Patent ID: 6925171

Claim:
A codifier/decodifier filter circuit connected in a subscriber line interface circuit, the subscriber line interface circuit connected to a subscriber instrument via a tip lead and a ring lead of a subscriber loop across which a subscriber loop impedance exists and to a digital switching network, the codifier/decodifier filter circuit, comprising: a transmit section for converting differential voltage audio transmit signals representing voice transmissions from the subscriber instrument into encoded digital data for transmission to the digital switching network; a receive section for converting encoded digital data representing voice signals switched through the digital switching network to differential voltage audio receive signals for transmission to the subscriber instrument; the subscriber loop and subscriber instrument being operable to reflect the digital voltage audio signals to the transmit section; an impedance section connected between the transmit section and the receive section disposed to provide an audio band feedback signal between the transmit section and the receive section for synthesizing a source impedance for the subscriber line circuit that matches the subscriber loop impedance; wherein the impedance section comprises a programmable digital filter coupled to the transmit section having a transfer function equal to H ⁡ ( z ) = ( R 2 ⁢ T ) ⁢ ( 1 + z - 1 ) ( R 1 ⁡ ( T + 2 ⁢ C 2 ⁢ R 2 ) ⁢ ( 1 + ( T - 2 ⁢ R 2 ⁢ C 2 ) ( T + 2 ⁢ R 2 ⁢ C 2 ) ⁢ z - 1 ) ) where R 2 is the second subscriber loop impedance, C 2 is the subscriber loop capacitance, R 1 is the first subscriber loop impedance, T is the sampling rate of the analog-to-digital converter and z is the frequency of the signal, and a summer circuit coupled between the programmable digital filter and the receive section, the summer circuit arranged to sum the single-ended audio receive signals from the digital switching network with the audio band feedback signal output by the programmable digital filter.