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I have two Macs on the desk and want to use one set of microphone/speakers with both. One machine, a Mac mini, has a Focusrite audio interface (Scarlett 8i6, love it) connected by USB-C and that has an XLR microphone and powered stereo speakers connected. I want output from a nearby MacBook Pro to play on the main speakers (not its internal speakers) and to send microphone output from the audio interface to the MBP as input (instead of using internal microphone).

No problem getting output. A simple stereo - double TS (mono) cable lets me connect it to the Focusrite box as one of the inputs. I can mix levels in the Focusrite application, plays on main speaker outputs. Sounds good.

Microphone output from the Focusrite is mono. A TS-TS cable connects the output to the mic input of the splitter cable. Doesn't work!

Here's the wiring of my splitter cable - it follows "iPhone headphone" in the picture. Output cable, plugged into the headphone jack of the splitter, follows "Standard stereo headphone". Ignore "iPod AV Cable", it just happens to be in the picture:

TRRS Splitter cable wiring

That's verified by meter testing. As soon as this cable is connected MBP lets me set output to "External Headphones". However I never get the option "External Microphone".

The output from the microphone is good, if I connect it to the microphone input of a nearby PC no issues at all, good levels, clear audio from microphone. I just never get the option to select it on the MBP.

Just what is the input on a MBP 15,1 for if not for this?

Is the Mac doing some tricky resistance or 1-wire interrogation of the connected device and refusing to allow anything but a device that gives the right answer?

If I had a set of TRRS AirPods with included microphone would that microphone work? Does anything work as input on MBP?

I really don't want to have to use another USB interface for this.

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  • If I'm understanding correctly [& I may not be, 1st coffee of the day;), you've got a TS plug in the Mac's headphone socket to use as the mic input - that's not going to work for mic as you're shorting mic to ground. You need a TRRS with the last RS connected. I'd say this is a DIY soldering job, TS to [TR]RS. – Tetsujin Nov 12 '20 at 8:33
  • Yes, it was a soldering job - cut the plug off a splitter that was not wired as the above, and made it so with a new TRRS jack. The splitter is mono (mic) + stereo (headphones). If the jack is TR1R2S the splitter is mic = S + R2 (common), headphones = R2 (common) + R1 (right) + T (left). I did have a driver installed "Crestron Audio" and once I deleted it I saw an external mic. I still think levels are far too low so would not quite call this solved but there is progress. – Adam Eberbach Nov 12 '20 at 21:40

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