Abstract

An autoequalizing hybrid circuit for providing a cancelation path to minimize the two-wire receive path signals that are reflected onto a two-wire transmit path from a subscriber line interface circuit is disclosed. The hybrid circuit includes a variable impedance balance network. A tone generator injects a set of equalization signals into the receive path which are used to control the setting, or equalization, of the balance network. The tone generator also generates a supervisory signal into the receive path used to monitor the performance of the balance network. A detect circuit monitors the level of the reflected equalization sidetone signals and produces balance signals whenever the equalization signals fall to a null level. The detect circuit also monitors the supervisory sidetone signals and produces a recalibration signal whenever an abrupt change in the signals indicates that the balance network is no longer adequately cancelling their reflected signals. A microprocessor controls the generation of the equalization and supervisory signals and is connected to the detect circuit for monitoring production of the balance and the recalibration signals. When the hybrid is first activated, the microprocessor initially sets the balance network impedance with reference to the balance signals. After the balance network is equalized the microprocessor directs the tone generator to produce the supervisory sidetone signals. The microprocessor then monitors the detect circuit for recalibration signals and if they are produced, reequalizes the impedance of the balance network.