The Firehouse

Until the keynote a few days ago, the only remote control available on the iPhone over music playing was the microphone button, which could be used to play/pause or advance songs. The new $79 in-hear headphones feature volume up and down, apart from the center button that now adds a three-click function, which goes back one song.

How is all this done over a normal four-pin connector? On the “regular” headphones, two conductors carry the left and right audio, one the microphone input, and the fourth conductor is ground. Since the electret microphone used in the headset is fed a voltage bias, the phone can detect if the microphone line is shorted to ground. In this case, a single digital line into the main microprocessor will detect this short and trigger an interrupt.

If we now want to send more than a single click, such as volume up or down, a series of resistors of different values are placed in series with each button. When you press a button, the voltage drop across the resistor can be measured by the microprocessor, and thus the particular button function executed – with two conditions. The input line must be analog, not digital, thus capable of reading voltage variations, and the microprocessor’s firmware must be adapted to detect changes in voltage, rather than a digital 1 or 0.

Does the iPhone, or first-generation iPods have this arrangement? It’s hard to tell, but my first test on a Touch with firmware 2.1 doesn’t look promising. The center button on my V-Moda headset is not detected at all. This could mean that the firmware has been adapted to detect voltage changes, and so a remote with resistors is now needed. In any case, there either is no input at all in first-generation iPod Touch, or there is but has been configured to detect particular resistance values. I’ll have to wait to get my hands on one to really find out.