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

[dsimic]

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.

I had originally dismissed this possibility, but the two of you convinced me I had prematurely dismissed it and should actually test it. So I did two phone calls where I turned the earpiece speaker volume all the way to zero so that there would be no audio spill except via RF pickup in the earpiece speaker and then producing sound.

On a 4G call (I think it was 4G, but it might have been 3G), there were no problems. The recording was nice and quiet and the frequency comb was not there as seen in the spectrogram below.

However, in a call with a worse signal and on 2G, the frequency comb is still there as seen by the spectrogram below.

From this, I think there is an issue independent of audio spill. Now, that isn't to say that audio spill couldn't also be a problem with a similar fequency comb and just adding more noise on top. But, there is definitely at least something there that isn't audio spill.

Thank you for doing this additional testing!  Based on the test results, I agree it's safe to conclude that the root cause of the issue isn't an audio spill.  In the above-described tests, the issue was clearly demonstrated to be caused by changing the network access type and the strength of network reception, while the audio spill was completely ouf of the equation.

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 see I misunderstood you. I probed the modem and I am definitely running some version of the EG25GGBR07A08M2G firmware, but it is possible I am not running the very latest version of it. I am going to try to update it to this version for some next tests and see if that does anything. Hopefully I can do this soon, but I have been a bit busy lately.

As a note, the latest available version of the proprietary modem firmware should be EG25GGBR07A08M2G_01.002.01.002.