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Non-standard VGA mode setting.

deathshadow

deathshadow

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Jan 4, 2011
Messages
1,378
I could have sworn there was a way to set a non-standard 768x512 mode on a standard VGA. I have the easy 720x480 mode up and working, but attempts to narrow the front/back porch are immediately taking things out of sync.

I know there's the VESA 768x576 that most clones can easily handle the timings for if the display can, but how far can one push a vanilla out of the box VGA like that in... well, this model 30-286 I have sitting here?

Again, my ideal would be 768x512 for this... project I'm working on. It's kind of a "what if".
 
VGA can be tweaked into 768x576 (30.3 kHz/49 Hz), and even 800x600 (29.3 kHz/45 Hz).
Both work for me on a regular SVGA monitor (30+ kHz? or 31+ kHz?), but I don't know about others.
 
You'll want to look at the source code of Fractint - that supports a lot of tweaked modes on standard VGA up to and including 800x600. It was also popular enough that the timings were probably tuned for maximum compatibility. However, my experience with it was that it was highly monitor-dependent - many monitors didn't cope with the lower sync rates of higher resolutions. And even the resolutions that gave a stable image sometimes had artifacts like audible whining from the monitor, the beam being (partially) visible during retrace, and bending or even "folding over" at the edges. So by all means play with it and even support it in your software but please give users an option to use standard resolutions if their monitors can't cope with the tweaked ones.
 
Xacalite said:
VGA can be tweaked into 768x576 (30.3 kHz/49 Hz), and even 800x600 (29.3 kHz/45 Hz).
Both work for me on a regular SVGA monitor (30+ kHz? or 31+ kHz?), but I don't know about others.
Click to expand...

Yeah, I'm hoping for someone to already have the 55hz 768x512 -- I actually "need" the 3:2 aspect ratio. (well, I don't "need" it, but it would be nice).

reenigne said:
You'll want to look at the source code of Fractint
Click to expand...
Thanks, that's the codebase I was partly remembering but it's been so long I couldn't remember it's name. I could have sworn there was another that set a 360x512 mode that I applied the same values to the 16 colour 720 and it worked...

But since it has the 768x576 I hope I can just decrease VT/VDE/VBS/VBE to numbers that make sense and be done with it.

reenigne said:
So by all means play with it and even support it in your software but please give users an option to use standard resolutions if their monitors can't cope with the tweaked ones.
Click to expand...
I will also be implementing the option of the 720x480 mode which I know is safe on everything, even non multisync displays; though it's going to be using a TSR driver for most video control so adding/removing modes is not a big deal.

It's just 768x512 is the "ideal" resolution for what this is supposed to be... strange as that might sound.

Also having fun turning the 16 colour mode into a 4 colour mode with paging. Dunno if you folks ever saw that trick -- you change the palette to page-flip. When page 0 is set you make all colours for the bottom 4 bits show ignoring the top four, then for page 1 you show the top 4 bits ignoring the bottom 4.

For example if you wanted 4 scale grey:
Code: 
Page 0
0x00 = 0b00
0x01 = 0b01
0x02 = 0b02
0x03 = 0b03
0x04 = 0b00
0x05 = 0b01
0x06 = 0b02
0x07 = 0b03
0x08 = 0b00
0x09 = 0b01
0x0A = 0b02
0x0B = 0b03
0x0C = 0b00
0x0D = 0b01
0x0E = 0b02
0x0F = 0b03

page 1
0x00 = 0b00
0x01 = 0b00
0x02 = 0b00
0x03 = 0b00
0x04 = 0b01
0x05 = 0b01
0x06 = 0b01
0x07 = 0b01
0x08 = 0b10
0x09 = 0b10
0x0A = 0b10
0x0B = 0b10
0x0C = 0b11
0x0D = 0b11
0x0E = 0b11
0x0F = 0b11

Which is shockingly easy to control using the page mask registers. Just need to be careful as the page flip can trigger the appearance of some odd colour flashing (since you're changing the palette) -- so I wait for the blanking period.
 
Not to come back months later, but I finally had some time to play with this. (two other projects are ahead of this)... and I'm an IDIOT!

768x576 is 3:2 and aspect correct. The thing I was trying to "one-up" has this bizarre 12:9 ratio despite the wild historical claims of it being a perfect fixed PPI of square pixels. Just another bald-faced lie from a company who built their entire reputation and product lines on bald-faced lies I guess.
 
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