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I built an audio 4 connector 3.5mm splitter. The goal is to play a sound clip through a phone's mic input across a call, while recording the same sound clip on the computer. I used the TRRS configuration of: Tip = Left audio, Ring = Right audio, Ring = Ground, Sleeve = Mic. This is apparently the configuration for an Apple headset. The recording part works on most phones, but the playing of a sound clip into the mic input of the phone only works on a select few. By the way, I am connecting speaker output on the one side to the mic input on the other side.

I realized that each phone has different impedance requirements. How do I adapt the cable to work on any phone? When I do impedance matching, how do I get to the total value of eg. 23 Ohm specified in the technical specifications of headsets? Is there something I can buy that will do the impedance matching for me or should I build it myself?

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If you have a sound-clip why would you want to record "the same sound clip" onto your PC irrespective of whether you feed it into the mic or not? Is this part really relevant to your question? Is your question really more like "why do some phones not transmit a sound clip outputted to the microphone connections"? Maybe I'm being stupid? –  Andy aka May 16 '13 at 14:17
Yes that could be true. We are not only recording the sound clip, we are recording the noise across the call. We can record the call on the output side as well. The recording part works, but not the mic. So yes the question could be more like "why do some phones not transmit a sound clip outputted to the microphone connections" –  user23984 May 22 '13 at 6:58
I think you should edit your question to raise the question directly about some phones working and some not, unless there is another problem. BTW it's not impedance matching that is the issue - it maybe that you need to connect to the microphone via capacitors on each wire you use. –  Andy aka May 22 '13 at 7:05

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