Computer microphone disconnects when connected to wave generator

Question

I am using the Visual Analyser virtual oscilloscope as both an oscilloscope and a wave generator (on two different computers). I connected audio jacks (1 TRS and 1 TRRS) to a breadboard to connect the two.

When I connect the ground of the oscilloscope mic to the ground of the generator, the wave from the generator shows up on the oscilloscope for a second, and then the microphone disconnects. Why is this?

Answer 1

The PC soundcard is probably switching off the microphone input because it thinks you have connected something other than a microphone to the microphone input.

The microphone input has a DC voltage on it intended to power the electret microphone in a typical headset. Some soundcards monitor that DC voltage to detect what is connected.

• Full DC voltage = nothing connected
• A small drop in voltage = microphone connected
• Bigger drop = speaker connected (switch to multioutput instead of simple stereo out.)

You can probably trick it into working right using the following circuit:

1. The resistor from microphone to ground (2k) is approximately the DC resistance of a typical electret microphone. It makes the soundcard "think" there's a microphone attached.
2. The capacitor keeps the output from the other soundcard from causing a DC short to the microphone input. That DC short is what is currently causing the microphone input to switch off.

The down side to all this is that the resistor and capacitor make a high pass filter. You will lose some of the lower end frequencies (below about 100Hz.)

You could try using the line-in on your sound card instead of the microphone input. That won't have the problem with the input switching off.

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You don't have to use two PCs. You should be able to connect the line-out from on soundcard to its own line-in input. I do this kind of thing all the time.