Northern Telecom Limited

Guy J Fortier

Christopher Michael Forrester

David A Pepper

Norbert J Diesing

W J McCready

Sidetone in a four-wire telephone station set that is coupled to a two-wire telephone line through a four-to-two wire hybrid circuit is automatically and cancel the phase 'to a predetermined value', set by incrementally adjusting a balancing impedance connected to the circuit until an optimum impedance match between the telephone set and line is achieved. Under software control, during idle intervals between dialed digits when a call is initiated, a microcontroller mutes the telephone transmitter and receiver on the telephone side of the circuit and substitutes for these components a tone signal generator to produce a sidetone signal that is applied to a full-wave rectifier and therefrom to a peak voltage detector to generate a sidetone envelope. Individual ones of several impedance balancing networks are then sequentially connected to the telephone side of the circuit and the sidetone envelope for each balancing impedance is digitally encoded and stored. A mutual comparison of the stored data indicates a minimum sidetone value which corresponds to an optimum impedance match that is retained unless a subsequent telephone call results in a change of balancing impedance.

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Kanata, Canada

Guy J Fortier