TV Sensitivity to Rolling

[Kylearan]

How sensitive are TVs to infrequent, small variations in number of scanlines?

 

I see Thomas notifying other people when their kernels have variable scanline counts, and found a few discussions on rolling screens on TVs so I guess it's fairly important to have a constant scanline count. So far my PAL kernels manage to generate a constant 312 scanlines according to Stella. The only problem is when I switch from one Kernel to another, which only happens every 30 seconds or so. Then there is one single frame with a different number of scanlines, before it will be 312 again next frame. Is this single frame already enough to cause screen rolling?

 

 

Related question: Debugging (or even detecting) scanline variations is hard if the number of scanlines only varies in one single frame every 20-30 seconds. Is there some way to tell Stella to break if the number of scanlines in a frame deviates from a specific number? Or to generate a trace/log of scanline counts, so I can see the number of scanlines per frame and find out in which frame the number varies?

 

How do you debug this kind of problem?

 

[RevEng]

If it's a big variance in the single frame, then yes, you could get a full roll, partial roll, or jitter, depending on the TV. A small variance in a single frame will usually visibly shake the screen, but not on always, depending on the TV.

 

You should work on getting the same line counts for both kernels, even during a hand-off. If you have the same number of visible lines in both kernels, it probably means your overscan or vblank time/counts are different, or you're setting one of those timers earlier or later for one of the kernels. (assuming you're using timers)

 

RT wrote up a very good summary of keeping track of scanline counts in stella. It includes instructions on how to set up a stella breakpoint if the count exceeds a certain number.

[Thomas Jentzsch]

1. Yes, some TVs are very sensitive. And PAL will loose all color when an odd number of scanlines is displayed.

 

2. Debugging: Stella supports breakif. Check the debugger documentation, the possibilities are huge! The problem is, that when you use a break you cannot step backwards a frame or so. So it it usually better to step by using frame+1. As soon this becomes too long, step back one frame and then you can debug the frame causing the problem.

[Kylearan]

OK, I'll take extra care then not to have a single frame with different scanlines.

 

Thanks for the link! I knew about breakif, but somehow missed the scanline var. Nice! Stella is one awesome piece of Software. :-)

[+Gemintronic]

My roommates TV was sensitive to rolling. I found that out in the morning after I crashed into his room drunk and slept on the floor by mistake.