For those who are interested, automatic gain control on VHS players was exploited by Macrovision to prevent the copying of commercial tapes. Analog video signal

For those who are interested, automatic gain control on VHS players was exploited by Macrovision to prevent the copying of commercial tapes. Analog video signals contain synchronizing pulses, which are necessary for correct sweeps. These pulses are not part of the visible lines shown by the TV. A Macrovision tape contains spikes of varying amplitudes within the synchronization pulses. The spikes are placed far enough from the visible lines (usually the first 40ms of a frame), so that the TV's AGC is not affected, and the picture remains stable. When a VCR is receiving a Macrovision signal to record, the AGC (which unlike the TV does care about the first 40ms of a frame) amplifies the signal, causing the visible lines to be heavily altered in an erratic fashion. This sometimes appears as "snow" on the duplicated tape. Some modern "VHS-To-PC" device drivers/conversion software will detect a Macrovision signal and refuse to accept it. It was defeated in numerous ways, but the hassle free way to duplicate a Macrovision tape was simply to use an older VCR without an AGC circuit.