An audiovisual subscription system includes means for aperiodically inverting the lines of a transmitted video signal on a frame-by-frame basis and for decrypting encrypted PCM audio information which is transmitted along with the aperiodically inverted video information....http://www.google.ca/patents/US5185794?utm_source=gb-gplus-sharePatent US5185794 - System and method for scrambling and/or descrambling a video signal
