49. In a system for identifying signals for determining audience ratings having means for operating on the signal to be identified in order to extract a feature string, means for comparing the feature string with various feature strings corresponding to known signals and for identifying the signal when the feature string of the signal to be identified correlates with one of the feature strings corresponding to a known signal within a predetermined tolerance, the improvement wherein the feature string extracting means comprises: means for monitoring predetermined changes in a predetermined parameter of the signal to be identified; means for determining the time intervals between the predetermined changes in the predetermined parameter; and means for utilizing the time intervals between successive predetermined changes to generate the feature string, the feature string being a digital representation of a predetermined number of said time intervals.
50. The improvement recited in claim 49 wherein the signal to be identified includes an audio signal, and wherein said feature string extracting means includes means for monitoring the amplitude of the audio signal and determining the time intervals that the amplitude of the audio signal departs from a predetermined level of amplitude.