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Does anyone know what the criteria for a MacBook switching to external mic input is?

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  • Does anyone know what the criteria for a MacBook switching to external mic input is?

    Hi there,

    Teaching a lot online at the moment, as the situation prescribes, I've been trying to simplify my setup (my good audio interface doesn't work with the platform my school has chosen, another to hand is outdated and causes the OS to lock up as not officially supported any more)

    I'm trying to avoid buying a newer simple interface just for this period!

    I'm on a 2013 MacBook, the analog audio port is trrs, ctia standard I THINK.

    I made a simple splitter that connects tip, and ring 1, as L and R audio (that works fine) and has a mic input connector on the sleeve and 2nd ring is ground (as per ctia standard).

    The MacBook recognises an external mic has been attached if I attach a headset with mic.

    If I connect my splitter, using a desk to feed what should be the mic in on the sleeve connector, then the Mac doesn't recognise a change and the internal mic is still selected. (external input not available as a choice)

    Does anyone know; firstly, what conditions the MacBook headphone port is expecting to allow receiving audio on the line-in mic connector. Maybe seeing a specific impedance or similar which connecting it to my desk is not giving..

    or secondly, if I can use a terminal command to force the Mac to switch to the external (ie, wired input on the headphone jack connector) source for input..

    There are many YouTube videos on the matter, but they all involve buying a splitter (which I'm happy to do, but how is that splitter wired? And surely I could replicate it!) and then often bodging around with a trrs to trs convert for one of the inputs. In short something I'm sure could be navigated easily if the conditions are known, and hard to discern with some guy plugging in a few random converters until it shows up as working with no clear (real/electronics) logic!

    any info gratefully received!

  • #2
    This is an old article but may apply to your model Macbook.


    • #3
      Hey man!

      Thanks for the time taken to reply, unfortunately they removed the software 'option' in later iterations of the OS. Now it's all automatic...

      But! I found the solution! I had tied the ground connector on my audio-source (a desk) to the input's ground. But on measuring the mic/ground resistance on a headset I have lying around it was a huge resistance, 10M or so, so got me thinking, perhaps the mic circuit on a headset doesn't reference the connectors ground, or at least it's expecting to see a very large resistance to be detected. So I removed the ground connection, so my hot 'mic in' signal is just floating, and the computer detects it as an external source and seems to work!

      Edit - ok, it detected once but the audio in was high passed, now it won't detect again, oh well, may leave this one
      Last edited by OwenM; 06-13-2020, 12:59 AM.


      • #4

        Ok, I should have guessed it was a bit more complicated than what I initially hoped, particularly as the headset has volume and track skip controls on it too. So there must be more going on that just L, R, Mic, Ground.

        Theres plenty of random schematics for headsets floating around, it seems the mic input needs to see some capacitance to recognise a mic is plugged in. Theres lots of people trying to create their own receiver for headset trrs, but not so many that have a receiver and want to make a 'headset' out of other devices, ie a little system that looks like a headset to my computer trrs input..

        I haven't found any schematic that looks much like what I want to achieve. Or at least, I've found plenty that are what I have already tried but haven't worked, and none of the more complex designs are applicable to my situation. Ie

        Click image for larger version  Name:	image1.jpg Views:	0 Size:	20.4 KB ID:	907319
        Simple, and includes controls, but basically what I have tried already

        Click image for larger version  Name:	HTCHeadsetSchematic.png Views:	0 Size:	135.2 KB ID:	907320

        Maybe closer to something useable...


        • #5
          I did a bit more digging and certain Macbook models detect an external Mic (from an Apple headset) when a 2K resistor is applied between Mic In and Ground. This switches off the internal Mic.

          I drew up a schematic of what I think will solve your problem. I've inserted a 15dB pad to adjust signal levels, and a blocking capacitor. Give it a try!
          Click image for larger version  Name:	Apple-Audio-Interface.jpg Views:	0 Size:	20.7 KB ID:	907323
          Last edited by Wal_zz; 06-13-2020, 04:59 AM.


          • #6
            Originally posted by Wal_zz View Post
            I did a bit more digging..
            Thanks very much as ever! I had a go at it, and it's intermittent, sometimes it works but the audio in is high-passed, sometimes it just buzzes madly on the input, it locked up the audio software a couple of times. At one point it briefly sounded like it was passing correct, full range audio! So maybe it's possible

            At the moment, despite restarts and plugging in in different orders it's not recognising, though my headset still works fine so the software hasn't crashed. Mic to ground on my headset reads about 7Mohm, and you can get it to read as a diode biased ground (anode) to mic (cathode). Probably impossible to discern anything new from just that but theres some further circuitry going on in the headset thats for sure!

            I tried upping the mic-in to ground to 7Mohm to mimic my headset (didn't work), and also put it down a bit lower to 1.5k (same behaviour)

            EDIT - Well, after a bit of fiddling around it seems to work 4 out of 5 times, as long as I leave the desk unplugged until the external input is detected.

            However, it sounds like the audio is intentionally high passed and has noise reduction on it, so not ideal, but a good learning process!

            I'll have another go tonight and see if I can get it any better
            Last edited by OwenM; 06-13-2020, 11:21 PM.


            • #7
              Sounds like you have a 'ground loop' problem. An audio isolation transformer between Mixer Out and the Mic port on the laptop would solve that.
              Something like this would do the job..
              600 Ohm line isolating transformer

              Click image for larger version  Name:	600R-isolation-transformer.jpg Views:	0 Size:	20.2 KB ID:	907446

              Also came across this article about resolving issues with Mac audio interfacing that may hold a few clues..
              Last edited by Wal_zz; 06-14-2020, 07:39 AM.


              • #8
                If using the headphone port is still problematic I'd purchase an external USB sound interface. While a cheap and cheerful USB dongle costing a few pounds *may* do the job, I suggest something like this.. ( 31 with free shipping, from Amazon UK )
                Behringer UCA202 2 In/2 Out USB/Audio Interface

                Click image for larger version  Name:	UCA202-USB-Audio.jpg Views:	0 Size:	25.4 KB ID:	907471
                Last edited by Wal_zz; 06-14-2020, 08:34 AM.


                • #9
                  Wal_zz Thanks a bunch as ever (look at me, getting down with the new forum features!), I've actually got a really nice Lundhal transformer that I tried to make into an in line isolator about ten years back. Looking over it I have no idea what I was doing, so I'll take that out and give it a go.

                  Thanks for the bits box link though, haven't came across that site yet and just looking around for a minute I found loads of little useful things that would be buried in the pile of Farnell or mouser etc, at least if its me looking anyway! Thanks for the Sweetwater link too! Resetting the PRAM and NVRAM is like the 'turn it off and on again' cure all for a Mac in many cases, so might give that a go too.

                  Yeah, I've considered just buying a newer interface. My reluctance is down to this only being for 5 more weeks of teaching online. I already own a very nice Metric Halo ULN which works fine on everything but the teaching program. I have the schools focusrite Saffire's which also don't like the teaching software at all. I'm using the Mac version of a mainly windows focused program and I'm not sure if it's a core audio issue, or just down to my interface needing the 64bit upgrade (400, so not for now!)..

                  So I think, if the transformer doesn't work I'll just go back to using the built in mic and headphone output and adjust my lessons. The whole reason for starting this is simply that the teaching software doesn't allow me to stream audio during lessons, ie 'listen to this by Beethoven' etc. And I wanted a way to use a mixer so I could combine my mic, a guitar, and some kind of music player and send it down my audio stream to the students... So not a big deal all in all! It seems apple really didn't want this to be called an audio input, its not documented that way anywhere, I think they want it to only be thought of as offering headset functionality.

                  Thanks for all the help!


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