Playing around with my iPhone 3GS I discovered that the mic portion of the headphone jack is active when video recording. If you have your iPhone headset, with built in microphone plugged in, the audio recorded is from the mic on the headset, not the one on the iPhone itself.
That got me to thinking. It should be possible to put any audio signal, even a professional microphone signal, into the iPhone. I began to look for the wiring diagrams and such for the iPhone plug.
Today, I ran across a post about using a standard AV cable from a camcorder. Plug the cable in, and the red RCA cable becomes the mono audio input. The other two RCAs may even pass audio signal back. So I tried it.
Let me apologize for the lack of detail in the video. It exists because you have to record it when testing. Later I will create a more in depth look at how this works. You are looking at the back of a mixer that happened to have RCA record outputs. I plugged the red cable into one of those. Initially, the audio is from the mic on the iPhone, then I plug in the cable and, the music is recorded. Very short, very simple, but sort of a proof of concept.
The audio level is very hot in the recording. The iPhone mic is looking for a mic level input, and the signal I was sending was at line level. Even pulling the level way back, I am on the edge of distortion. I tried using the meter on the voice memo app to set levels, but the difference was too great. This signal on that meter showed to be well below distortion. But as you can hear, it was red lining in the camera app.
But, the experiment brought me one more step closer to an XLR input for the iPhone. It should be able to handle a normal mic level input. Now I know that an AV cable can be rewired to serve as the plug for the mic input. Now to work on a transformer for a balanced audio signal to an unbalanced one.