Circuit eliminates PC echoes

Hans Krobath, EEC, Nesconset, NY -- EDN, 3/30/2000

Long-distance-telephone services available via the Internet often require the PC user to wear headphones of a headset to prevent echo caused by the microphone's picking up the loudspeaker outputs. The circuit in Figure 1 eliminates the echo while using the existing PC microphone and speakers for a comfortable conversation. The interface is between a standard electret condenser microphone and the microphone input of the PC. The loudspeaker output of the PC serves to mute the microphone input. R₁ and R₂ provide biasing for both the electret microphone and the Q₁ emitter follower. Q₂, a p-channel FET, acts as a switch that opens with the application of a gate voltage greater than 6V and closes with a gate voltage of 0V. Q₃ compensates for the switching-circuit losses and buffers the output. R₉ and R₁₀ provide an appropriate input impedance to Q₃ and limit the output to 5V p-p, thus preventing any possible damage to the PC's microphone input.

IC₁, which acts as low-level retriggerable monostable multivibrator, controls the Q₂ FET switch. Loudspeaker voltage levels as low as 15 mV from the PC cause comparator IC_1A's open-collector output to discharge C₆ via R₁₈. The falling voltage of C₆, passing the threshold of comparator IC_1B, produces a high output that turns off Q₂. Any input from the PC's loudspeaker output discharges C₆. The absence of an input allows C₆ to charge within approximately 40 msec to the IC_1B threshold, producing a low-level output and turning on Q₂. LED D₂ lights whenever no loudspeaker output is present, and the microphone input to the PC becomes enabled. D₁ reduces the Q₂ gate voltage to 0V when the IC_1B output saturates. You should set R₁ such that approximately a 100-mV p-p microphone input just triggers IC₁, as indicated by the LED's extinguishing. This level prevents any noise from the PC's loudspeaker output from falsely triggering the monostable multivibrator. (DI #2508).