Voice coding conversion circuit between office exchange and ISDN keyphone system

A voice coding conversion circuit between an integrated service digital network (ISDN) keyphone system and an office exchange in an ISDN system having a line termination unit interposed between the ISDN keyphone system and the office exchange, a network termination equipment (NTE) and an intermediate multiplexer (IMUX) connected to the ISDN keyphone system, a primary multiplexer (PMUX) and a primary network termination equipment (PNTE), comprising:

• a buffer for buffering pulse code modulation (PCM) data received from the network termination equipment connected to an ISDN terminal or a non-ISDN terminal via said ISDN keyphone system;
• a PCM interface controller for interfacing with a line termination unit connected to the office exchange, so as to transmit and receive PCM data to/from an ISDN analog subscriber at a required time slot;
• an A/μ-Law converter interposed between the buffer and the PCM interface controller, for converting PCM data output by said ISDN keyphone system to PCM data corresponding to a voice coding mode of said office exchange and for converting PCM data input to said ISDN keyphone system from said office exchange to PCM data corresponding to a voice coding mode of said ISDN keyphone system;
• a clock driving circuit for providing a frame sync signal and a fundamental clock signal to the A/μ-Law converter;
• a central processing unit connected to the A/μ-Law converter, for generating a control signal for designating a voice coding mode according to a voice coding mode of the office exchange, so as to convert the PCM data input and output to/from the A/μ-Law converter in the designated voice coding mode;
• and
• a keyboard for inputting a specific command to said central processing unit to enable said central processing unit to generate said control signal for controlling said A/μ-Law converter.