Buzzing audio with RCA

[Nallebeorn]

Hello! There are a bunch of posts and articles around about similar issues, but the exact circumstances seem to be different from everyone and I don't really have the knowledge of electronics and audio signals to confidently apply it all to my exact case.

My setup is as following:

• Raspberry Pi 4B
• A Trinitron PVM as screen, with mono audio input
• 3.5mm->RCA cable connecting the Pi to my monitor
• Official Raspberry Pi PSU (I've also tried powering the Pi from a (powerbank) battery, as sometimes suggested for ground loop issues, but it makes no difference.)
• disable_audio_dither=1 set in config.txt

Whenever I have both the video and one of the audio cables plugged in, there's an overbearing buzzing noise. It goes away if I remove the audio cable, and it actually also goes away when removing the video cable. And, it goes away while I touch the loose third audio cable (monitor only has mono audio) to any of the metal input ports or even a screw on the monitor. Properly plugging it in to an unused port doesn't help though, it needs to touch the outside.

So, that touching thing suggests it's some kind of grounding issue (and gives me a workaround by taping the cable to the metal :p). But if someone more knowledgeable could help me, I still have some questions:

• What exactly is happening here? Why does the buzzing go away and appear in the exact cases it does?
• Is this something that could potentially be fixed with a higher-quality cable?
• Or a separate speaker? (if I can find one with RCA input, is that common?)
• Or is there any other way to fix this more properly?