28. A method for identifying signals for determining audience ratings comprising the steps of operating on the signal to be identified in order to extract a feature string, comparing the feature string with various feature strings corresponding to known signals and 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 tolerence, wherein the step of extracting the feature string from the signal to be identified comprises the steps of: monitoring predetermined changes in a predetermined parameter of the signal to be identified; determining the time intervals between the predetermined changes in the predetermined parameter; and utilizing the time intervals between successive predetermined changes to generate the feature string, the feature string being a digital representation of a predefined number of said time intervals.
28. A method for identifying signals for determining audience ratings comprising the steps of operating on the signal to be identified in order to extract a feature string, comparing the feature string with various feature strings corresponding to known signals and 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 tolerence, wherein the step of extracting the feature string from the signal to be identified comprises the steps of: monitoring predetermined changes in a predetermined parameter of the signal to be identified; determining the time intervals between the predetermined changes in the predetermined parameter; and utilizing the time intervals between successive predetermined changes to generate the feature string, the feature string being a digital representation of a predefined number of said time intervals. 29. The method recited in claim 28 wherein the signal to be identified includes an audio signal, and wherein the step of extracting the feature string includes the step of 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.