A channel unit interface circuit for interconnecting a two-wire, bidirectional signal transmitting means and a four-wire digital signal transmitting means in a communication system, said interface circuit comprising:
- a two-wire port having tip and ring terminals adapted for connection to said two-wire means;
- a signal receive port adapted for connection to a first signal transmission path of said four-wire means for receiving incoming tip and ring terminal DC to 60 Hz ("DC band") frequency signals and incoming tip and ring terminal 200 Hz to 3.5 KHz ("voice band") frequency signals;
- a signal transmit port adapted for connection to a second signal transmission path of said four-wire means for supplying outgoing tip and ring terminal DC to 60 Hz ("DC band") frequency signals and outgoing tip and ring terminal 200 HZ to 3.5 KHz ("voice band") frequency signals from said two-wire means;
- circuit means including voice codec means, A/D signal processing means, and D/A signal processing means coupled to said four-wire means for supplying said incoming tip and ring terminal "voice band" frequency signals of said four-wire means at a first node connected to said codec means and for supplying said incoming tip and ring terminal "DC band" frequency signals of said four-wire means at second and third nodes connected to said D/A signal processing means;
- tip terminal current line driver means connected between said first node and said tip terminal and being responsive to said incoming tip terminal "voice band" frequency signals of said four-wire means arriving at the first node from said codec means for applying to said tip terminal corresponding outgoing tip terminal "voice band" frequency signals;
- ring terminal current line driver means connected between said first node and said ring terminal and being responsive to said incoming ring terminal "voice band" frequency signals of said four-wire means arriving at the first node from said codec means for applying to said ring terminal corresponding outgoing ring terminal "voice band" frequency signals;
- voice sense amplifier means having first and second inputs coupled to said tip and ring terminal current line driver means and across said tip and ring terminals and being responsive to incoming differential "voice band" frequency signals from said two-wire means for generating an output voltage;
- said tip and ring terminal line driver means being responsive to said output voltage for generating an AC input impedance across said tip and ring terminals;
- said codec means of circuit means being connected to an output of said voice sense amplifier at a fourth node for supplying said outgoing tip and ring terminal "voice band" frequency signals of said two-wire means for transmission over the second path of said four-wire means;
- said tip terminal current line driver means being responsive to said incoming tip terminal "DC band" frequency signals of said four-wire means arriving at the second node from said D/A signal processing means for applying to said tip terminal corresponding outgoing tip terminal "DC band" frequency signals;
- said ring terminal current line driver means being responsive to said incoming ring terminal "DC band" frequency signals of said four-wire means arriving at the third node from said D/A signal processing means for applying to said ring terminal corresponding outgoing ring terminal "DC band" frequency signals;
- first feedback impedance means coupled between the tip terminal and an input of said tip terminal current line driver means and being responsive to incoming tip terminal "DC band" frequency signals from said two-wire means for generating a simulated tip terminal impedance;
- second feedback impedance means coupled between the ring terminal and an input of said ring terminal current line driver means and being responsive to incoming ring terminal "DC band" frequency signals from said two-wire means for generating a simulated ring impedance;
- said A/D signal processing means of circuit means being connected to said first impedance means at a fifth node for supplying said incoming tip terminal "DC band" frequency signals of said two-wire means for transmission over the second path of said four-wire means;
- said A/D signal processing means of circuit means being connected to said second impedance means at a sixth node for supplying said incoming ring terminal "DC band" frequency signals of said two-wire means for transmission over the second path of said four-wire means;
- and
- balance network means operatively connected to said voice sense amplifier means for preventing the incoming "voice band" frequency signals in the first transmission path arriving at said receive port from said four-wire means from being returned to said transmit port.