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When I plug in my Apple EarPod earphones, my phone sets the volume to whatever it was the last time, but when I plug in the auxiliary cable in my car (just a simple jack, no fancy infotainment system) it puts it to full volume, like the last time I plugged in that cable. Is there something special about the Apple EarPod cable? Or does the iPhone do something even cooler?

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An line-level input, which is what your car's aux in jack is, presents a relatively high impedance to the source - generally 10k ohms. Your 'phones have a much lower impedance, typically 32 ohms for PMP phones. It's possible to distinguish between them in that way.

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    The EarPod earphones also have a mic and remote connections, which will differentiate them from a normal stereo jack. – heavyd Apr 8 '16 at 14:12
  • That works too, but would not allow distinguishing between ordinary headphones (with a TRS plug) and a standard TRS patch cable to a line input. We don't know if the iPhone in question can actually make that distinction, but I do know that the impedance method is what works for the most cases. (muttered: Though it would be just like Apple to use something that would only do that trick with their own branded earset.) (And you meant "...from a normal stereo plug", not "jack". The things with holes are jacks; plugs fill holes. If you have a hole in your boat you want to "plug" it, right? ;) ) – Jamie Hanrahan Apr 8 '16 at 18:38

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