Macrovision Decoder/Blanker

44 EE October 1988 MACROVISION DECODER/BLANKER First used by CBS-Fox on PAL VHS tapes of the action movie Crocodile Dundee, the MacroVision encryption system is gradually being introduced by film and video rental companies to prevent customers making copies of prerecorded video tapes. This article describes the basic operation of the MacroVision system, and proposes a circuit that negates the copy protection signal. Invisible lines In the PAL system, a television picture is transmitted (and recorded) as 625 in- terlaced lines. Actually, the picture, or frame, is transmitted as two rasters of 312.5 lines, at a speed of 25 per second (50 rasters per second: the field fre- quency is 50 Hz). Not all lines are, how- ever, visible on the screen. The vertical blanking interval (VBI) comprises the vertical (raster) synchronization pulse, and about 17 blank lines, which produce a black bar at the top of the screen when the picture is shifted downwards with the vertical picture position con...