Hybrid balance and combination codec filter circuit

Abstract

A hybrid balance and combination codec filter circuit used in a subscriber line interface circuit over which a connection is established between a digital switching network and a subscriber instrument. The hybrid balance and combination codec filter circuit in combination includes a transmit section that converts differential voltage audio signals representing voice transmissions transmitted from the subscriber instrument into Pulse Code Modulated (PCM) encoded digital data, for transmission to the digital switching network. A receive section converts PCM encoded digital data representing voice signals switched through the digital switching network into differential voltage audio signals for transmission to the subscriber instrument. An included hybrid balance and echo cancellation section connected between the transmit section and the receive section reshapes the audio band signals output by the receive section. The reshaped audio band signals are applied to the transmit section, where they cancel any reflected audio band signals not completely transferred to the terminating impedance. Finally, an impedance section provides an audio band feedback signal between the transmit section and the receive section for synthesizing a source impedance that is matched to the impedance of the tip and ring leads of the subscriber loop.