Subscriber line interface circuit for serving ISDN subscribers using echo cancellers and POTS subscribers

A subscriber line interface circuit for a digital telephone switching system serving both ISDN (Integrated Services Digital Network) subscriber stations having an echo canceller and POTS (Plain Old Telephone Service) subscriber stations, comprising:

• a switched hybrid circuit operable during a POTS mode when an associated subscriber is using a POTS subscriber station and an ISDN mode when said associated subscriber is using an ISDN subscriber station, said switched hybrid circuit having a two-wire port coupled to a loop of said associated subscriber, a four-wire transmit port and a four-wire receive port, said hybrid circuit comprising:
• a resistance element coupled to said two-wire port;
• a network balancing impedance element arranged to be coupled to said resistance element during said ISDN mode;
• a first differential amplifier circuit for amplifying a signal from said two-wire port with a unity gain and applying the amplified signal to said four-wire transmit port during said POTS mode, and for differentially combining a signal from said two-wire port with a signal at said network balancing impedance element and applying the combined signal to said four-wire transmit port during said ISDN mode;
• a second differential amplifier circuit for differentially combining signals from said first differential amplifier circuit and said four-wire receive port according to a predetermined voltage dividing ratio during said POTS mode, and for amplifying a signal from said four-wire receive port with a unity gain during said ISDN mode;
• a third differential amplifier circuit for differentially combining a signal from said second differential amplifier circuit and a signal from said two-wire port and applying the combined signal to said resistance element during said POTS mode and for amplifying a signal from said second differential amplifier circuit with a unity gain, and for applying the amplified signal to said resistance and impedance elements during said ISDN mode;
• an analog-to-digital (A/D) converter for converting analog signals from the four-wire transmit port of said hybrid circuit to digital signals;
• a digital-to-analog (D/A) converter for converting digital signals applied thereto to analog signals and applying same to said four-wire receive port of the hybrid circuit;
• first memory means for storing a first program for effecting network balancing computations and codec filter computations during said POTS mode;
• second memory means for storing a second program for effecting echo canceller computations and equalization decision computations during said ISDN mode;
• a digital signal processor configurable according to each of said first and second programs, the processor receiving digital signals from said A/D converter and pulse code modulation signals from said switching system, processing the received signals according to the program supplied from one of said memory means and applying a processed version of the signals from the A/D converter to said switching system and a processed version of the signals from the switching system to said D/A converter;
• and
• means for causing said switched hybrid circuit and said first and second memory means to be switched from the POTS mode to the ISDN mode.