Use of external mic thru audio jack
Can you please tell me if the Nexus 7 supports the use of a headphone/mic combo so that when the audio jack is inserted the internal mic is bypassed and the external mic provides the source of audio. In other words does it work the way any ordinary cell phone does? I ask because I have received 2 units and neither works in this way (the internal mic continues to be used when I insert a plug that includes a mic). Google seems to think they muct be defective (e.g., a solder joint is bad) but given that you have taken the tablet apart, I thought you could provide a definitive answer.
The Nexus 7 has a normal 3-pole headphone-only jack at the bottom, and two microphones: one on the top left (on a little daughterboard plugged into the mainboard) and one on the lower left edge (soldered directly to the mainboard) about 1" above the docking pins.
I don't know why they chose to do that. It is possible to replace the jack with a 4-pole and reroute the top microphone through it so it behaves as you would expect, but it's not a simple hack and it requires a willingness to shred your warranty and the equipment & skill to work with small SMD parts and flex-cable. Also, physically fitting and securing a 4-pole jack will probably involve epoxy and a dremel in addition to soldering little jumper wires.
The Realtek ALC5642 - the (micro|head)pho
I suspect the second microphone strips out ambient noise, such as the noise of the speakers - which it is much closer to - when the top mic is being used... that would be an easy way to prevent feedback, but it wouldn't be necessary when using a headset w/microphone. If it doesn't cause any problems I'll leave it alone so everything works the same without headphones.
I'll be tearing into my N7 next week to experiment with this stuff... I'll post results when I have them.