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

[dsimic]

By the way, back at the time when the modem calls initially started to work on the PinePhone, the dialer application didn't perform the audio routing.  As a result, there was a loud noise during the call, resemblling some sort of weird, metallic, pulsating noise that made it impossible to hear the other end of the call, or the other end to hear me.  After executing the audio setup utility, during the call, this noise disappeared and the audio quality was rather good afterwards.  I wonder if this is somehow related?

It is related to that sound. When I first heard the recorder, I thought it was something very different. But listening to my more recent recording, I think they are related. The pulsating sound has the same spectral characteristics as the more continuous sound, so I think they are the same. Since my Nexus 5 makes similar sounds but doesn't have the issue, I am inclined to think it isn't direct audio coupling from the sound to the microphone. This is why I think it is electronic. I could of course be wrong on this.

So, I think what you just brought up is worth investigating further.

I might be wrong, bit I think that at least some of the weird noises I've described above were caused by the voice of the called party being played through the PinePhone's built-in speaker.  That surely created an audio spill, which at least contributed to those weird pulsating noises.

Could an audio spill be one of the reasons for the original problem you're experiencing?  If so, something inside the phone could be picking up the audio output signal and emitting it, in some way or form, through the built-in speaker.  The microphone could be picking that, creating a spill.

Edit: I somehow managed to forget mentioning that the proprietary modem firmware can be updated, which you're probably already aware of, so you should check that as well. You can find the instructions, the latest firmware, and the release notes in this repository.  Further information, including the source code of the update utility, can be found in this forum thread

I am familiar with it and looking forward to where the firmware will eventually go, but I am not quite ready to dive down that rabbit hole yet. A lot of things I have to get setup before I can go there.

That all said, that firmware might be able to have some effect on the problem (e.g. use settings that reduce it).

Sorry, I wasn't clear enough...  The above-linked repository contains the latest version of the official, proprietary modem firmware, which is provided by Quectel.  It would be advisable to update the modem firmware to the latest available official version; just have a look at the mile-long update release notes.

I don't know if the sampling frequency in the modem would have much effect given that the frequency comb is seen at the CPU. But, there is a chance it might due to interactions between the different close components in the CPU's sound unit due to their close proximity electrically.

If everything else fails, it might be worth trying to change the sampling frequency.

I managed to get a wired headset since the store is open now and do a recording with it. I wasn't able to get a very long recording yet (important for getting very good spectrograms) but was able to some preliminary ones.

The bad news is that the problem is still there. The good news is that it is A LOT quieter with the headset. The spectrogram is below. There are a lot more spikes visible in it because the problem is quieter overall (easier to see other spikes). Also, the recording was shorter and that means a noisier looking spectrogram.

This might support my above-described "theory" that an audio spill might be contributing to the issues you're experiencing, because an increased distance between the phone's built-in speaker and the headset's microphone would decrease the spill.  This "theory" could be easily verified by taping up the built-in speaker and using a wired headset to make a test call...  It might be worth trying, if you agree.

I'm not sure if i understand the exact layout of the pinephone audio! On phosh (manjaro-arm beta 10) audio test it sounds like there are 2 speakers on each side, left/right!? Why would there be a speaker on the bottom near the mic? Doesn't this create the echo as the bottom speakers are beside the mic?

There's a single speaker in the PinePhone, as visible in the daughterboard schematic, but for some reason the audio test in Phosh shows the left and right channels.  Actually, the two channels should be mixed into a mono output by the audio codec, and fed to the mono speaker.

The built-in speaker is disabled during phone calls, and the earpiece is enabled.  When the speaker is not disabled, it surely creates an audio spill, as I've described it above.  What happens when the speaker(phone) is activated during a call?  I'm not sure, I haven't tried that.