Oh, forgot to add, this was tested on ATI Mach64, fairly fast card. Although, I don't think this would be graphics card dependent (except maybe due to framebuffer timing/speed reasons).

I used to play Pinball Fantasies with PC speaker sound a lot back then and I never noticed it (I had ISA only 486DX2-80 with no sound card). Might be some configuration problem.

Interesting to know. Yeah, might be some odd combo.

I now recall there is a sound quality setting in SETSOUND.EXE. Tried setting Low Quality, but that did not affect the glitch at all (the video above was recorded with PC Speaker setting at MAXIMUM). Also tried toggling PC Turbo button while game is running, that didn't seem to modulate the glitch either. So it is somehow behaving quite speed independent.

Pinball Fantasies is quite picky when it comes to video cards- are you able to test it using a different one?

Pinball fantasies synchronizes the timer IRQ to the horizontal sync (maybe not at 31.5kHz, but at 15.7kHz). The complete drawing engine is locked to the frame rate. As you correctly suspect, this game doesn't render each frame completely, but it just draws the ball and the flippers as required. The ball is supposed to be redrawn during vertical blank, but it seems the game engine is not locked correctly to the vertical timing, suspecting the vertical blank when in fact the display is at the center. I wonder whether something goes wrong with counting the scan lines twice, so the end of frame is suspected at scanline 200 instead of 400. Redrawing the ball at scanline 200 (just after the displayed area) is the correct thing to do if Pinball Fantasies set to the "standard" video mode, but the wrong thing if Pinball is set to the "high-res" video mode.

If a different video card doesn't fix the issue, I suspect your copy of Pinball Fantasies might be broken. You don't need a fast video card for Pinball Fantasies: I played it with DAC-on-LPT sound on a 12MHz 286 computer with a mediocre ISA video card without any issue, so I am not surprised that processor or bus performance is not the root cause of the issue.

I switched from the PCI ATI Mach64 to ISA Headland HT208 (the card that was topic on this thread: Can your VGA card provide a hardware source for random numbers? ), and the glitches do go away completely. Very odd.

Looks like blood streaks on the ball. Maybe it considers running that wonderful sountrack and special effects through the PC Speaker as an evil act 😀

Really though I don't have a clue.