2023-06-05:
In order to debug vsync placement in regular and interlaced modes, I ended up building a custom VGA cable to direct the video and vsync signals to my computer's sound card, while it's also outputting to a display so I can see what I'm doing. I purchased a passive VGA splitter (which you shouldn't actually use with two monitors each with a 75 ohm termination), plugged the base and one end into my regular VGA port and cable, and stuck wires into the green, vsync, and ground holes in the other output. Since I didn't have protoboard, I hot-glued these wires to a piece of cardboard, using surface-mount resistors to divide the vsync signal to a safer voltage for microphone jacks. This proved to be a mistake, with the cardboard being so floppy that I repeatedly snapped the resistors' metallized terminals off their bodies.
My first VGA probing rig had some truly terrifying signal properties, far worse than plugging my Wii's component 480p luma signal through a RCA splitter into