Hi, running MODEP on my RPi 3b+, I get a very annoying high pitched electronic noise from my USB sound card. The noise is infrequently changing its pitch slightly between two frequencies. This alternation could be caused by any electronic module on the board, but I am not sure about it. The noise is also present with the guitar not plugged in.
As it’s not, or less present using a different OS and guitar amp software, it might be a MODEP related issue?
Trying to find the cause for this noise I tried the following so far:
Set very conservative jackd settings (-p 512 -n3)
Updated firmware using rpi-update
Changed the following settings in /boot/config.txt one by one:
Set USB-1.1 mode: dwc_otg.speed=1
Use alternative pwm mode: audio_pwm_mode=2
Disable audio dither: disable_audio_dither=1
Constantly played silence: aplay -t raw -r 48000 -c 2 -f S16_LE /dev/zero
Used different power supplies, headsets, cables, etc.
Powered RPi with a battery pack to eliminate issues with socket power supply.
Put slight pressure on the different electronic components of the RPi to make sure none of them where vibrating/resonating.
Removed HDMI and keyboard, leaving only the USB sound card.
When I booted the RPi with a different Raspbian Stretch image running Guitarix, it also produced some noise, but not this high pitched electronic noise. Disabling auto gain in alsamixer got rid of the noise.
Disabling auto gain using the MODEP image only lowered the volume of the high pitch sound.
Then I booted Android (Lineage OS) and used a guitar amp app. No noise at all. Just clear and brilliant sound!
Conclusion: None of the hardware is causing this issue.
So I booted back into the MODEP image and muted the bcm2835 device just to make sure.
Then I muted the mic and mic capture device of my USB sound card, which to my surprise didn’t have any effekt and my plugged in guitar was still amplified.
Muting the speaker and auto gain control however worked. I saved these settings (alsactl store).
Next I stopped Modep ‘systemctl stop mod-ui mod-host jack’. The noise was gone.
Then I started MODEPs components one by one.
‘systemctl start jack’ The noise appeared, but very faint
‘systemctl start mod-host’ No change
‘systemctl start mod-ui’ The noise got amplified and was noticeable as before. Unplugging the virtual patch cable in the UI reduced the noise to the level when only jack was running.
I’m running out of ideas here. Any help is appreciated.