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Special-information-tone frequency detection

The ability to distinguish recorded announcements from other call types is an essential part of the mechanized service-evaluation process. To do this, all announcements will have a special-information-tone prefix. A frequency detector — which is based on the correlation functions of the received signal — would be used to decide which announcement was triggered by a specific call attempt. This paper evaluates the performance of the frequency detector in the presence of additive noise and frequency shift induced by the announcement machine. The theoretical results, based on a calibration frequency, are very encouraging. To verify that use of this frequency is feasible in practice, an algorithm is proposed and its performance evaluated to show that it compares favorably to the theoretical one.