I got interested in this after I saw SquareUp use the headphone jack on the iPhone to send credit card data.
What's the average bandwidth of the headphone jack on the iPhone, average notebook, and average mobile device?
Can it be doubled by sending different data streams on the different channels (left/right)?
One issue is the bandwidth of audio cables, which I won't go into here. As for audio ports, assume a soundcards with a maximum sample rate of 44,100 or 48,000 samples/s at 16 bits/sample/channel, resulting in a maximum bandwidth of 22.05 or 24 kHz (basically a result of the Nyquist-Shannon sampling theorem, though for sound sampling, the sampled signal would also have to be continuous-amplitude for this theorem to apply) and a transfer rate of 176.4 or 192 kBps for stereo.
According to Studio Six Digital, the line-in on the iPhone supports a max sample rate of 48 kHz. The mic on the 3G version also runs at 48 kHz, while the 1st gen iPhone's mic sampled at 8kHz. I haven't been able to find bit depth specs for the iPhone, but I believe it uses 16 bit samples. 24 bit samples is the other possibility.
According to Fortuny over at the Apple forums, who was quoting an Apple Audio Developer Note, the line-in on a MacBook support up to 24 bit samples with a 96 kHz sample rate, for a data rate of 576 kBps. Apple's MacBook External Ports and Connector's page lists the max sample rate as 192 kHz, but they may have switched that with the max sample rate for digital audio using the optical port.
For a rate comparison, phone systems had a sample rate of 8 kHz at 8 bits/sample mono, resulting in a max data rate of 8 kbps. FM has a sample rate of 22.05 kHz at 16 bits/sample/channel and is stereo, resulting in a data rate of 88.2 kBps.
Of course, the above calculations ignore the problem of synchronizing the data stream and error detection and correction, all of which will consume a portion of the signal.
20Khz is pretty much the max on any circuit intended to carry audio, because it's pretty much the top of the human ear's frequency response. Given the Nyquist limit, you're probably looking at 10Kb/sec tops. Of course, Back In The Day(TM), we though 9600b/s was high speed, so it might be good enough. And yes, you could double it using stereo output.
Typical audio device maximum is 48Khz stereo, lots of devices can handle 96 Khz.
But course what comes out of the headphone jack is analog, not digital, and it runs through some filters as well on the way out, so some sort of tone modulation is the way to go. There may be some crosstalk between the stereo channels - how much crosstalk will be very device dependent.
0ld style telephone modems could send 9600 baud over standard analog lines that aren't even as clean as your typical headphone jack. And that's MONO. I would think you could get 2400 baud per channel without working too hard.
You might be able to go as high as 100K baud if you were very clever at signal processing. Credit card validation systems were designed to run at 2400 baud mono last time I looked at them, It wouldn't surprise me if they still were given how much inertia there is in point of purchase systems.