I'm working on an audio application for mobile phones. At this point in time I'm trying to measure audio latency of a mobile device by feeding colored noise into its microphone jack and having it play it back through the headphones, and correlating/deconvolving the two to obtain the time delay.
This all sounds great on paper, but my problem is that the phone will only accept inputs of certain impedance as microphones, or at least that's my guess. I've tried several microphones (actual transducers), and some of them worked while others didn't. As a rule of thumb, headset-like mics worked. Vocal-like dynamic mics didn't. I'm guessing there has to be enough bias current for a microphone to work. When simply taking an audio cable from the output of a computer or MP3 player and putting it into the microphone jack of the mobile phone it never works. What I mean by that is that the device does not recognize presence of an external microphone and does not switch to it as default input.
I don't know a whole lot about current biases and such, but do you suppose there's a way of building a circuit to somehow "trick" the phone into thinking that there's a microphone plugged into it when it's a line in instead?