Date : Fri, 02 Apr 2004 13:14:33 GMT
From : "Thomas Harte " <thomasharte@...>
Subject: Re: Aspect ratio question
> > Because to be square would require a clock rate substantially
> > different to those available to any other chip,
> > and would make the CRTC6845 default values in the BBC appear
> > something of a nonsense.
>
>I disagree with that! See, for example, the URL which Richard quoted:
>
> http://www.howell1964.freeserve.co.uk/logic/video_clone.htm
>
> notably the paragraph headed "Square Pixels" (but ignore the obvious
> typo of 56µs where 52µs is clearly intended, in the first diagram :-))
"I found the BBC micro in 320x240 pixel mode (8 MHz pixel rate) gave almost
square pixels"
Now, supposing Tom Seddon is right, and the image is ~1.19 times as wide
as tall. This means that characters
are 95% as wide as tall. On a 12" screen such as your monitor, in Mode 4,
if 48µs (and a similar proportion of
scanlines) of PAL image were visible then the entire display is just 11mm
off being genuinely square. Each
individual mode 4 character is 0.28mm thinner than a square one would be.
Looking at it another way - "a small error in either setup or measurement
will account for much of [any]
difference". Therefore, could it not possibly be that the 'true' output is
not quite square, but since 99% of
people with their imperfect screens will see the image stretched a little
one way or the other, they decided it
was square enough that it wasn't worth bothering about?
From the same page's table of default 6845 values:
Horizontal:
Total number of characters on a line minus one = 63
The number of displayed character cells per line = 40
Position of horizontal sync = 49
Sync width = 4 chars
implies:
Total time over which pixels are output = 59 chars
left border: 10 characters (output chars - position of sync)
pixels: 40 chars
right border: 9 characters (position of sync - number of chars)
For the image to be square, if overscan didn't exist horizontally, we therefore
need 40/59 * 4/3 of 312
scanlines to be visible. Which is 160/177 of 312 lines, or 282 (and a bit)
i.e. we need the full defined PAL visible
area to be 59µs and 282 scanlines.
That page says PAL active display area is 56µs, so what we genuinely want
40/56*4/3 of 312 - 20/21*312 - 297
active display scanlines.
The defined PAL visible area in scanlines is 288, so if we had a TV screen
that displayed all 288 scanlines, we'd
then want it to display just over 55.23µs of active display to get square pixels.
My point? That I can easily believe that what in an ideal perfect world is
not square pixels is close enough for
nobody to care less about the difference when producing software. However,
it would be correct behaviour for
an emulator to produce the not quite square output as people will continue
to view it through a not quite ideal
perfect monitor. Albeit one they can adjust themselves.
> I think we're going to have to agree to differ. You will not convince
> me that a BBC Micro's displayed image in Mode 1 or similar is not
> designed to have square pixels (or virtually square pixels)
I'm not trying to persuade you they aren't _virtually_ square. I'm not even
persuaded we can agree to differ,
because it doesn't look to me like we do. The differences we're talking about
are very minimal. But I'm nothing if
not petty about making my emulator accurate!
-Thomas
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