What is crystal sync?
Internet Filmmakers' FAQ
Historically, when shooting on film sound is recorded separately on tape stock (magnetic or digital). Because of power fluctuations, tape tautness, and many other factors, the speed of either the camera or the sound recorder may vary slightly during shooting/recording. The result of this is that one part of the process may be running at a slightly different speed to the other, creating problems in synchronisation between sound and image.
Pilot tone was the original method for keeping cameras and sound recorders in sync, however it comes with one major limitation – it requires a physical connection between the two devices.
The problem can be alleviated by using a motor that is regulated by highly accurate oscillations of certain quartz crystals. Since both devices use motors with the same crystals, they can be forced to stay in sync without the need for the physical connection (since the crystals oscillate nearly identically in both devices creating a "base-line" for the motor to maintain its speed from).