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ST display ratio

Posted: Tue Mar 27, 2018 9:56 am
by BlankVector
Hi, I wonder what is the correct display ratio for ST Low / Medium video modes. The goal is to use correct ratio in screenshots / videos.

You can easily help by measuring the actual size of the 320x200 screen on your monitor.

My monitor is 16/10, and the image is slightly distorted, even in 4/3 mode, because it wrongly advertises it is a 16/9 monitor, while it is actually a bit taller. Anyway.

So on my distorted monitor, when I measure the 320x200 screen (without borders), 50 Hz, I get:
Width = 326 mm
Height = 217 mm
Display ratio = 326 / 217 = 1,502

What about your monitor?

I consider that the standard ratio is the one obtained on a normal 4/3 cathodic TV. It should be pretty constant (I hope).
I remember that the Atari color monitor has a button to adjust the height of the image. I fear that people used much that button, and got a distorted image most of the time. It would mean there is no standard display ratio...

Re: ST display ratio

Posted: Tue Mar 27, 2018 10:24 am
by Moulinaie
Hi,

If you consider that pixels should be squares : 320/200 = 1,6 for the ratio.

Guillaume.

Re: ST display ratio

Posted: Tue Mar 27, 2018 12:04 pm
by Foxie
You should be able to calculate this from standard PAL timings:

Line total time: 64us
Line active time: 52us
Lines total: 625 (626 on ST)
Lines active: 576
Aspect: 4:3

The ST's display area is:
Line active time: 40us (77% of the active line)
Lines active: 400 (70% of the lines)

Therefore the screen display area has a 1.1 aspect ratio. The screen is wider than it is tall. This is a 4.4:3 aspect.

Medium resolution will be identical, except with half the pixel width. High resolution is probably completely different.


Moulinaie wrote:If you consider that pixels should be squares : 320/200 = 1,6 for the ratio.


The pixels aren't square, however. This is why ST graphics look so distorted on an IBM compatible.

Update: corrected calculation mistake

Re: ST display ratio

Posted: Tue Mar 27, 2018 12:07 pm
by troed
ST pixels are not square - they're 0.9/1 (low resolution).

Re: ST display ratio

Posted: Tue Mar 27, 2018 1:45 pm
by joefish
Yes, it's a bit of a shame they're not square. The ZX81 / ZX Spectrum / MSX-1 / Oric have square pixels with a 256x192 display which is exactly a 4:3 ratio. On a lot of CRT TVs, that meant that their pixels aligned perfectly with the dots on the TV screen (although this could drift over time with RF modulator noise).

The ST aligns in height since the scanline spacing on a TV is the same, but in width it has 320 pixels in roughly the same space, so the pixels are squashed. Same happens with the Amiga, CPC and C64 in their equivalent resolutions.

Re: ST display ratio

Posted: Tue Mar 27, 2018 2:04 pm
by ranix
looks great on my 4:3 lcd monitor, which operates natively at 15khz 320x200. There are still 15 left on Amazon

https://www.amazon.com/gp/product/B00DZFOVZW/

Re: ST display ratio

Posted: Tue Mar 27, 2018 3:51 pm
by krupkaj
Unfortunately this BenQ does not display hires mode really well. I think it selects a wrong mode and scales slightly down.

Re: ST display ratio

Posted: Tue Mar 27, 2018 4:09 pm
by Foxie
joefish wrote:Yes, it's a bit of a shame they're not square. The ZX81 / ZX Spectrum / MSX-1 / Oric have square pixels with a 256x192 display which is exactly a 4:3 ratio. On a lot of CRT TVs, that meant that their pixels aligned perfectly with the dots on the TV screen (although this could drift over time with RF modulator noise).

The ST aligns in height since the scanline spacing on a TV is the same, but in width it has 320 pixels in roughly the same space, so the pixels are squashed. Same happens with the Amiga, CPC and C64 in their equivalent resolutions.


Are you sure TVs have a standard shadow mask spacing? I thought they tended to be all over the place. Being an analogue system, they wouldn't align very well anyway. It's probably best to use a tube with the finest dot pitch you can find - I noticed a huge improvement moving from a TV to a monitor in dot pitch.

The Amiga has a different screen area aspect to the ST. This arises because it uses a 7.1MHz pixel clock - instead of 8MHz like the ST. The 320 pixels fill more of the screen horizontally, leaving a much smaller border. The Amiga also displays 256 scanlines normally - with more actual screen height than the ST.

Amiga screen width: 86%
Amiga screen height: 89%

So the Amiga has a 3.9:3 display area. However, the pixels still aren't square. They're a different kind of rectangular than the ST. If you display ST graphics on an Amiga, it will be stretched horizontally.

In order to have completely square pixels on a PAL screen, you'd need a resolution of 384 x 288 pixels (with overscan). A 320x240 display area would be 4:3 with square pixels, and would leave a decent border. The problem is, this would require a pixel clock of around 7.4MHz. That's a pretty unusual frequency, and I doubt there are many computer manufacturers who used it.

If the Spectrum really does have square pixels, the screen must be no wider than 67%. That's pretty narrow, it would leave a huge left and right border - bigger than the ST.

Re: ST display ratio

Posted: Tue Mar 27, 2018 6:13 pm
by Moulinaie
Hi again,

For my STE, I am using a video converter to display the color modes on a VGA monitor. I don't lknow if that changes the ratio... The VGA monitor is a 4:3 one.
In centimeters, I get:
24,7 x 17,3 close to 1,43.

Guillaume.

Re: ST display ratio

Posted: Wed Mar 28, 2018 1:00 am
by ranix
krupkaj wrote:Unfortunately this BenQ does not display hires mode really well. I think it selects a wrong mode and scales slightly down.

That is true, it doesn't do quite a perfect job of hires mode

Re: ST display ratio

Posted: Wed Mar 28, 2018 9:57 am
by joefish
Well, for a start, the alignment of the horizontal pixel rows with the scanlines on the TV is fixed in all cases.
And secondly, not all the 625 lines of the PAL format are visible on a TV screen, which means the borders don't appear as big as they are in theory.
But on the ZX Spectrum, the borders are quite wide. You can see another 40 pixels worth or so of border to the left and right of the display.

Close inspection shows there aren't really 625 distinct lines of phosphor dots on a typical CRT TV screen. There's half that, but every second dot is offset vertically by half a dot so the alternate PAL frames do sort-of target different dots.

There is no 'standard' dot pitch for a TV either, but the number of scan lines is fixed, and so if they make the phosphor dots square (which most seem to do, for want of a better idea) then many are going to have the same horizontal pitch as vertical. So a well-timed properly 4:3 picture can hit them exactly. Also bear in mind that most 8-bit and 16-bit machines' clock speeds aren't exactly the quoted speed - they're all deliberately set some small fraction off the stated number of MHz so as to get the timing exactly right for the TV image they're trying to render.

And it does work - hook up a ZX81 to a good-quality CRT TV like my old Sony 14", bypassing the RF modulator to get a composite signal, and you can draw a chessboard 10101010 / 01010101 pixel pattern on the screen with no distortion. Do it with an ST and you get vertical bands of blurring.

Re: ST display ratio

Posted: Wed Mar 28, 2018 9:38 pm
by Foxie
joefish wrote:And it does work - hook up a ZX81 to a good-quality CRT TV like my old Sony 14", bypassing the RF modulator to get a composite signal, and you can draw a chessboard 10101010 / 01010101 pixel pattern on the screen with no distortion. Do it with an ST and you get vertical bands of blurring.


I haven't noticed any vertical bands of blurring in low res. I just see a fairly sharp chequerboard pattern. In medium res, they just blend into a solid colour. Are you sure your ST is connected via RGB?

There does exist an issue with the ST and the colour carrier, because the pixel clock is 8MHz and the colour carrier is 4.43MHz. There's a synchronisation circuit, but it can't synchronise every cycle. This could create some very bad artifacts on composite video.

The actual alignment between the pixels and the shadow mask varies depending on so many factors. There's the H and V picture size controls, which are always set a little differently on every TV or monitor. Also, the EHT is often not regulated on TVs and monitors - so the picture size shrinks and swells significantly depending on how much white is on the screen.