular maintenance of these machines was not optional. Good editors knew their interlock equipment like a craftsman knows their tools.
With digitalization, interlock operation disappeared as a physical practice, but the principle lives on in modern NLEs—where it's called video lock or synchronized playback. However, those still editing archival material from 16mm or 35mm will encounter interlock machines. The few studios still working analog keep these machines alive, not out of nostalgia, but because they work and preserve the characteristic image quality.