Station line interface circuit for a telecommunication network

Claim 1

A telephone station subscriber line interface circuit for connecting a telephone subscriber station to a telecommunications transmission line, the interface circuit comprising, in combination: (a) a loop current sensing circuit for providing an output signal proportionately indicative of a change in the DC resistance in the telecommunications transmission line; (b) a ring-side driving circuit being coupled to said loop current sensing circuit and being responsive to said output signal and including source means for providing an alternating current voltage source and a controllable direct current voltage source to a transmit amplifier in periods of high constant DC resistance in the telecommunications transmission line, the ring-side driving circuit maintaining the output of said alternating current voltage source constant to the transmit amplifier and automatically modifying said direct current voltage source form an equivalent constant direct current voltage source to an equivalent constant direct current current source in the event of a decrease in the DC resistance in the telecommunications transmission line sensed by the loop current sensing circuit; and (c) a power denial circuit, the power denial circuit, in response to a control signal, deactivating the ring-side driving circuit, thereby causing the ring-side driving circuit to provide no current to the telecommunications transmission line.