title: Bandwidth from headphone/microphone jack
body: <p>I got interested in this after I saw Square use the headphone jack on the iPhone to send credit card data. </p>
  
  <p>What's the average bandwidth of the headphone jack on the iPhone, average notebook, and average mobile device? </p>
  
  <p>Can it be doubled by sending different data streams on the different channels (left/right)?</p>