How Square implemented data transfer to iPhone?

Some days ago I saw a interesting device for iphone, square, here: https://squareup.com/

you can plug it into iphone's earphone socket, and it can transfer data to iphone. A running App on iphone can receive it.

does any one know how it implemented? I guess it can encode data to audio stream and "sing" it, and Ap开发者_如何学JAVAp on phone can record the sound and decode it. but how to? is there a protocol or SDK?

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The implemention is likely to be no different to that of a simple acoustic modem. The relevant APIs include Audio Units (low-level) or Audio Queue Services (higher level).

Matt Gallagher has written an excellent (as always!) post on creating an iOS tone generator, which is one way of enabling what you are after.