Duplex audio communication system circuit

A simple duplex audio communication system circuit can be build using this electronic scheme designed using conventional operational amplifiers and electronic components.
To be functional,Duplex audio communication system circuit require two identical circuits.
Operational amplifier A1 is intended to serve as a buffer and transmitter. The signal is transmitted over the cable by means of R4. Resistance R4 makes cable tension to be half of the output voltage of A1. Meanwhile, the presence of R4 does not allow signals from the other end of the link to reach the exit of A1 (otherwise they would have shorted it out).

The receiver is a differential amplifier consisting of operational amplifiers A2-A4. Signal cable is applied to one differential amplifier inputs and the output signal (halved) from A1 - at the other. Since the differential amplifier has a gain of 6 dB, the received signal applied to K2 has the same level as initially signal.
Circuit will be calibrated by connecting the cable to him and twin mounting, and by injecting a sinusoidal signal with a frequency of 1 kHz and 5 V input level. Line input of the other circuit must be short circuited when making calibration. Adjust P1 for a minimum signal at K2. Then increase the input signal frequency to 10 kHz and adjust the C5 for minimum signal K2. Repeat same operations for the other (twin)circuit.