VHS and DVD Copy Protection using Macrovision
"MacroVision" copy protection is a copy-protection scheme employed on video media (VHS and DVD) to stop people from making illegal copies of the contents of the media. It is named after Macrovision Corporation, the company which sells licenses to video media manufacturers to use their protection scheme.
Macrovision in action
Macrovision copy protection is intended to stop copying video material to VHS tape on a VCR. If video from a Macrovision protected DVD or VHS tape is fed into a VCR and the recording attempted on the VCR, the brightness of the video fluctuates to such an extent that the video becomes unusable.
Principle of Macrovision protection
The VHS standard includes a feature called Automatic Gain Control (AGC). AGC works by storing brightness information in the part of the recording corresponding to the vertical retrace intervals (the time during which the electron beam on the TV screen goes back to top left corner after reaching lower right corner of the picture). This information is used by the VCR to adjust the brightness of unusually dark or bright images to provide a relatively consistent brightness level to the viewer.
Macrovision works by mis-using the AGC feature by deliberately putting in incorrect AGC information. This makes the AGC circuit think that the image is too bright or too dark and compensate for that, producing a picture which is too dark or too bright. Macrovision continues putting in different levels of false AGC information, so that the picture fluctuates in brightness.
The AGC circuit in VCRs kick in only when in recording mode. Hence, Macrovision does not take effect unless recording is attempted.
TV sets use a different kind of AGC than VCRs do. Thus, Macrovision protected video signals fed directly to TV sets also produce normal picture.
Some non-VHS VCRs (e.g. 8mm video VCRs), TV-VCR combos and older VHS VCRs are not affected by Macrovision because of non-dependency or incomplete implementation on AGC.