Unique digital codes are encoded on a video signal, the codes are retrieved at receivers and precise information concerning the time of occurrence, length, nature and quality of a monitored broadcast at a frame by frame level, is generated. The codes are inserted on scan lines of the video, and vary...http://www.google.de/patents/US5319453?utm_source=gb-gplus-sharePatent US5319453 - Method and apparatus for video signal encoding, decoding and monitoring
