System and method for copy protecting computer graphics

Abstract

Copy protection is provided for the now-unprotected computer monitor port of a computer in two ways: (i) delaying synchronizing signals by a time variable amount and (ii) generating pulses during non-active video. Delay of the synchronizing signal is performed by selecting a fixed offset and selecting either the fixed offset of the pseudorandom delay to be sent to a variable delay generator which delays the horizontal synchronizing signal by the selected amount. This new delayed horizontal synchronizing signal is encoded by the CRT Controller and is then sent to the computer monitor which uses the delayed synchronizing signal as encoded to produce its display in conjunction with a data signal. Thus, if these signals are intercepted by a VGA to TV converter the converter (or any downstream device) is unable to lock onto the correct frequency in order to reproduce the image properly. Horizontal pulse bars may also be generated to render a signal unfit for taping purposes. The horizontal pulse bars are generated to replace the portion of the video signal containing no picture information after vertical picture contraction has occurred. The amplitude of the input signal is reduced, and the amplitude of the pulse bars is increased to be greater than the amplitude of the input signal The horizontal pulse bars are then combined with the attenuated signal When a VHS recorder detects the amplitude of the modified signal, it lowers the amount of gain added to the signal because of the high amplitude of the added pulses. As the data content of the signal is at a much lower amplitude than the added pulses due to the attenuation, the viewable picture after taping is extremely dim and of poor quality.