TT Software overscan?

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mrbombermillzy
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TT Software overscan?

Post by mrbombermillzy » Tue Jun 05, 2018 10:16 pm

Ok, this is something that doesnt really come up much anywhere. Not that I can find anyway.

Can the TT display into the L/R borders? (Im not really concerned about upper/lower as its pretty much there anyway).

Im guessing because all the modes are essentially VGA, you cant just do the ST mono switch and frequency change tricks to achieve this?

Has anyone ever attempted this?

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frank.lukas
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Re: TT Software overscan?

Post by frank.lukas » Wed Jun 06, 2018 4:00 am

Overscan TT was a Hardware for the VME Bus and Software -> http://dev-docs.exxoshost.co.uk/OverScanTT_Manual.pdf

Petari
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Re: TT Software overscan?

Post by Petari » Wed Jun 06, 2018 7:52 am

Can I ask why you need TT SW overscan ?
And the reason for that way how it is done in ST(E) works not on TT (and Falcon too, of course) is not only because VGA mode, but whole video circuit is pretty much different.
There is 2 kind of people: one thinking about moving to Mars after here becomes too bad, the others thinking about how to keep this planet habitable.

stephen_usher
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Re: TT Software overscan?

Post by stephen_usher » Wed Jun 06, 2018 8:29 am

What I'd really like is something which brings the TT's VGA output within the standard dot clock range so that LCD monitors can sync correctly and give a good display. However, I think this would have to be some sort of hardware which buffered and replayed each scan line slightly slower.

Atarian Computing
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Re: TT Software overscan?

Post by Atarian Computing » Wed Jun 06, 2018 9:36 am

stephen_usher wrote:
Wed Jun 06, 2018 8:29 am
What I'd really like is something which brings the TT's VGA output within the standard dot clock range so that LCD monitors can sync correctly and give a good display. However, I think this would have to be some sort of hardware which buffered and replayed each scan line slightly slower.
You mean TT's 32Mhz pixel clock vs VGA's 25Mhz?

Petari
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Re: TT Software overscan?

Post by Petari » Wed Jun 06, 2018 10:08 am

VGA pixel clock is not only 25 MHz. At higher resolutions it goes over 80 MHz .
The problem is not only with TT, but any ST in mono-res mode (640x400) too. Horizontally you see some periodical luminance change. Almost like vertical bar. The reason is of course that pixel clock is not 'syncronised' with dots (pixels) of display :D Some kind of aliasing.
User can only to set horizontal width and phase (pos) to make it better.
You don't see it when run PC on VGA via analog cable, but it is present. Only that resolution of video signal is much bigger.
In theory, best would be to do some converter, which produces HDMI digital signal. But that's not simple. Probably simpler is to change TT's video stage to HDMI type, and it will give best results.
There is 2 kind of people: one thinking about moving to Mars after here becomes too bad, the others thinking about how to keep this planet habitable.

Atarian Computing
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Re: TT Software overscan?

Post by Atarian Computing » Wed Jun 06, 2018 10:23 am

Petari wrote:
Wed Jun 06, 2018 10:08 am
Some kind of aliasing.
User can only to set horizontal width and phase (pos) to make it better.
My NEC 1990SXi is really good at "fixing" TT's resolutions but many monitors can't do much about it.

mrbombermillzy
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Re: TT Software overscan?

Post by mrbombermillzy » Wed Jun 06, 2018 8:25 pm

Petari wrote:
Wed Jun 06, 2018 7:52 am
Can I ask why you need TT SW overscan ?
The reason I wanted to do this is because of the problems detailed above. (i.e. very big borders which could be better used for more screen size).

I believe I read somewhere that the VGA signal standard was changed just after Atari set it into silicon for the TT, hence the very oversize L/R borders.

Ray has done a demo which displays 1280x960 with 32 colours per scanline.

I havent had time to analyze the code, (theres no source code provided unfortunately) but Im guessing he is using TT med res with L/R border removal and doing a palette change in the middle of a scanline.

Im just surprised that no one has looked into this yet, as that demo was coded a good few years ago now.

Im not able to get to my TT ATM, but even when I do, without knowing any tricks for fooling the shifters display into not blanking the borders by whatever means Im not sure I'd know where to start.

Atarian Computing
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Re: TT Software overscan?

Post by Atarian Computing » Wed Jun 06, 2018 8:41 pm

mrbombermillzy wrote:
Wed Jun 06, 2018 8:25 pm
Petari wrote:
Wed Jun 06, 2018 7:52 am
Can I ask why you need TT SW overscan ?
The reason I wanted to do this is because of the problems detailed above. (i.e. very big borders which could be better used for more screen size).

I believe I read somewhere that the VGA signal standard was changed just after Atari set it into silicon for the TT, hence the very oversize L/R borders.

Ray has done a demo which displays 1280x960 with 32 colours per scanline.

I havent had time to analyze the code, (theres no source code provided unfortunately) but Im guessing he is using TT med res with L/R border removal and doing a palette change in the middle of a scanline.

Im just surprised that no one has looked into this yet, as that demo was coded a good few years ago now.

Im not able to get to my TT ATM, but even when I do, without knowing any tricks for fooling the shifters display into not blanking the borders by whatever means Im not sure I'd know where to start.
I believe it's 1280x480 and also interlaced 30Hz. http://s390174849.online.de/ray.tscc.de/misc.htm

mrbombermillzy
Posts: 25
Joined: Sun Jun 03, 2018 7:37 pm

Re: TT Software overscan?

Post by mrbombermillzy » Wed Jun 06, 2018 8:50 pm

Atarian Computing wrote:
Wed Jun 06, 2018 8:41 pm
I believe it's 1280x480 and also interlaced 30Hz. http://s390174849.online.de/ray.tscc.de/misc.htm
You are correct.My mistake. But even still, what trickery is going on here to get the 1280 H pixels?

Reducing the vertical scanrate would give some extra bandwith for horizontal pixels, but how could the horizontal displayed pixels be adjusted from the shifter/glue?

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