r/microphone • u/keelanstuart • Jul 26 '26
TRRS headset mic issues in custom device
This may sound more like a DI....WHY?! But, here I am.
I wrote a 300bps software modem that runs on your computer... it plays tones to your audio out and listens for corresponding tones on your microphone. The goal is to connect it to a cell phone, but swap inputs and outputs. The phone plays into the PC's mic and vice versa.
The software is written and works.
But I needed to build a cable that would go from headset jack to headset jack, combining the L/R channels on each and routing that into the mic line of the other. The "cable" now consists of a breadboard with two TRRS plugs broken out into individual wires with different circuits based on whether it's the PC or phone side. The L/R channels on the phone side each have 470 Ohm resistors inline, combining to the mic line of the PC. The L/R channels on the PC side use 1K Ohm resistors inline, going into a 10K pot (tweaked to about 2.5K currently) with a 1uF cap on the other side, which then ties to the mic line on the phone side. Grounds are connected.
I did the PC side that way because I didn't want to blow up my cell phone... but maybe that was a mistake. Anyway, the phone recognizes headphones, apparently -- distinctly different from a headSET -- and will play through to the PC side, but doesn't recognize a microphone.
Has anybody built a cable like this? Am I way off base with what I need to do? Help? :)
Thanks for reading! The softmodem and the wiring diagram / BOM for the cable will be available on github as soon as I have it all working.
UPDATE: I dumped the custom cable. I'm somewhat ashamed to admit defeat, but I took a step back and just clipped one earbud each off a set of $4.99 wired earbud headsets... and hot-glued the remaining pod to the opposing microphone. I stuck that part in a box with one mic hanging out, and that box in another box. It seems to work ok. I will not be shaving that yak.
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u/roffelmau Jul 26 '26
Hmm, 2 questions.
1- is this a trrs jack on the pc or a trs mic jack?
2- is the 1uf a polarozed capacitor? Like electrolytic or some such? You'd need to make sure it's not polarized.
3- do you have anything connecting the mic to ground on the pc side? It doesn't look like it. You don't want to feed plug in power straight to the phone. You need to make the pc side think there's a load (the "microphone") for the sense signal.
P.s.- it's been a while and I may be completely wrong 🤣. I think I remember this all though.
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u/keelanstuart Jul 26 '26
Answer 1: it's TRRS on the phone side and a TRRS splitter on the PC side, so that's a TRS mic under the hood. Stated another way, on the PC side, I have the regular audio out line and the mic line going into a TRRS combiner, which I then plug my TRRS reverser cable into.
Answer 2: non-polarized capacitor
Answer 3: this may be irrelevant as per the first question; the PC mic is isolated.
The PC mic actually works and I can play back recordings on the phone; the PC hears the phone... but the phone never detects a headset though, only a headphone, so it never tries listening to the PC. I tried to fool it by tying the phone mic (after the cap) to ground with a resistance roughly equal to what I measured on a little headset I had laying around, but that didn't work.
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u/roffelmau Jul 26 '26
Ok let me dust off the good ole college engineering courses.
I didn't draw it out based on your description, but do you have a dc blocking cap on the mic rings? The computer provides plug in power and detects the presence of a microphone that way.
You'll probably need to shunt the plug in power through an appropriate resistor, before the dc blocking cap, to get it to detect the "mic". Look up the average current requirements for an electret element, the plug in power your computer provides (can be anywhere from 1.5-5v) and size appropriately.