Way back in the '80s I cut my programmers teeth on a Microbee 32k. Back on a classy green and black CRT, with an audio cassettes for permanent storage.
One of the things I remember noticing was that while playing a tune (1-bit square wave), if I held down a key on the keyboard that the frequency would drop slightly. This was presumably due to the poor 2MHz CPU having to stop counting how long it has spent on each waveform to go and service the keyboard IRQ. How quaint.
Now, as I sit at a PC that's at between three and five orders of magnitude more powerful, depending on the measure, I've just noticed an errily familiar behavior.
With my sound turn up, I hear a faint low-pitched hum even though no sound is playing. Now, whenever I move my mouse it turns into a faint high-pitched squeal. I actually get one of two high pitches depending on how fast I'm moving the mouse.
Not 100% sure what's going on here, but as they're USB powered speakers, I'm guessing that the transmission of mousing data is causing some interference on the power rail. I guess it shouldn't be surprising given that people have figured out how to capture keystrokes from EM emissions.
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