TV jitter

[DirtyHairy]

No description provided.

[sa666666]

When we get to this, the following ROM will be useful in comparing output to the old core:
jitter.zip

Use cursor keys to adjust scanlines, and press space to use the selected count. The effects in pre4 are already quite close to the old core, but still incomplete since the 'tv.jitter' and 'tv.jitter_recovery' options aren't being used.

[sa666666]

Refer to this thread on AtariAge for more information.

[DirtyHairy]

I have started to look into this and am confused by the behavior of the implementation in the old core. What I do understand is that, if frame length changes, a CRT requires some time to lock onto the new sync timings, causing the screen to "roll into place".

However, I would have assumed that, if frame lenght increases, the next frame will start too early and thus appear shifted towards the bottom and then roll into place. However, what I observe in Darrell's example is precisely the opposite behavior. In addition, things get even more funky above 169 lines or 89 lines (again in jitter.bin):

• more than 169: Only every odd frame is drawn, the screen is aligned at the bottom (!) of the kernel (top is cut off)
• less than 89: Roll direction is partially reversed, so both pressing and releasing fire causes the screen to roll in place downwards.

Bevore I invest more time in understanding and possibly reproducing this to the bits: is the old implementation just an effective implementation that rolls in a superficially convincing way, or is there more to it?

I can't test atm (away from home) and don't have a CRT at any rate, so I am asking instead from testing 😏

[SpiceWare]

bingo - "just an effective implementation that rolls in a superficially convincing way"

I did it after encountering one too many posts like this where the programmer had no way to test on real hardware and was surprised when others encountered jitter. I just made it so the problem would be readily apparent so the programmer would know they needed to fix it.

My last three blog entries about Stella are on the jitter implementation.

[SpiceWare]

Also note that what happens at the extremes totally depends upon the TV/monitor. Some end up showing 2, or even 3, stable images. See reply #76 & 77

[DirtyHairy]

😄 That's about what I expected. In this case, I'll aim for the same effect, without attempting an exact recreation .

[DirtyHairy]

This commit reimplements jitter. The new implementation works differently, but the effects look similar 😏.