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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.
JRE
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Krzysztof Bociurko
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  • 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. – Krzysztof Bociurko Sep 16 '20 at 23:31
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    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. – JRE Sep 17 '20 at 08:25
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    The video doesn't apply to Android phones. It applies to PC soundcards. – JRE Sep 17 '20 at 08:42
  • @JRE, didn't think that a PC soundcard speaker/headphone and an smartphone headphone output would be that different. – Krzysztof Bociurko Sep 17 '20 at 13:38
  • PCs have a TRS jack for speakers, and a TRS jack for line in. The microphone jacks are usually only TS. – JRE Sep 17 '20 at 13:43
  • So, is it distorted audio or just lack of low frequencies (bass) in voices? – JRE Sep 17 '20 at 13:44
  • Most laptops have TRRS jacks now. From what I understood from the CTIA/OMTP standards, this is just two ports combined into one (and sometimes even three ports with Toslink added to the mix) – Krzysztof Bociurko Sep 17 '20 at 13:49
  • Not sure what is the exact nature of the distortions, its feels a bit as if the volume was changing with words begining to be louder and then quiter near their end, with the next word starting loud again. Maybe comparing input/output waveforms would shed some light into this. Will try with bigger resistors from your answer. Maybe this time I'll record something so I can show it here. – Krzysztof Bociurko Sep 17 '20 at 13:51

1 Answers1

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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.

JRE
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  • Can you please help me on my question ? Someone replied to my question but it is kind of half baked cake. https://electronics.stackexchange.com/questions/560694/how-to-connect-one-android-mobile-audio-output-to-mic-of-another-android-mobile – Dheeraj Chauhan Apr 20 '21 at 03:39
  • @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. – JRE Apr 20 '21 at 06:15
  • @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. – JRE Apr 20 '21 at 06:19
  • I need it for myself also as I am also part of that "Hard of hearing" community. If we can achieve it using the single mobile then it is bestEST, but as per my current understanding about the Android, I don't think achieving it using single mobile will be so easy (Although google have already implemented it https://www.youtube.com/watch?v=7hb3p8LZIq8 but it has its own limitation which I mentioned in my original post). So as of now that adapters/circuit way seems more promising. Can we get into chat and discuss it further. I don't have enough reputation to create chat room :( – Dheeraj Chauhan Apr 20 '21 at 07:10
  • Please let me know if you can create chat room for discussion as well as timing so that we can get in touch. I am doing all these just to help myself and needy people around me. I am not going to do this for any profit making. Let us join hands to make it happens ! If you can provide me some points by anyway then I can also create chat room. I am just 30 points away from that. – Dheeraj Chauhan Apr 20 '21 at 07:13
  • Something also suggested here ( https://electronics.stackexchange.com/questions/239708/headphone-out-to-microphone-in-for-mobile-phones?rq=1 ) to make the sound output of earphone compatible with MIC, but I am not sure about that. You must have great SOLDER skill, in case you can build that at your home and check once. And if you do so please don't forget to make a video :) Waiting for your reply. – Dheeraj Chauhan Apr 20 '21 at 07:15
  • Just tagging you, I am not sure whether you are getting any notification for my comments. – Dheeraj Chauhan Apr 20 '21 at 08:01
  • @DheerajChauhan: I saw your notifications. I'm at work now. I'll get back to you when I have time. – JRE Apr 20 '21 at 08:22
  • any comment/feedback/update ? – Dheeraj Chauhan Apr 22 '21 at 08:21
  • I haven't had a chance to do much with this. I've got other projects running. I haven't forgotten about it, but I haven't done much with it either. – JRE Apr 22 '21 at 08:22
  • Shall I try the circuit answered in this question ? https://electronics.stackexchange.com/questions/239708/headphone-out-to-microphone-in-for-mobile-phones?rq=1 ? I have 5K ohm resistores as well microfraud capcitor. Or any modification you suggest ? – Dheeraj Chauhan Apr 22 '21 at 08:42
  • I am referring to this circuit - https://i.stack.imgur.com/7M8mw.png – Dheeraj Chauhan Apr 22 '21 at 08:44
  • That looks good, but you'll have to build it, and I thought you wanted to avoid building circuits. – JRE Apr 22 '21 at 08:45
  • Absolutely. Solution should be as simple as possible. But that adapter as well software based things are not moving since a week and that circuit seems quite simple, I can ask some local shopkeeper to give it a try. Will you be able to review hardware output of this circuit to be sure that shopkeeper did correct ? I want your suggestion for the circuit so that I can ask the shopkeeper to try it once. – Dheeraj Chauhan Apr 22 '21 at 08:49
  • I need to think about this. The circuit you linked is OK, but doesn't have the resistor to tell the phone to use the microphone. I need to sit down and build one to make sure it does things right. I haven't had a chance to do that. I need to see if I have connectors here (I don't think I have any at hand) and probably order a couple of them and build the cable and test it with a couple of phones and the software. Then I'll have to write up a description of how to use it. I also want to see if it can be done with purchase cables. – JRE Apr 22 '21 at 08:55
  • You have a nice idea, and I'd like to help. This is **not** like giving advice to other folks here on the site. Them I can throw ideas and concepts at and expect them to either know what I mean or else expect them to figure it out from the suggestion. You need a complete, finished product (or at least the instructions to build one,) and I can't expect you to pick up and run from a vague suggestion. – JRE Apr 22 '21 at 08:58
  • Thanks for contributing to the noble cause and your eagerness to spread the smile on the face of needy in this pandemic situation ! Highly appreciated. Sometime we need to think beyond our boundary and professionalism and you proved that you are the genuine eprson who want to make it success. It is possible for you to create a chat room and invite me there so that we can have little chit chat there instead of here in comment section. Although any important point of our chat- I will keep it posted here for other reference and help. Thanks again ! – Dheeraj Chauhan Apr 22 '21 at 09:04
  • Got some time to get your hands dirty to help ? COVID situation is worst at my side so not able to connect with anyone to build even that simple circuit which we discussed. SO just asking for you if any update from your side if any ? – Dheeraj Chauhan Apr 29 '21 at 09:59
  • I've gotten some of my other projects wrapped up, and gotten a bit of much needed sleep in the last couple of days. I'll see what I can come up with this evening. – JRE Apr 29 '21 at 10:00
  • "@JRE" - Yes sure. Take your time. – Dheeraj Chauhan Apr 29 '21 at 10:38
  • "@JRE" - The local shopkeeper made the adapter according to the circuit diagram. But It did not work. I clicked some photos as well as well one video. I need to explain whatever is my update till now. Where else can i explain as well as send videos photos ? comment section will not work. Please suggest where can i send details for your reference? – Dheeraj Chauhan May 02 '21 at 08:39
  • @DheerajChauhan: You can reply in the [comment section of my blog.](https://josepheoff.github.io/posts/androidtranscribe1) It will accept pictures, and links to videos. The circuit wasn't done yet, though. – JRE May 02 '21 at 08:45
  • "@JRE" - I have made a document and uploaded the same on my google drive . Please check the link ( https://drive.google.com/file/d/1Ue35TzvLzkzxaN4wBerzUCfloY058Cro/view?usp=sharing ) to download the word document which have my observation along with necessary pictures. – Dheeraj Chauhan May 02 '21 at 18:04
  • @DheerajChauhan: I have put up [a new blog post with a complete circuit.](https://josepheoff.github.io/posts/androidtranscribe2) It has a complete and functional circuit. We'll have to discuss the best way for you to have it made. – JRE May 02 '21 at 23:04
  • "@JRE", Thank you very much for the detailed explanation, very much appreciated. Unfortunately full lockdown started here from today so I will not be able to buy the components, so need to wait till then. But you mentioned that transcribe expect slow speaking as input for best work but in case I directly speak in phone mic , google live transcribe write very fast and it can catchup almost all speeds. Maybe you were referring something else in your blog. – Dheeraj Chauhan May 03 '21 at 10:14
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    @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." – JRE May 03 '21 at 10:50
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    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. – JRE May 03 '21 at 10:55