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I've notice that when I plug mobile headphones (with a built-in mic) into a computer, the microphone output doesn't come from the actual microphone, but from something inside the speaker itself (the ear bud).

This is extremely weird, how is this picking up any input in the first place? It's also very quiet and picks up only loud sounds (with sound boosting), I've used some intuition to guess that this could be some sort of noise cancellation microphone?

My question is, why do computers use this "mic" instead of the actual microphone, when I plug the headphones into my phone, it uses the correct microphone and sounds great.

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    Microphones and speakers operate on the same principle, but reversed. A speaker can be used as a microphone, and vice-versa. I realize that's not an answer to your question, but it's at least a partial explanation. To make the headset work with your computer, you probably need to find a driver for your operating system, specifically made for that device. – boot13 Nov 29 '15 at 13:02
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This is partly because your computer does not have a good connection between your speaker "ground" and the system ground which can be a good thing for noise isolation from the rest of the computer, but a bad thing in this particular case, and also that your computer does not have a 4-pin socket that supports a Speaker+microphone headset jack.

The difference between the microphone and "standard" headphones is as below:

enter image description here
image half-inched from http://forum.allaboutcircuits.com/threads/reading-information-from-video-glasses.52135/

Your computer probably lacks the 4-pin connector and so the microphone connection occupies an area where the sound card expects the "ground" signal to be. Chances are that the "ground" contact in your laptop is over the "Microphone" pin meaning that whatever is on your microphone is acting as your speaker ground. Because speakers are driven by the potential difference between two lines this means that whatever is on your microphone will act as the signal line the speaker lines are referenced against. Voltage changes on the microphone line due to noise will result in an apparent signal on the speaker lines.

Basically for best performance you should only use 4-pin jacks with devices that have 4-pin sockets. If a device has both a microphone port and a speaker port then chances are it does not have 4-pin sockets.

If you really want to use your current headset with devices that have separate microphone and speaker outputs then I'd recommend getting something like this: StarTech 3.5mm 4 Pin to 2x 3 Pin 3.5mm Headset Splitter Adapter - F/M

enter image description here

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