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.
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. 51. The improvement recited in claim 50 wherein the predetermined level amplitude is a level that corresponds substantially to silence, and said time interval corresponds to the time interval between successive periods of silence in the audio signal.