Electronic – PC Headset crosstalk

The TRRS connectors are wired like this, in order:

• Tip: Left output
• Ring: Right output
• Ring: Ground/return
• Sleeve: Microphone input

That way the headphones will work plugged into a normal TRS jack, too.

The microphone input is meant to connect to an electret mic with a circuit like this:

The bias voltage, resistor, and DC-blocking capacitor are all inside the phone. So just connecting a signal source like a computer's sound card to the microphone input (with ground to the ground ring) should work.

If your sound source's DC output resistance is too low (normally a good thing), the phone could be interpreting it as a continuous button press (Answer or Play/Pause), because the headset button shorts the mic line's bias resistor to ground. Does it do anything to indicate that it thinks a button is being pressed? If this is the case, just put an electrolytic capacitor in series with your source, like 10 µF to 100 µF between your source and the microphone input. This will block the DC current from the bias resistor while allowing the AC signal to pass. Larger capacitance will have better low-frequency response.

For instance, if the input impedance of the phone is 2.2 kΩ, 10 µF will have a -3 dB cutoff frequency of 7 Hz, which is plenty.

Thanks to a certain theBear on IRC's freenode (#electronics), I learned that devices generally test for the presence of a microphone by checking the resistance across the bottom two pins.

There doesn't seem to be a consistent convention for pinout, but generally mic and ground occupy the two rings closest to the sleeve

so the device tends to check for the resistance between these bottom two rings to determine whether a microphone is present.

for a standard stereo plug (L+R+G), The resistance should be practically zero as there is just one single ring that will be connecting the G and M

so the host says ' if the resistance is < X, then we assume no microphone. '

clearly 100 ohm is too small a value for X

doubling up to 200 did the trick

I still didn't get it working on an android phone even trying 300. So I don't know what the deal there is.

in case anyone wishes to pursue this further, the advice I got was to try putting a potentiometer between the microphone and ground and turning it until the host registers that a microphone is connected

if you launch the sound recorder on your android device and sing while you are twiddling, at some point the spectrograph will just cut out -- this means that the device now believes a microphone is connected externally

the last suggestion was to string a capacitor in series with this small resistor. however I don't have any idea how to calculate what value to use

As a final note, the ratio of R1:R2 determines the ratio of V_in:V_out