Starting in
1985 with the video release of "
The Cotton Club", Macrovision has licensed to publishers a technology that exploits the
automatic gain control feature of VCRs by adding pulses to the vertical blanking sync signal. These pulses do not affect the image a consumer sees on his TV, but do confuse the recording-level circuitry of consumer VCRs. This technology, which is aided by U.S. legislation mandating the presence of automatic gain-control circuitry in VCRs, is said to "plug the analog hole" and make VCR-to-VCR copies impossible, although an inexpensive circuit is widely available that will defeat the protection by removing the pulses. Macrovision uses a legal strategy of patenting its video AGC system, giving it a more straightforward basis to shut down manufacture of any device that descrambles it than often exists in the DRM world.
