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Abstract

An interface between a telephone switch bus line and a dual tone multiple frequency (DTMF) type digital tone decoder is provided which minimizes false tone decoding due to filter-induced transient oscillations caused by spurious impulse noise and the like on the input bus line. The interface includes a limiter circuit for generating a uniform amplitude output pulse train and a circuit for automatically generating a variable threshold which is time responsive to input signals on the input bus line. The variable threshold circuit output signals are used by the limiter circuit to mask transient oscillations in the tone filter output. Only when persistence of the oscillations wherein the limiter threshold level signal is finally less than the peak amplitude of the input signals to the limiter from the tone filter will such input signals be sensed as valid switching tone signals.

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