Method and apparatus for processing a video signal so as to prohibit the making of acceptable videotape recordings

摘要

A video signal is modified so that a television receiver still produces a normal color picture from the modified signal, whereas a videotape recording of this signal produces generally unacceptable pictures. This invention relies on the fact that videotape recorders feature some form of automatic level control circuit. These automatic level control circuits measure the sync pulse level in the video signal and develop a gain correction signal for keeping the video level applied to an FM modulator in the videotape recording system at a fixed predetermined value. In accordance with the present invention either a positive or a negative pulse is added to the video signal immediately following the trailing edge of sync. The effect of these added pulses is to cause the automatic level control circuit in the recorder to assess the video level at either many times its actual value or as a fraction of its actual value. The gain correction signal applied to the amplifier in the automatic level control circuit thus causes recording of a signal which is either not strong enough to properly synchronize a television receiver's scanning circuits or is so high to result in clipping and signal distortion. The sync pulses themselves can also be at a reduced level, and means are disclosed for boosting the black level in the video signal so as to not degrade normal reproduction of the video signal on a television receiver.