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Abstract

Dual tone multi frequency signals, such as those used in Touch-Tone.RTM. telephones, are digitally detected. In an eight tone frequency regime of four low frequencies (697 hz, 770 hz, 852 hz and 941 hz) and four high frequencies (1209 hz, 1336 hz, 1477 hz and 1633 hz)) combinations of paired tones, one from each group, are detected. Each received tone is converted to a digital format and thereafter passed through parallel digital filtering in a microprocessor acting as a tone detector. Each suspect tone is examined twice, once for a short time interval in the range of 10 milliseconds and once for a long time interval in the range of 20 milliseconds. The short time span, generally not capable of resolving the discrete tones in each group one from another, only produces a broad power response to the signals present in each group thus giving a wide band filter. The long time examination is sufficient to discretely resolve each of the chosen four discrete frequencies thus giving a narrow band filter. The signal from each of the examinations, the long time interval and the short time interval, are then compared to each other. The signals are passed if the ratio of the signal strength of the long time span detected signal to the sum of the short time span detected signals of the frequency group is 8/10 or greater. Frequencies outside of the specified tones by a margin exceeding 3.5 percent cannot be passed with the prescribed ratio of 8/10 of the long time span signal to the short time signal. Provision is made to avoid dial tone interference by utilizing a digitized raised window in the order of a cosine window as a multiplier to shape the input signal.

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