Automatic line impedance balancing circuit for computer/telephone communications interface

Claim 1

Automatic line balancing circuit for a telephone/computer interface comprising: a modulator/demodulator arranged to send and receive FSK modulated carrier signals including at least one transmitted carrier signal and at least one received carrier signal, to and from telephone lines of varying impedance, a microcontroller connected to the modulator/demodulator arranged to transmit and receive digital data therefrom, an isolation transformer coupled to said telephone lines for receiving and transmitting said carrier signals, a duplexer connected between the isolation transformer and the modulator/demodulator, said duplexer having a first variable gain operational amplifier having a first input connected to receive said transmitted carrier signal and supply it to said isolation transformer through an output impedance connected to a junction, said duplexer having a second operational amplifier with a first input connected to said junction to amplify said received carrier signal from said transformer and a second input connected to the first input of said first operational amplifier, a balance adjust-null detection circuit connected between said second input of the second amplifier and the output of the second amplifier and adapted to provide an output signal upon presence of transmitted carrier signal through said second amplifier, and means for varying the gain of the first amplifier in response to output from said balance adjust-null detection circuit, said gain varying means comprising a line balancing program controlling said microcontroller to periodically sample the output from said balance adjust-null detection circuit and to periodically change the gain of said first amplifier to reduce line unbalance at the telephone/computer interface.