Telephone answering apparatus having two integrators of different time constants

Claim 1

A circuit for discriminating a command signal for operating a telephone answering apparatus adapted to be connected to a telephone line to transmit therethrough a recorded message to a calling station and to receive said command signal therefrom, said command signal including a series of tone bursts of a varying number signifying a particular operating mode of said apparatus, the discrimination circuit comprising: a mechanical filter for passing components having the frequency of said received command signal; a first integrator working cooperatively with said mechanical filter and responsive to the output thereof, said first integrator having a large time constant value for integrating said components passing through said mechanical filter to generate a continuous output signal during the reception of said tone bursts; a second integrator having a small time constant value for integrating said received command signal to generate an output signal having an envelope substantially identical to the envelope of said tone bursts; and a coincidence gate having first and second input terminals respectively coupled to the outputs of said first and second integrators for generating a coincidence signal when there is a match between the output signals of said first and second integrators.