1
\$\begingroup\$

I'd like to create a serial cable link library between two devices called Playdates. They have a TRRS audio jack, and my idea is to use a few cables as shown below to connect each Playdate's audio out into the other's microphone in, allowing for a full-duplex connection, and the creation local multiplayer games like the Gameboy Link Cable.

Audio jack configuration

"L" is left audio channel, "R" is right audio channel, "G" is ground, and "M" is microphone. Those with 4 letters are TRRS, and with 3 are TRS. I don't know why TRS mic jacks have an "M" tip and ring however; perhaps for stereo mic input?


I don't know much and have only recently found out about TRRS vs TRS, and the above diagram is made with my understanding of how TRRS to TRS splitters and mergers would work.

Before I begin development, I have a few questions to make sure this isn't an impossible endeavour, and to make sure my approach actually just makes sense :)

  1. Would the above arrangement work from a connecter perspective? Are such TRRS to TRS converters made in a way so the connectors would line up as in my diagram?

  2. Would this work from a device hardware implementation perspective? While researching around I saw that jack microphones work by applying a resistance and detecting small changes (somehow), so if one device "played audio" from its jack (whatever that actually means), could the other device pick it up as though it were a microphone input?


Some additional relevant info/resources:

  • When programming the Playdate, you only have access to the microphone through their C API or Lua API, which means I'm limited in what I can do with the input/output data. The C API for mic gives you a stream of sample values, so there is some level of processing happening between the hardware and software which I'm unable to intercept (afaik).
  • iFixit teardown of Playdate
  • What I think is the headphone chip for the Playdate, from the above iFixit guide

Thanks for your help!! Software is my stronger point so this aspect of the project is where I need the most help, so your time is appreciated :) - Dan

\$\endgroup\$
2
  • \$\begingroup\$ just wire LRGM to LRGM directly ... what would be the point of adding multiple intervening contacts? \$\endgroup\$
    – jsotola
    Commented Apr 29, 2023 at 19:09
  • \$\begingroup\$ @jsotola I want to allow other non-technical people to play games via this cable link method, so they can just go and buy the adapters and plug-n-play rather than needing to create their own mic-headphone jack swapper cable. but from what Justme said in their answer below, it doesn't seem like this will work regardless ;-; \$\endgroup\$
    – dwb
    Commented Apr 30, 2023 at 7:35

1 Answer 1

0
\$\begingroup\$

If the connectors have only standard headset audio, that is really not a serial link cable, just audio cable.

From connector point of view, you can connect them in any way. But from signal point of view, your suggestion will be shorting L and R outputs directly together to mic input, which is not really recommended.

The audio output amplitude is also much larger than typical microphone input amplitude is, so directly connecting audio output to mic input may have too large amplitude, may cause damage or just won't work.

Generally crossover cables may bring down amplitude with resistors, and have proper biasing to fake a microphone so common devices can actually detect that a proper mic is connected.

Perhaps just buy a ready made TRRS to TRRS audio crossover cable.

\$\endgroup\$
3
  • \$\begingroup\$ Do you have a link to such a "crossover cable"? I've googled that and similar terms but can't find anything - could something like this loopback cable be what you mean? \$\endgroup\$
    – dwb
    Commented Apr 30, 2023 at 8:04
  • \$\begingroup\$ NVM I don't think that cable would work - looks like it's meant to go into a "line in" port which is less sensitive than a mic port \$\endgroup\$
    – dwb
    Commented Apr 30, 2023 at 8:11
  • \$\begingroup\$ No, that's just a standard TRS to TRS cable, there is nothing special that makes it specifically a loopback cable, as all TRS to TRS cables can be used as loopback cables if you loop an output on a device to input on the same device. And LINE and MIC all have same connector. No I can't provide a link and suggest a product. If you can't find one, take two TRRS to dual TRS adapters and then use two TRS cables to route output to input, and if too loud or L+R are shorted then modify a simple cable. \$\endgroup\$
    – Justme
    Commented Apr 30, 2023 at 8:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.