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Yeah, I'd expect some funky harmonics out of PC speaker tones. Would be a fun thing to record and see. Legacy ones are definitely driven via a single digital output; I wonder what happens with modern HDA setups though.
I think for HDA there's two ways, either you feed in the i8253 signal into a pin on the codec, or the BIOS emulates the whole thing using an HDA widget to internally generate the tones inside the codec. I wonder if either or both of those approaches have band-limited output.
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Well I just tried both on my laptop. Neither is bandlimited. The i8253 path at 10kHz (i8253 -> HDA codec PCBEEP input -> line out) has harmonics up to at least 290kHz before I hit the noise floor, and the internal gen at 12kHz (HDA codec beepgen -> line out) goes beyond 1MHz (!).pic.twitter.com/QroMccNeNk
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Incidentally, that's 12kHz instead of 10kHz because the uber-crappy HDA internal generator works using an 8-bit divider from 12kHz (48kHz/4). Which makes PC Speaker music completely out of tune if played through it.
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