I’d like to have a portable setup that I can use to record conference calls for work, in particular I want to cover the cases where I want or need to use headphones.

I often use more than one kind of setup I work off of, for example some possible combos for the headphones use case:

Headphones w/mic TRRS 3.5mm —-> Apple TRRS-USBC adapter —-> iPad Pro
Headphones w/mic TRRS 3.5mm —-> Apple TRRS-Lightning adapter —-> iPhone
Headphones w/mic TRRS 3.5mm —-> Macbook
Headphones w/mic TRRS 3.5mm —-> TRRS to TRS mic and TRS phones for PC front panel —-> Desktop PC running Windows or Linux

I’d like to have a device that sits at the position of the first arrow which splits the TRRS signal out into one 3.5mm output which I will plug into the recording device.

So the TRRS is dealing with 3 channels: L, R, and Mic with a shared ground (4 wires). My main goal is to achieve the mixing of these 3 channels together into either a single mono signal or to mix the mic channel evenly on top of the L and R, as both would be acceptable, or I suppose if I could somehow find a recording device that takes this kind of 3 channel input then I can get an off-the-shelf TRRS splitter and be done with it.

The question is, assuming I only have a simple run-of-the-mill audio input device to use and I want to record my mic input mixed together with the headphones output into that device, what type of (portable!) product would I need to use to achieve this?

  • Something that strikes me as a possibility without requiring additional hardware is to assemble a “special” TRRS splitter which routes the mic into the Right channel, so that the recorder will receive the Left channel as normal and the Mic on the Right channel, dropping the headphones’ right channel entirely. This is electrically sound and should get the job done as well, but I am thinking that if a product exists that is not bulky that can do proper mixing, I’d prefer that. – Steven Lu Jul 8 '19 at 16:18
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    The main issue is that you are dealing with massively different voltage & impedance structures on input vs output. There are software solutions that would have no trouble with it once you were past the DACs & purely in the digital domain, but idk of anything to do that in analog. [or, not that would fit in your jacket pocket] – Tetsujin Jul 8 '19 at 16:20
  • @Tetsujin could you help me out and comment a bit more on the differing voltage and impedance on input vs output? Does that mean that my comment is also completely off-base? – Steven Lu Jul 8 '19 at 16:37
  • tbh, I don't deal with the 'mechanics' of sound engineering too often, I deal with it once it's on the board/computer, but very roughly a line out for a headphone or amp is maybe 0.5 - 2 volts. A mic is going to be more like 50 millivolts. It would be like throwing an ice cube in a pan of boiling water. – Tetsujin Jul 8 '19 at 16:51
  • Yes, i see, thank you. I now understand that I can make a suitable circuit for adjusting the line level signal into something suitable for a recording device expecting mic level input by using 3 resistors and one capacitor, which is not ideal but also not impractical to make it portable. Also I have learned that sometimes the mic plug has a DC voltage on it (thats what the cap is for). – Steven Lu Jul 8 '19 at 17:01

It's easiest to do this digitally. For example, on Mac OS, you can use Audio Hijack to record the audio streams. Similar software is likely to be available for your other platforms too.

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  • This software looks to be more powerful than I need. I was able to implement something that seems to work by creating an Aggregate Device (which fuses the corresponding input device and the Built-in Output) using the Audio MIDI Setup app available in OS X/macOS, then using QuickTime Audio recording with that device selected as input. The only thing about this is that if I reduce the system volume to 0 the recording will not catch it. But I believe that this is the correct behavior anyway! – Steven Lu Jul 25 '19 at 3:36
  • Ah, crap. So, the Aggregate Device is great with built-in output when no headphones are plugged in. once headphones are plugged in, the computer's speakers are muted, and so is this Aggregate Device. – Steven Lu Jul 25 '19 at 3:42

This is too impractical to do digitally under all cases because sometimes I want to record a phone call and sometimes a call within software running on a computer. In particular it does not work well when I would need to set up a convoluted software setup on each device that this applies to.

The solution appears to be a 4-channel audio recorder, where the stereo output from the computer can be split into my headphones and the recorder line-in, and being a 4-channel, the recorder's own mics would be able to also record simultaneously. This has several benefits: It does not have to interfere with the TRRS 3.5mm headphone/mic setups, and it captures my voice in high quality as well as the sound directly from the device, and splits it into separate recording channels, which can be useful as I can easily perform the mixing after the fact.

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