What is Macrovision?

Macrovision is an anti-piracy mechanism that was commonly found on VHS tapes, and is found on DVDs so that they cannot be easily copied to VHS or with a DVR. Basically, it creates a modulation in the non-viewable section of the screen. This is invisible during normal playback, but when a VCR tries to record it, the gain control goes nuts and the screen flashes constantly.