Telephone line interface

Claim 1

A telephone subscriber's line interface for a system in which the telephone instruments and the connections therefrom to the exchange use analogue techniques while the exchange uses digital techniques such as pulse code modulation, in which the line is coupled to an analog network in said interface and via a two-wire to four-wire conversion circuit to separate GO and RETURN channels, in which the GO channel is coupled from the conversion circuit to analogue-to-digital conversion means which produces the digital output for use in the exchange, in which the RETURN channel is coupled from digital-to-analogue conversion means which re-creates speech for transmission to the subscriber's lines and said two-wire to four-wire conversion circuit, the invention in which said analog network includes a monitoring circuit having a high impendance connected to and individual to one of said analogue lines so as to monitor the line current and/or line voltage, means for coupling the output of the monitoring circuit via further analogue-to-digital conversion means to the input of a microprocessor common to a plurality of line interfaces to react to monitored line change conditions at said plurality of line interfaces under control of the output of the respective monitoring circuits and under a control connection from within the exchange, and further means for coupling the output of the microprocessor via which various signalling and other conditions applied to the line are coupled to said line via further digital-to-analogue conversion means and via a line driver circuit within said analog network.