05-23-2021, 04:13 PM
Going to break things into smaller pieces.
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).
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.
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).
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).
Both of these are good ideas to narrow down the problem.
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.
(05-23-2021, 02:54 PM)dsimic Wrote: Based on the way audio is routed, wouldn't it be expected to make the issue reproducible with no actual call in place, if the root cause is the microphone or something along the microphone-to-SoC path?
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).
(05-23-2021, 02:54 PM)dsimic Wrote: If that ends up as not reproducible, something must be wrong either along the CPU-to-modem path, which is probably unlikely, or within the modem itself.
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.
(05-23-2021, 02:54 PM)dsimic Wrote: It could also be something up to the way power is supplied to the modem, but it's highly unlikely because the modem is pretty much directly connected to the battery.
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).
(05-23-2021, 02:54 PM)dsimic Wrote: If the root cause turns out to be the modem itself, the microphone could be placed out of the picture entirely, by having the CPU play a .wav file or something else containing a "test pattern" for the call, so to speak. Playing a "test pattern" instead of speaking into the microphone would also make it possible to perform precise and repeatable analysis of the audio received on the other end of the call.
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).
Both of these are good ideas to narrow down the problem.
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.
