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This question already has an answer here:

An audio port on a MBP is a combination output. If you plug in 4-conductor 3.5mm connector (TRRS) like the one on Apple earbud with microphone, it appears that that port can be use as an input.

I just bought a bare male TRRS audio jack. (without cable, it is for soldering your own) When I plug just that into the MBP, the MBP did not change the output to "External Microphone" as when I plugged an earbud with microphone to it. So I am wondering how did the hardware detects the mic? Does it communicate with the other end by some means?

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marked as duplicate by dim, Passerby, Daniel Grillo, JRE, brhans Oct 9 '16 at 1:49

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  • \$\begingroup\$ It might measure the load on the jack. If it was an unassembled connector its still an open circuit. \$\endgroup\$ – Wesley Lee Oct 7 '16 at 11:53
  • \$\begingroup\$ This is speculation, but it is probably the low impedance of the microphone compared with the infinite impedance of nothing or just a plug with nothing connected. Also, unlike ear-buds, there are dramatically different types of microphones which likely have very different impedances. So some may work and other not. \$\endgroup\$ – st2000 Oct 7 '16 at 11:55
  • \$\begingroup\$ Other Apple devices expect 1.6 kilohms between Mic and Gnd (oddly, with Mic being the sleeve and Gnd being the inner ring. Perhaps the Macbook Pro is the same? \$\endgroup\$ – Brian Drummond Oct 7 '16 at 13:47
  • \$\begingroup\$ Dupe because Apple uses the same tech/detection in both the I devices and macs. \$\endgroup\$ – Passerby Oct 7 '16 at 15:26
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The phone senses the impedance between the 2nd ring and the sleeve. Plugging in a "raw" connector is no different than nothing plugged in at all. There is still practically infinite impedance between the 2nd ring and the sleeve. That is why it doesn't detect anything.

Note that if you insert a traditional TRS plug into the jack, the sleeve will "short out" the 2nd ring position. That indicates to the phone that earbuds (or headphones, or interface cable of some sort) has been connected. If you connect an electret microphone capsule between the sleeve and the 2nd ring, then the phone should recognize it.

Note also that earbud/mic gadgets with remote-control buttons use the microphone connection to temporarily insert different resistors into the mic circuit. The phone monitors the impedance and when it sees it change, it identifies the button press and proceeds to take the appropriate action (skip, volume up/down, etc.)

Ref: https://en.wikipedia.org/wiki/Phone_connector_(audio)#PDAs_and_mobile_phones

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I don't know Mac specific but generally it works by the rings, if you notice the microphone headsets have an extra ring in the plug, this is for the microphone.

The name actually explains itself, a normal headphone jack is a TRS (tip, ring, sleeve) and a microphone is TRRS (tip, ring, ring, sleeve), so it has the extra band in the jack.

When it is headphones only, the last ring and sleeve are either shorted or open (depending on whether you have a plug in mic option on the headphones). When it is mic and headphones the last ring and sleeve have an impedance of the mic.

SO to answer your question, without soldering anything to it there is no change from something being plugged in or not in the eyes of the MBP.

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