Chase-Interlock
Digital recorders are precisely clocked, and therefore in most cases, recordings can be made simultaneously on different devices, which can then be imported onto a timeline, referenced to a common point, for example a slate or a 2-beep, and they’ll run in perfect sync. However, the same isn’t true for analog recorders.
When interlocking analog machines, it’s necessary to have some type of control track, like timecode, to keep multiple machines in sync. SMPTE timecode, which is a digital clock, originally developed by NASA, can be either an audio signal, or in the case of video, can be inserted in the vertical interval, between the frames. This type of code is known as VITC or vertical interval time code.
In order to keep multiple machines running in sync, a chase interlock system is used, which includes one or more synchronizers and usually a controller. One machine is designated as the master, and the others are the slaves.
The synchronizer compares the incoming timecodes (one from the master and one from that particular slave machine). It first locates the slave machine to the timecode of the master, or to the master’s timecode with a predetermined offset. This is the “chase” part of the operation. Then, during the “interlock” portion, the synchronizer keeps the two devices locked by making subtle speed adjustments to the slave machine.