Simple Full Duplex Audio Line Circuit

A Full Duplex Audio Line Circuit is like a super powered conversation highway.

It lets two people chat back and forth over a single wire, just like on a phone call.

This is different from a regular walkie talkie, where only one person can talk at a time.

With full duplex you can both chat freely, like a normal conversation.

Circuit Working:

Parts List:

The principle is just a basic:

Two transmitters, one at each end of the line transmit the signals.

The voltage at the line is the sum of the two signals U1+U2, although in the circuit this is effectively half of the sum.

At each end of the line, one signal is recovered while the other is rejected necessitating the construction of two identical circuits.

The op amp A1 serves as an impedance converter and transmitter.

Resistor R5 protects the output of A1 from signals coming from the transmitter at the other end of the line.

The sum of the signals is taken from the circuits output and fed to the non inverting input of the differential amplifier composed of op amps A2, A3 and A4.

The right signal passes through the voltage divider network R4, R11 and P1 and is extracted from the sum of the signals appearing at the OUT output of the circuit.

To achieve the highest possible same phase suppression low tolerance resistors should be used.

This suppression is approximately 80dB at 1kHz and 60dB at 20kHz.

For improved suppression with long cables C3 can be adjusted.

Calibration involves connecting the IN input to a signal generator feeding a 5kHz sinusoidal wave with 1 Veff to the circuit attaching the twisted pair line cable to the IN and OUT point of the circuit and shorting the input of the second circuit at the opposite end of the cable.

The signal frequency is then increased to 10kHz and C3 is adjusted for optimal results.

Formulas:

TL084 op amp is used in the design of a Full Duplex Audio Line Circuit to create distinct routes for incoming and outgoing audio signals, preserve signal integrity, and avoid feedback.

The TL084, a quad op amp integrated circuit, is commonly employed in audio applications because of its large bandwidth and low noise properties.

Full duplex operation, which is comparable to a phone line or a two way communication system, is the simultaneous transmission and receiving of audio signals.

Formula Considerations:

Gain Calculation: Depending on the configuration required for each stage, use either the normal non inverting or inverting op amp gain calculation.

Regarding a non-inverting setup: G = 1+ ​R_f / R_in​​

For inverting configuration: G =−R_f​​ / R_in​

where,

• R_f is the feedback resistor and
• R_in is the input resistor.

Bandwidth and Frequency Response: Based on the intended frequency response of the audio signals, determine the values of the capacitors for AC coupling and filtering.

Note:

To achieve clear and dependable two way audio transmission, careful consideration of gain settings, signal routing and isolation strategies is necessary when designing a full duplex audio line circuit utilizing the TL084 op amp.

Because of its qualities, the TL084 is suited for audio applications, offering the high fidelity and low noise amplification required for these circuits.

How to Build:

To build A Simple Full Duplex Audio Line Circuit following are the steps to follow:

Operational amplifiers op amps:

• Use op amps such as LM741 or similar.

Resistors:

• Choose resistors with low tolerance for accurate performance.

Capacitor:

• C3 can be a standard capacitor with its value adjusted for optimal performance.

Twisted pair cable:

• Use a twisted pair cable for the audio line.

Signal generator:

• Use a signal generator to provide input signals for testing and calibration

Connect the op amps:

• Follow the schematic to connect the op amps A1, A2, A3, A4 as shown diagram.
• Use the datasheets of the op amps to determine the correct pin connections.

Connect the resistors:

• Connect the resistors R4, R5, R11, P1 as per the diagram.
• Adjust P1 preset for calibration as described.

Connect the capacitor:

• Connect capacitor C3 as shown in the schematic.

Connect the twisted pair cable:

• Connect the twisted pair cable to the circuit.

Test and calibrate:

• Use a signal generator to input signals and test the circuit.
• Adjust P1 for optimal performance, as described in the circuit description.

Finalize the circuit:

• Once the circuit is tested and calibrated finalize the connections and ensure everything is securely connected.

Power up the circuit:

• Connect the power supply to the circuit and power it up.

Note:

Please note that this is a basic overview and additional considerations may be needed based on your specific requirements and components used.

Safety precautions should always be followed when working with electronic circuits.

Conclusion:

To conclude, a Full Duplex Audio Line Circuit facilitates two way audio communication over a single twisted pair line allowing simultaneous transmission and reception of audio signals.

By employing simple yet effective techniques this circuit enables seamless and efficient communication without the need for complex technology.

References:

Party-line / Full-duplex intercom – mixing audio signals

Duplex (telecommunications)

Datasheet IC TL084