Thanks to a generous contribution, I now have an 800 for testing and have finally been able to investigate firsthand the elusive mystery of why the 800 produces such odd artifacting colors. It's been known for a long time that this model is odd in being able to produce green/blue artifacting, which is unlike other models that produce opposite colors. I'd determined a while ago that simulating this with linear filters wasn't possible and had been wanting to track down the cause. However, when I hooked up the 800 to a Commodore 1702, I was a bit disappointed:
Brown/blue artifacting, opposite colors. But then on a whim I hauled the machine into the living room and hooked it up to a Samsung TV, and was shocked to see this:
A Dell U2711 monitor shows similar:
But, then, capturing with an All-in-Wonder USB device produces this:
This is similar to what I see on an 800XL.
Mind you, this is the exact same machine, just swapping the composite output over to each monitor. I've never seen anything as crazy as this. To double check, I repeated the same test with a 130XE, and all four displays showed similar brown/blue artifacting colors.
Anyone familiar with composite video or the output circuit who might have insight into why the displays vary so much in decoding artifacted colors from the 800? The one thing I've been able to determine so far is that the 800 has greater skew between the luminance bits, which manifests as pretty dark bars between luminances 3-4, 7-8, and 11-12 in a color ramp. This causes the artifacting colors to vary slightly at different luma levels. That makes me suspect that there might be something odd about the color burst, like an uneven duty cycle. Still, I can't think of what would cause the artifacting colors to vary by as much as 90 degrees between displays.