SUBSCRIBER LINE INTERFACE CIRCUIT FOR SERVING ISDN SUBSCRIBERS USING ECHO CANCELLERS AND POTS SUBSCRIBERS

摘要

A subscriber line interface circuit includes a switched hybrid circuitoperable during a POTS mode when the associated subscriber is using aPOTS station and in an ISDN mode when the subscriber is using an ISDNstation. The hybrid circuit includes a first amplifier circuit for coupling asignal at the two-wire port thereof to its four-wire transmit port, and asecond amplifier circuit for coupling a signal at the four-wire receive portof the hybrid to the two-wire port. A high termination impedance issynthesized at the two-wire port during the POTS mode and a lowtermination impedance at the two-wire port during the ISDN mode. Thesignal from the four-wire transmit port is converted to a digital signal andapplied to a digital signal processor which is configured differentlyaccording to DSP running programs respectively stored in first and secondmemories for operations during the POTS and ISDN modes. The DSPprocesses signals received from the hybrid's four-wire transmit port as wellas from the telephone switching system according to the programsupplied from the first or second memory and applies the process signalsto the switching system and the four-wire receive port.