15. A system for identifying a signal received by an audience member's television receiver for determining audience ratings comprising: means for detecting the occurrence of a first event in the functional operations of the audience member's television receiver; means responsive to the detected first event for detecting the occurrence of a second event in the signal to be identified; means for extracting a signature from a single frame of the video signal to be identified after the occurence of the second event; means for storing the signature and the time of occurence thereof; and means for comparing the stored signature with reference signatures that occur at approximately the same time as the stored signature.
15. A system for identifying a signal received by an audience member's television receiver for determining audience ratings comprising: means for detecting the occurrence of a first event in the functional operations of the audience member's television receiver; means responsive to the detected first event for detecting the occurrence of a second event in the signal to be identified; means for extracting a signature from a single frame of the video signal to be identified after the occurence of the second event; means for storing the signature and the time of occurence thereof; and means for comparing the stored signature with reference signatures that occur at approximately the same time as the stored 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.