DTMF Signal receiver

Claim 1

A dual tone multiple frequency (DTMF) signal receiver for telephone systems, said receiver comprising an amplifier with automatic gain control for receiving and amplifying DTMF and audio signals, a series of bandpass and notch filters receiving the output from said amplifier for producing bandpass and notch outputs in response to the DTMF frequencies, the first filter in said series receiving the output from said amplifier and each succeeding filter in said series receiving the notch output of the preceding filter with each filter being tuned to a prescribed DTMF frequency so that the notch outputs of said filters successively eliminate signals with the prescribed DTMF frequencies, the bandpass output of each of said filters including only the particular DTMF frequency to which that filter is tuned, a plurality of detection means each receiving the bandpass output from one of said filters for producing signals representing the presence or absence of a signal of each prescribed DTMF frequency in the output from said amplifier, a DTMF decoder receiving the signals from said detection means and producing output signals in response to the simultaneous detection of preselected pairs of the prescribed DTMF frequencies, A voice guard circuit receiving the notch output from the last filter in said series and comparing the amplitude of that notch output with a predetermined threshold level for producing a disable signal whenever the amplitude of said notch output is greater than said predetermined threshold level, said disable signal being supplied to said DTMF decoder for disabling the production of said output signals by said decoder and thereby preventing the detection of invalid DTMF signals.