Pictured to the right is a Cowon iAUDIO E2 music player. I have one, but I’ve lost the proprietary PC/charging cable long ago. It’s a really neat little device, so I decided to figure out the pinout.
The cable is really weird: it’s a passive USB A to 3.5mm 4-pin barrel jack cable. Pretty sure that violates every possible part of the USB specification. The one good thing is that it required the manufacturer to insert all kinds of protection circuits—just think of what happens when you plug it in!
Anyway, I made a handy diagram.