21. A system as recited in claim 15 wherein said signal is a television signal and said means for extracting said signature includes means for low pass filtering a video signal of the television signal to provide a video envelope signal, means for periodically sampling said video envelope signal to provide a series of video samples, and means for digitizing said video samples to provide said signature.
21. A system as recited in claim 15 wherein said signal is a television signal and said means for extracting said signature includes means for low pass filtering a video signal of the television signal to provide a video envelope signal, means for periodically sampling said video envelope signal to provide a series of video samples, and means for digitizing said video samples to provide said signature. 22. A system as recited in claim 21 wherein said low pass filtering means has a cut-off on the order frequency of approximately 2 to 6 kHz.