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Abstract

A DTMF signal receiver, for telephone systems, includes an amplifier with automatic gain control for receiving and amplifying DTMF and audio signals. A series of bandpass and notch filters receive the output from the amplifier for producing bandpass and notch outputs in response to the DTMF frequencies. A detector receives the bandpass output from each of the filters to produce a signal representing the presence or absence of a signal of each DTMF frequency in the amplifier output, and a DTMF decoder receives the signals from all such detectors and produces output signals in response to simultaneous detection of preselected pairs of DTMF frequencies. A voice guard circuit receives the notch output from the last filter in the series and compares the amplitude of that notch output with a threshold level to produce a disable signal whenever the amplitude of the notch output is greater than the threshold. This disable signal is supplied to the DTMF decoder for disabling the production of output signals by the decoder and thereby preventing the detection of invalid DTMF signals.

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First Claim