Analog Protection System

The process works by adding pulses to analog video signals to negatively impact the AGC circuit of a recording device.

In set top boxes trigger bits are incorporated into Conditional Access Entitlement Control Messages (ECM) in the stream delivered to the STB.

Macrovision's legacy analog copy protection (ACP) works by implanting a series of excessive voltage pulses within the off-screen VBI lines of video.

These pulses were included physically within pre-existing recordings on VHS and Betamax and were generated upon playback by a chip in DVD players and digital cable or satellite boxes.

In addition, these voltage pulses caused some TVs to lose track of the vertical blanking interval, meaning they no longer knew when one frame ended and the next began.

The signal has also been known to confuse home theater line doublers (devices for improving the quality of video for large projection TVs) and some high-end television comb filters.

CGMS-A consists of a "flag" within the vertical blanking interval (essentially data, like closed captioning) which digital recording devices search for.

The principle of their function lies in detecting the vertical synchronization signal, and forcing the lines occurring during the VBI to black level, removing the AGC-confusing pulses.

United States fair use law, as interpreted in the decision over Betamax (Sony Corp. v. Universal City Studios), dictates that consumers are fully within their legal rights to copy videos they own.

On October 26, 2001, the sale, purchase, or manufacture of any device that has no commercial purpose other than disabling Macrovision copy protection was made illegal under section 1201(a) of the same controversial act.

Macrovision pulses in an otherwise unused video line. Here they are large, forcing a VCR's auto contrast circuit to make the picture darker.
A couple of seconds later, the pulses have reduced in amplitude, forcing a VCR's auto contrast circuit to make the picture lighter. A couple of seconds later still, the pulses return to their original amplitude, darkening the picture once more.