05-18-2021, 12:20 PM
An update. I did the same tests again today but coordinated with the other person to make sure one of my calls went to the recorder to be recorded so I could listen to it later.
First, the issue was a little better today on both ends. The connection was a little better based on the connection indicator. May have been the weather (more water weakening the signal from my carrier's tower enough that the modem decided to roam to another carrier's tower that was closer), less phones connecting to the same tower, or something else. Not sure. Calls consumed less power too, which further supports the hypothesis that the signal was stronger today.
But the issue was still there.
Listening to the recorded call, it is definitely sounds like pickup from a switching power supply. It isn't quite a pure tone like some 5 kW frequency drives can make, but more smeared out around a central frequency like a small switching power supply. What is interesting is that the intensity was inversely correlated with how loud I was speaking. During pauses in my speech or if I spoke in a lower volume, the sound got louder. When I was speaking at high volume, it was quieter. From this, I think the pickup is occurring before the amplifier. The gain is auto adjusted based on the volume. When I speak louder than the pickup, the gain is reduced and the pickup is quieter since it isn't amplified as much. When I stop speaking, the gain is increased and the pickup is amplified. It is of course possible there is some pickup after the amplifier, but it seems like most of it is before the amplifier.
Now, this does suggest a possible software mitigation. If the gain is set low, or at the very least the software is set to increase it only very slowly except at the beginning of a call; then the noise shouldn't be amplified too much. One way to do it would be to let the autogain setting run at the beginning, find the lowest gain needed in say the first few seconds or so, and then lock the gain or only let it increase very slowly but decrease quicker.
I don't have a wired headset to test with, sadly. I do have wired headphones. But, your suggestion made me realize a potentially important catch. I've had DIP switch 6 set to UART instead of headphone/microphone. It is possible that the particular setting of this switch affects how much pickup can occur. I will make another test tomorrow with the switch set to headphone/microphone mode, and if I remember I will also try with headphones.
On the hardware front, in future revisions, if changing the board for better isolation isn't possible or doesn't do quite enough, another possible mitigation would be switching whichever power supply to one with a sharper frequency spectrum and putting a notch filter for that frequency on the microphone line either right before the amplifier or right before the digitizer (that way there is little room for pickup after the filter). Such a filter could in principle be done in software as well, but that would consume more power. Also, a stronger filter right before and after the switching power supply would also do the job (may not be forward propagated noise, but instead noise propagated backwards to the power source) and maybe a small RF can if absolutely necessary (though that adds space, weight, and cost).
I am also going to see if I can find a way to record the sound to a file so I can get the spectrogram. The spectrogram might help determine which component on the board is causing the sound, and also how to mitigate it (knowing the frequency is needed for a hardware or software filter, and is useful for any potential board revisions).
I was hoping I was the only one (i.e. better that my phone has a defect and no one else is effected than everyone being effected), but it seems that I am not the only one. Guess this was good to bring up.
First, the issue was a little better today on both ends. The connection was a little better based on the connection indicator. May have been the weather (more water weakening the signal from my carrier's tower enough that the modem decided to roam to another carrier's tower that was closer), less phones connecting to the same tower, or something else. Not sure. Calls consumed less power too, which further supports the hypothesis that the signal was stronger today.
But the issue was still there.
Listening to the recorded call, it is definitely sounds like pickup from a switching power supply. It isn't quite a pure tone like some 5 kW frequency drives can make, but more smeared out around a central frequency like a small switching power supply. What is interesting is that the intensity was inversely correlated with how loud I was speaking. During pauses in my speech or if I spoke in a lower volume, the sound got louder. When I was speaking at high volume, it was quieter. From this, I think the pickup is occurring before the amplifier. The gain is auto adjusted based on the volume. When I speak louder than the pickup, the gain is reduced and the pickup is quieter since it isn't amplified as much. When I stop speaking, the gain is increased and the pickup is amplified. It is of course possible there is some pickup after the amplifier, but it seems like most of it is before the amplifier.
Now, this does suggest a possible software mitigation. If the gain is set low, or at the very least the software is set to increase it only very slowly except at the beginning of a call; then the noise shouldn't be amplified too much. One way to do it would be to let the autogain setting run at the beginning, find the lowest gain needed in say the first few seconds or so, and then lock the gain or only let it increase very slowly but decrease quicker.
(05-18-2021, 06:50 AM)dsimic Wrote: This is a really weird but important issue, thank you for describing it in detail! Maybe you could try using a wired headset to make a test call to the landline under the same circumstances, instead of using the built-in microphone and the earpiece? It would be interesting to see it that makes a difference.
I don't have a wired headset to test with, sadly. I do have wired headphones. But, your suggestion made me realize a potentially important catch. I've had DIP switch 6 set to UART instead of headphone/microphone. It is possible that the particular setting of this switch affects how much pickup can occur. I will make another test tomorrow with the switch set to headphone/microphone mode, and if I remember I will also try with headphones.
On the hardware front, in future revisions, if changing the board for better isolation isn't possible or doesn't do quite enough, another possible mitigation would be switching whichever power supply to one with a sharper frequency spectrum and putting a notch filter for that frequency on the microphone line either right before the amplifier or right before the digitizer (that way there is little room for pickup after the filter). Such a filter could in principle be done in software as well, but that would consume more power. Also, a stronger filter right before and after the switching power supply would also do the job (may not be forward propagated noise, but instead noise propagated backwards to the power source) and maybe a small RF can if absolutely necessary (though that adds space, weight, and cost).
I am also going to see if I can find a way to record the sound to a file so I can get the spectrogram. The spectrogram might help determine which component on the board is causing the sound, and also how to mitigate it (knowing the frequency is needed for a hardware or software filter, and is useful for any potential board revisions).
(05-18-2021, 07:58 AM)Rainer Wrote: hi, i'm still working on getting my cellular network to function reliably but based on the few calls i did make people complained about a loud, echo-y sound on their end to the point where they just hung up on me.
I was hoping I was the only one (i.e. better that my phone has a defect and no one else is effected than everyone being effected), but it seems that I am not the only one. Guess this was good to bring up.
