Friday, August 19, 2016

What's wrong with this picture?


Look at my Atari ST desktop... can you spot two things that are wrong?

Okay, I've been experimenting with a few GEM programs that allow larger resolutions and more. Some support interlace to double the vertical resolution but that can look a little odd and hurt the eyes! (use 60Hz!!). Others can create a virtual screen by using the hardware scrolling of the Atari STe. But do you really wanna scroll?

Interestingly, there are also programs that enable overscan and it was this that sparked my curiosity. I managed to activate this and using the border worked perfectly. There doesn't appear any slowdown or problems when using most GEM programs. In fact, it's cool so why didn't Atari push the boat a little further back in 1985?

I've made a bundled download of these programs - please let me know if there are others I've missed.

6 comments:

  1. The download is no longer valid, any chance we can find this somewhere else?

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    1. Hello James. The link works... What error are u getting as I'm worried it has been failing all along for people :(

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  2. I'd be interested to hear of anyone's real-world experience with getting DBL to work, because as far as I was aware the ST (and STe) was incapable of generating interlaced video. Nothing that actually increases the true usable screen resolution anyway; what that program seems to do is simply buffer an image twice the height of the actual display then flicker alternate lines on top of each other, which might fool a casual observer but is no good if you actually want to use the increased detail lace would actually give you (especially on an LCD which will iron out all the shimmer...)

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  3. yeah there's no default option in the hardware so it's all done in software, be it to double the screen or interlace it or use the borders. Some CPU time is used but not much

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    1. Actually I have a theory it might be possible to produce actual interlace using some heavy demo wizardry, essentially implementing syncscroll (which slightly shortens scanlines, IIRC) over a great many lines instead of just one... or maybe switching to either 60 or 50Hz (which ever is opposite of what's used over the rest of the screen) for several lines (50hz lines are slightly longer than 60hz ones). Switching between that and normal video with each successive field. With a fixed vertical scanning rate, you eventually send the scanlines of the affected frame half a vertical space out of true with the normal frame. Whether this would actually work remains to be tested. Issues with it are that you can only put the altered-length lines at the top of the screen because Vsync resets the start position - biasing the image downwards and reducing the number of usable scanlines, though 200 active scans should still be possible (thus 400i), and that it will almost certainly only work on a CRT (because it works on the basis of physically skewing the electron beam path out of true across the course of a frame, which an LCD would likely automatically compensate for and snap the pixels into the next nearest line, vs the usual method of starting Vsync halfway across a line which is what LCDs look for when setting progressive/interlace mode and deciding which field is which), which limits its usefulness in terms of any sustained hi-rez use because of flicker... unless you happen to have a long persistence phosphor monitor (maybe colour, maybe greyscale) ... but really if you're going to that trouble, why not just get a proper mono screen and be done with it?

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    2. ((also that method should allow 640x800i in hi-rez, if syncscroll can be engineered in mono... might even be useful with a long-persistence portrait 1-page monitor, then.))
      ((actually thinking about it, if you could force the wrong colour mode for a line or more without also causing a reset, a mono line is a little less than half a colour one, which might make producing the vertical skew a bit easier if your screen will withstand a line or three at an erroneous 35kHz or 15kHz instead of whatever its usual is...))

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