ULA Plus and Interlace.

joefish

May be a bit late to suggest this, but here goes.

Did the original Speccy ULA actually output a proper PAL interlaced image, with timing pulses on alternate frames to tell the TV to do the upper or lower half of the interlace? Or did it always just output the upper half every frame?

Now, does ULA Plus generate a proper interlaced image, or just repeat the upper half-frame?

And if it does generate a proper interlaced image, is it possible to expose a bit to the Speccy indicating whether the current frame is the upper or lower half of the interlace, thus allowing the programmer to alter what's on screen and generate hi-res interlaced graphics?

csmith

joefish wrote: »

May be a bit late to suggest this, but here goes.

Did the original Speccy ULA actually output a proper PAL interlaced image, with timing pulses on alternate frames to tell the TV to do the upper or lower half of the interlace? Or did it always just output the upper half every frame?

The Speccy always produces Frame 1.
http://www.zxdesign.info/vertcontrol.shtml
It also cheats and produces a single low vsync pulse instead of the broadcast standard of a chain of 2 and 30us pulses.

An interlaced computer display generally "jitters" due to the slight vertical offset of frame 2, so it's not generally done. I don't know of, and doubt any 80's computers did this. The Sonys PS1 and 2 also generate a single frame for example.

Now, does ULA Plus generate a proper interlaced image, or just repeat the upper half-frame?

A single frame. It's based on the Harlequin design, which turns out to be very close to the Sinclair design anyway. It currently generates a specification vsync signal however, which guarantees that it works with modern TVs.

And if it does generate a proper interlaced image, is it possible to expose a bit to the Speccy indicating whether the current frame is the upper or lower half of the interlace, thus allowing the programmer to alter what's on screen and generate hi-res interlaced graphics?

That would be possible, but we're never going to have an interlaced screen as it would induce fits!

What you really want is the ULAPlus to display alternate frames from different memory banks, I guess? Both being frame 1 so there is no jitter. What you'd see is the blending of the two images, but done in hardware not software as is done with the 128K?

Edit: I see what you're thinking, doubling the vertical resolution? It might be possible, but will consume a lot of "hardware", and there must be a reason why it's not generally done with TV displays - it could just be the jitter issue, can't think of anything else right now.

Chris

Fred

FWIW the Amiga produced an optional interlaced display with twice the normal resolution, but indeed flickered like buggery.

Winston

csmith wrote: »

An interlaced computer display generally "jitters" due to the slight vertical offset of frame 2, so it's not generally done. I don't know of, and doubt any 80's computers did this. The Sonys PS1 and 2 also generate a single frame for example.

The BBC Micro uses the interlace frame to generate a higher vertical resolution for MODE 7 (teletext mode), and by default, interlace is on for all other modes too. The jitter isn't too bad because by default the Beeb has a black background. You can turn the interlace off on the BBC Micro. In MODE 7 this results in very ugly jagged characters, but it looks fine in all other modes (which don't in fact take advantage of the extra resolution anyway, there just isn't enough RAM). Quite a few games turn off the interlace to give a nicer display.

joefish

csmith wrote: »

The Speccy always produces Frame 1.

What you really want is the ULAPlus to display alternate frames from different memory banks, I guess? Both being frame 1 so there is no jitter. What you'd see is the blending of the two images, but done in hardware not software as is done with the 128K?

Edit: I see what you're thinking, doubling the vertical resolution? It might be possible, but will consume a lot of "hardware", and there must be a reason why it's not generally done with TV displays - it could just be the jitter issue, can't think of anything else right now.

I wasn't really thinking of doing anything involving pulling data from anywhere other than the standard screen memory. I mean, there's a whole load of things that can be done along that route (e.g. a screen pointer, or a single screen swap bit), but then swapping screens is perfectly possible on the 128 so it'd just make the 48 less like the real thing. I realise that's not the intention. (As an aside though, I'm curious if it'll be possible to switch a 48K Spectrum into the Timex 8x1 ATTR mode..? Or maybe I'm getting ahead of myself).

What I thought was, if the interlace stage was visible to software as just a single bit then that software could redraw part of the screen for hi-res effects the same way 8x1 ATTR effects are done in software. But, if it doesn't do interlace then there's nothing to be gained, so it's not worth persuing as it'll only lead to more flicker.

I was just reading up on how the Atari ST abused the PAL system and was wondering what the Speccy did with it. If it doesn't do interlace, and you forced it to, it could make those 'Gigascreen demos' look even worse, so bad idea.

joefish

Winston wrote: »

The BBC Micro uses the interlace frame to generate a higher vertical resolution for MODE 7 (teletext mode), and by default, interlace is on for all other modes too. The jitter isn't too bad because by default the Beeb has a black background. You can turn the interlace off on the BBC Micro. In MODE 7 this results in very ugly jagged characters, but it looks fine in all other modes (which don't in fact take advantage of the extra resolution anyway, there just isn't enough RAM). Quite a few games turn off the interlace to give a nicer display.

So if proper PAL was done on the Speccy, then you'd want it off by default. You'd need some way to activate/deactivate it as well as a way to detect the frame you're on. It's probably still stretching things to devote two separate screen memory areas to it though. On it's own it'd just be an effect that degrades gracefully on an ordinary Speccy but you can tweak for best effect on a compatible ULA. You'd do it all in software still. Though there's still a question of whether anyone would notice the difference and whether it'd be worth the trouble.