How could you transfer data over an audio line?

I'd like to pass some data over an audio cable (i.e from audio card to my cellphone) with a short cable.

I'm looking for some ideas for algorithms for achieving this. They of course should include the proper modulation technique to transfer the data over audio signal, so the cellphone could interpret it over it's microphone jack.

Ideas? ;)

(P.S - my current idea is using Biphase mark code. is it any good for this?)

Thanks.

Answer

Here is an example project using FSK: http://sree.cc/electronics/arduino-as-an-fsk-modem

An FSK modem basically convert a digital signal into analog signal or sound signal by frequency shift keying technique. Here we can make our Arduino as an FSK modem by uploading a simple code. This project produce and play a sound corresponding to a digital signal and demodulate a sound signal into the corresponding digital signal.