A tone detector for detecting particular frequencies within an incoming signal,such as dial tone on call waiting tone received from exchange equipment. The detectorcomprises a bandpass filter or analog-to-digital converter, a microprocessor, a bufferand an analyser. The analyser periodically examines samples of the incoming signalstored in the buffer for the presence of the particular frequency by means of a modifiedFourier Transform adapted to allow the use of a simple low-cost microprocessor.
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