Patent ID: 6842518

Claim:
An interface circuit for interfacing between a pair of subscriber tip/ring lines and a central office of a telecommunications network, the interface circuit comprising: (a) filter circuitry configured to separate low-frequency and high-frequency signals appearing on the tip/ring lines, wherein the filter circuitry comprises a blocking capacitor that affects the low-frequency impedance of the tip/ring lines; (b) high-frequency interface circuitry configured to process the high-frequency signals; and (c) low-frequency interface circuitry configured to process the low-frequency signals, wherein the low-frequency interface circuitry comprises: (1) a subscriber line interface circuit (SLIC) configured between the tip and ring lines; (2) a coder/decoder (CODEC) configured to encode and decode the low-frequency signals; and (3) an impedance warping circuit (IWC) configured between the SLIC and the CODEC, wherein: the IWC has first, second, and third differential ports, each port different from the other ports; the IWC is configured to receive a first differential signal from the SLIC at the first differential port and a second differential signal from the CODEC at the second differential port and generate a third differential signal provided to the SLIC at the third differential port; and the IWC tends to compensate for the effect of the blocking capacitor on the low-frequency impedance between the tip/ring lines.