EFF: DeepLinks:

Macrovision’s “Analog Copy Protection” (ACP) technology is intended to degrade the quality of video copies made on analog VCRs. It does this by intentionally adding noise to the vertical blanking interval of analog video signals. This noise confuses the automatic gain control (AGC) circuit used by analog VCRs. In short, Macrovision’s ACP is an exploit against a weakness in analog VCRs. Thanks to Section 1201(k) of the DMCA, VCR makers are now forbidden by law from fixing the weakness, which means that analog VCRs have remained vulnerable to ACP. In other words, Macrovision’s ACP is an antiquated DRM technology that owes its effectiveness in the analog world to a government mandate.