Full duplex audio line circuit

This full duplex audio line circuit enables two audio signals in opposing directions to flow simultaneously through a common twisted pair line. And it does that without complicated communication technology. This technique called full duplex is not really new. It is common in telephone technology. However, the telephone uses a carrier signal. This circuit on the other hand does the trick without a carrier signal.

The principle is simple: two transmitters, each one attached at each end of the line, feed the signals. The voltage at the line will be the sum of the two signals: U1 U2. In the actual circuit however, this is equivalent to only the half of the sum. At each end of the line, the signal is recovered while the other signal gets rejected. This requires that two identical circuits must be constructed.

TL074CN TMS320VC5409PGE100 TDA1308 TMS320LF2406APZA REF02AU OPA4227UA SN74LVC1G08DCKR OP177G LM337LM MAX211CWI MAX213CAI LM4880M The opamp A1 works as an impedance converter and at the same time functions as the transmitter. The resistor R5 protects the output of A1 from the signals coming from the transmitter at the other end of the line. The sum of the signals is taken from the output of the circuit and fed to the non-inverting input of the differential amplifier composed of the opamps A2, A3 and A4.

The “right” signal goes through the voltage divider network made of R4, R11 and P1. It is then taken out from the sum of the signals. This “right” signal appears at the OUT output of the circuit.
The resistors must be low tolerance type to achieve the highest same phase suppresion possible. This suppresion is about 80 dB at 1 kHz and 60 dB at 20 kHz. When using long cables, the suppresion can be improved by adjusting C3.

Full duplex circuit diagram

Calibration: Connect the (IN) input to a signal generator and feed a sinus wave of 5 kHz with 1 Veff to the circuit. Attach the twisted pair line (cable) to the (IN/OUT) point of the circuit and short the input of the second circuit at the opposite end of the cable. Raise the signal frequency to 10 kHz and adjust C3 to get the best result.