Play/pause will usually be connected directly to ground. The other functions connect to ground through different impedances. While each impedance level is roughly twice the last one (which makes the detection circuit simpler) having one level at 1 ohm and the next at 2 ohms is not practical due to noise. Also, due to the way that parallel impedances work, you do not get a binary encoding of the buttons pressed. (Do you ever really want to skip the track and increase the volume at the same time anyway?)
Microphone impedances start around 500 ohms, so if you go higher than that, you can't distinguish between a button and a working mic.
The current stackexchange question does not use the phrase "single wire" and more importantly nothing about your headphones will work with a single wire.
Its pretty common in the electronics world to not count the ground as a wire in a communication protocol, only wires carrying data are counted. For example the "1-Wire" inter-IC bus create by Maxim.
I tried to find some diagrams but was unable. Maybe this will help me.
We need a robust 3.5mm jack short cord extention. Something that would be robust in built but protect oryginal jack the same time. I think plenty of people that appreciate quality music on the go would chip in for such a cord. I would definitely give $15 to protect jack of my high quality headsets. If someone starts a kickstarter on this, I will pledge.
The details of the implementation aren't disclosed, but people have seen a whiff of it, e.g. here:
http://superuser.com/questions/107378/what-is-the-apple-mike...
Probably not used in headphones, but it could be.
