Abstract: 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.

Abstract: Changes in a telephone subscriber line voltage are detected in a telephone set by supplying a voltage dependent upon the subscriber line voltage to a potential divider having first, second, and third tapping points, the second tapping point being between the first and third tapping points, and comparing the voltages at the first and third tapping points with a smoothed version of the voltage at the second tapping point to produce pulses at respective terminals when the line voltage is falling or rising. An algorithm is described for responding to the pulses to distinguish changes in hook state of another telephone connected to the same subscriber line from interfering signals. The arrangement facilitates remote release from hold and privacy indication functions for the telephone set.