This idea popped up when I was surfing Taobao for a cheap condenser microphone for field vocal recording and encountered with one made for iPhone 4, the TASCAM iM2.
The problem is that this little gadget only has a iPhone 30pin Dock port. I googled a lot for a converter cable, with no promising results. One I bought from Taobao does not work either.
More googling reveals that the pin definition of the 30pin Dock port is public. I then disassembled the not-working cable which only consists of a little adapter board (the green one with a jack).
I got the definition from the site allpinouts.org, but how could I figure out which pins should I connect? Well, as the iM2 is a high-speed audio device, it could not be using the serial interface, which only supports up to 38400bps data rate. It cannot be using the line in either as the line in is not digital :) Finally the USB interface becomes the only option.
It seems that the accessory is powered not by the USB + port, but the 3.3V accessory power port. Thus I picked a DC-DC converter module to provide 3.3V to the accessory drawing power from the USB +5V supply. The schematic look like the following:
Though I am using a DC-DC converter, you can easily replace it with a cheap LM1117-3.3 linear regulator.
Insert the cable into the Mac, and see the magic happen!