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I'm looking for a way to wire the stereo output of a device into its own mono microphone input. This will be soldered as a 4 pin 3.5mm TRRS audio jack.

I need to record the output of my smartphone on the smartphone itself (without software modification.) I have screen recording apps available, but they take audio from the microphone.

I tried two circuits:

  1. Android official Audio Loopback Dongle (stackoverflow question about this circuit) - this works but distorts human speech. Perhaps incorrect resistor/capacitor values?
  2. Headphone Output/STEREO MIX To MIC Input For Recording, but wired input and output together, ground left not connected. Android doesn't detect this as an mic and uses internal microphone.
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  • \$\begingroup\$ Adding a 1.0uF (polarized) to a 0.1uF (ceramic) seems to fix the problem a bit, but still audio doesn't sound right sometimes. Haven't got bigger caps at the moment to check if making this value higher will help. \$\endgroup\$
    – Krzysztof Bociurko
    Sep 16, 2020 at 23:31
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    \$\begingroup\$ Scratch Youtube off your mental list of places to find technical information. Fuzzy videos of people mumbling to themselves while showing you a circuit drawn on a post pad just doesn't cut it. You need written descriptions and drawings that are legible. Youtube is great for funny cat videos. Leave it at that. \$\endgroup\$
    – JRE
    Sep 17, 2020 at 8:25
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    \$\begingroup\$ The video doesn't apply to Android phones. It applies to PC soundcards. \$\endgroup\$
    – JRE
    Sep 17, 2020 at 8:42
  • \$\begingroup\$ @JRE, didn't think that a PC soundcard speaker/headphone and an smartphone headphone output would be that different. \$\endgroup\$
    – Krzysztof Bociurko
    Sep 17, 2020 at 13:38
  • \$\begingroup\$ PCs have a TRS jack for speakers, and a TRS jack for line in. The microphone jacks are usually only TS. \$\endgroup\$
    – JRE
    Sep 17, 2020 at 13:43

1 Answer 1

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Don't mess with the capacitor if you have distortion.

Here's the "circuit" from your first link:

enter image description here

"TIP" and "RING" are the speaker outputs. "RING2" is ground.

The two 1k resistors form a voltage divider with the 100 ohm resistor connected to ground ("RING2.") The values given reduce the speaker signals by a factor of 10. That is, a speaker signal of 100mV is only 10mV when it gets to the capacitor. You need this because the speaker output signal is much larger than the normal microphone signal.

Distortion means the signal going in to "MIC" is too high. The solution is to use larger resistors in place of the 1k resistors suggested in the circuit. That will lower the voltage of the signal going to "MIC."

If you said "distortion" but meant that low frequencies are missing from the recorded audio, then you used the wrong word.

If low frequencies are missing, just say so.

The cure for missing low frequencies would be to use a larger capacitor.

A 1 microfarad ceramic capacitor would be simplest.

If you only have electrolytic capacitors, then the plus terminal should go towards the microphone. The microphone line has a DC voltage on it that is used to power the microphone capsule.

For various reasons, I had a look at the spectrum of the recorded audio using the Google "loopback dongle."

The 100nF capacitor causes considerable loss:

enter image description here

Red is the earphone, blue is going to the microphone.

Some experimentation showed that it takes a 4.7µF capacitor to flatten the frequency response:

enter image description here

The 50Hz spike is from my test setup - too much power line hum getting into things.

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  • \$\begingroup\$ Can you please help me on my question ? Someone replied to my question but it is kind of half baked cake. electronics.stackexchange.com/questions/560694/… \$\endgroup\$
    – Dheeraj Chauhan
    Apr 20, 2021 at 3:39
  • \$\begingroup\$ @DheerajChauhan: Just connecting the headphone output to the microphone input won't work properly. The microphone input expects a very small signal while the headphone output provides a very large signal. That's like having some one yell into your ear. \$\endgroup\$
    – JRE
    Apr 20, 2021 at 6:15
  • \$\begingroup\$ @DheerajChauhan: I'll have to think about this for a bit, and do some resesrch. You want (and need) a simple and inexpensive solution that you can just hand to people and have it work. That's more complicated than hacking something together for a one time use. I'll look into the software you mentioned and the Android specifications - as well as some other software. I'll get back to you and let you know what I find. \$\endgroup\$
    – JRE
    Apr 20, 2021 at 6:19
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    \$\begingroup\$ @DheerajChauhan: Whether I use the built in microphone, the headset microphone, or the complete adapter, the "Live Transcribe" doesn't always manage to follow me. Maybe my voice or my accent are too odd for it. At any rate, I have to make an effort to speak clearly to get good results from "Live Transcribe." \$\endgroup\$
    – JRE
    May 3, 2021 at 10:50
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    \$\begingroup\$ Sorry to hear that you are all in full lockdown. I'll write up some of the ways that a complete adapter can be built, and post that soon. \$\endgroup\$
    – JRE
    May 3, 2021 at 10:55

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