Date : Thu, 04 Jan 2001 11:41:47 +0000
From : jgh@... (Jonathan Graham Harston)
Subject: Aspect ratio of BBC screen?
"Mark de Weger" <mark@...> wrote:
> Can anyone tell me what the aspect ratio of actual BBC screens is? (I
> don't have a BBC at hand to test.) I thought I inferred from old Acorn
> documentation it's 1.25. However, in the emulators (BeebEm at least)
> it's 1.33. And then I read somewhere it varies according to the screen
> mode. Help!
You can work out what it /should/ be:
A BBC with a non-US display has a theoretical display of 80 columns by 32
text lines, that is 1280 by 1024 logical pixels in a 4:3 aspect screen.
This gives a 320:341.33 logical pixel size which resolves to 1.066.
A BBC with a US display has a theoretical display of 80 columns by 25
text lines, that is 1280 by 800 logical pixels in a 4:3 aspect screen.
This gives a 320:266.66 logical pixel size which resolves to 0.833.
In MODE 1 physical pixels are 4 logical pixels wide by 4 tall. On the
vast majority of monitors I have used pixels have visually looked square,
which 1:1.066 would appear. You can adjust your monitor so that the ideal
square actually appears square.
I also tended to ascribe a MODE 1 physical pixel to be 1mm by 1mm square,
making a MODE 1 screen 32cm wide and 25.6cm tall - about right. MODE 0
physical pixels can be ascribed to be 0.5mm wide and 1mm tall.
MODE 3 and MODE 6 have the same aspect ratio, it's just they have a lesser
number of text lines, but padded out with two blank lines between each. A
MODE 0 screen is 32*8=256 pixels high, a MODE 3 screen is 25*(8+2)=250
pixels high. The left over six pixels are accounted for by adjusting the
top-of-screen offset. The BAUG describes physical screen layouts quite
informatively.
--
J.G.Harston (JGH BBC PD Library) 70 Camm Street, Walkley, SHEFFIELD S6 3TR
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