How does Square Reader (squareup) device work via earphone jack in Android?
<p><a href="https://squareup.com/reader" rel="nofollow">https://squareup.com/reader</a> describes a hardware card reader that plugs into earphone jack on Android (and iPhone) too. </p>

<p>I wonder how the Square app reads the data from the earphone jack. Is it possible to send custom data via the earphone jack, or is it just some kind of audio decoding (like how dial-up modem works)?</p>
