this post was submitted on 16 Mar 2024
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For my desktop, I have a monitor with built-in speakers and the audio is carried by HDMI - but its speakers are such junk that I have powered speakers connected to it via a headphone cable. I haven't noticed any weird noises before texts or phone calls, but I do get a faint buzz that changes with the contents of my screen... Oddly enough, bright white makes most noise, dark is quiet. That would make sense for analog video, but for HDMI it baffles me.
It makes sense, the DAC is in the monitor, but the monitor is an electrically noisy place, so the IC that's responsible for the DAC/AMP is picking up noise from the power circuitry in the monitor. It's common with cheap DAC/AMP chips, like what they use in low end motherboards and monitors.
Basically as power usage is increased the noise should be greater. Unfortunately it's a design flaw, so your only real option is to replace it to resolve the problem. You may see improvement with a ferrule, or you could try doing some hardware hacking and adding shielding around the audio circuitry.
The ferrule is the easiest and cheapest, but it may not fully resolve the issue.
Oh no, I get why the interference happens, it's more the content of the interference that surprises me. With analog video, a white screen is basically a 15 kHZ full amplitude square wave, but HDMI is encoded so regardless of the balance of 0s and 1s in the original content, the data stream should be just noise either way.
You mentioned powered speakers plugged into it, which means the output from the display is unamplified, and it is amplified by the speakers. So the wiring from the DAC, decoding the HDMI signal to the jack where the speaker is connected is analog and surrounded by very noisy digital circuits, especially power circuits, which induces the noise.
Most digital speaker systems, such as Bluetooth headphones have a single IC for both DAC and amp which effectively eliminates any induction of noise.
The weakest point in the audio system is the unamplified analog signal. Any induced noise is amplified with the intended signal.
The frequency heard is going to be a function of what signal is causing the induced noise. Depending on the display, it could be any number of subsystems. Usually related to the backlight, but not always. The noise is very likely completely unrelated to the audio being played.