How does Square Reader (squareup) device work via earphone jack in Android? -

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https://squareup.com/reader describes hardware card reader plugs earphone jack on android (and iphone) too.

i wonder how square app reads data earphone jack. possible send custom data via earphone jack, or kind of audio decoding (like how dial-up modem works)?

square uses regular audio signals, in same way modem does.


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