PWM whine near earpiece in low signal phone call and loud sound sent to the other end

[vortex]

Yes, it could be done. The modem's transmit power would need to be gotten to high levels by some other means, but it would work. In such a scenario, the phone could do the recording itself. Not only is it a good check, it also makes it easier to get a spectrogram. Getting the transmit power high is just a matter of using mobile data in the background at a sufficient rate, particular upload (though it is possible that download would be sufficient).

Exactly, divide the problem into pieces and conquer.   You'd be able to do the testing of the microphone-to-SoC path much faster, easier and more often, with no need for someone (or something) to answer the call, do the recording of the call, and provide you with the recorded audio file.

Yes.

Within the modem itself is possible if the sound ever goes back to analog where it would be vulnerable. I would think that that isn't done, but who knows.

Right, there should be no reasons for the audio to go back to analog inside the modem, but who knows what happens inside that black box?  Maybe there's even some weird bug in the modem's DSP code, who knows?  If all else fails, you could also try changing the parameters of the PCM between the SoC and the modem, and see if that makes a difference.  It wouldn't be easy to do, but might be worth a try.

If the other things don't isolate the problem, these are possibilities.

It could be a DC-DC converter inside the modem itself if the transmitting unit needs a different voltage than the modem as a whole. Sadly, that would mean that it isn't possible to simply change out the part generating the noise in a board revision (still possible to better isolate microphone system from it).

Right, there isn't much that could be done to the power supplied to the modem by the phone, but the power-related issue might very well be inside the modem itself, which would make things rather difficult.  However, you could check out the latest results of the reverse engineering of the modem, and see if there are any exposed "knobs" that might be interesting.  I doubt there are any, because the audio processing isn't performed on the modem's ARM core, but it might be worth ckecking anyway.

Let's hope that the microphone-to-SoC path will turn out to be free from interference etc.

If the problem is a DC-DC converter inside the modem itself, there might be a hardware mitigation (beyond making the microphone lines more immune to pickup by caging or something similar) depending on how the noise gets into the microphone line (assuming it is that). Something a simple as a filter on the power connection to the modem itself might help. DC-DC converters send a lot of noise backwards to the source in addition to propogating it forward. Essentially, adding a line filter in a potential future 1.2c revision.

Playing a sound directly to the modem is also a good idea. Would make it easier to ascertain where exactly the problem is. If pickup is between the microphone and the CPU, then there should be no issues listening to the transmitted audio file on the receiving phone. If there is an issue listening to it, then there is a problem in the CPU to transmission end (possibly in addition to the microphone to CPU part).

As a note, configuring the required audio routing should be possible by using or modifying the audio configuration program that's available on Ondrej's web page.  I haven't researched that in detail, but I think it's rather easily doable once you get a grasp on the entire audio subsystem.  The hard part, dissection of the PinePhone's audio subsystem, has already been done by Ondrej.

That is what made me think it is doable and how to do it. Set things up like there with maybe a tweak or two to insert audio and start the call from mmcli with AT commands (can't use the call program since that would override the audio settings). But there will be quite a bit to learn in order to do it.

Also, the modem has a built-in text-to-speech generator, which I've tried out some time ago and it worked rather well, but for some reason the generated speech didn't seem to reach the other end of the call.  It might be worth having a look at that as well.

Didn't know it had one. Rather interesting. Not sure it would be useful.

I am going to attempt the first one (record audio without a phone call) first. I will setup a download of something big in the background. Already have the software.

Second idea is a bit harder to pull off. I will need to research the appropriate pulseaudio and AT commands to do it.

As already noted right above, I think that configuring the required audio routing shouldn't be too hard.

Hope so.