Connection headphones and microphone to Lenovo T540p laptop's 4-pole jack
It seems that there are basically two different pinouts of 4-pole jack: Nokia and the rest of the world.
Lenovo audio pinout
Nokia audio pinout
I have tried both wirings and both of them generates lots of noise to headphones (no noise when 3-pole jack
headphones are used). It looks like stupidly designed HW or some bug in audio SW for me.
I had an idea, that I can withstand the noise during rare events I'm using the mic, so I can short the
mic the same way as 3-pole headphone jack does when I'm using just headphones. So I have soldered the
parts together (using Lenovo pinout) and found, that the noise is still there! So the noise is not something
induced into the input, but something internal.
When it has been connected with shorted mic, everything seemed to be working properly. But then I have found,
that the laptop is using internal mic and ignoring the headphone one.
The progress is wonderful thing. I have wasted a few hours with thing which has never occured on computers
with just two jacks - one for mic and another for headphones. And nothing is solved still!
The saga continues
My friend pointed out, that I can check which pinout is used in my Lenovo just by measuring resistivity against
laptop's casing. It looks like good idea, but Lenovo is tougher, than it looks like. Both mic inputs has something
between 7 and 10 megaohms against the casing. Considering the fact, that at least USB and VGA casing should be
grounded it seems, that both sides of the mic are on Hi-Z inputs.
When mic-boost in alsamixer is set to 0 and mic around 80, the noise is acceptable. With mic on 65 the noise is
not hearable in headphones. The recorded sound is not very loud, but it's understandable.