Date   : Thu, 15 Feb 2001 22:02:19 +0000
From   : Philip Peake <philipp@...>
Subject: Re: Aspect ratio of BBC screen?

Isn't the video output of the Beeb based on the PAL TV specs? (and the PC's VGA
similarly based on NTSC)

This makes the 'natural' ratio the 640 x 512 (ie 256 interlaced) in Mode 0.

Which in itself is based on a 4:5 ratio -- the 'golden' rectangle invented by
some Greek chap (it might have been Pythagoras (?)) -- equals 1.25

Phil



"B. E. Newsam" wrote:

> In message <NDBBIEGJPLKLGGJFKFPDEELGCBAA.mark@...>, Mark de
> Weger <mark@...> writes
> >> "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!
> >
> >After the different answers I got, I did some measurements myself.
> >
> >First, I measured a number of screen shots from magazines. The average
> >aspect ratio was 1.24 (with quite a standard deviation--aspect ratios
> >varying from 1.13 to 1.35, probably due to different monitor settings).
> >
> >Then I did some measurements using Beebem, trying to get a perfect circle.
> >According to my calculations (based on slightly imprecise measurements), the
> >aspect ratio should be 1.253.
> >
> >So it seems quite likely the actual aspect ratio is 1.25.
>
> The reason that you get different answers to the question is that there
> are indeed several different answers.
>
> As far as "logical pixels" go, 1280 / 1024 is indeed 1.25, so the
> logical aspect ratio of the screen is *exactly* 1.25.
>
> In various screen modes, this is mapped onto physical pixels in various
> ways. If you draw a rectangle round the outside of the screen (0,0 to
> 1279,1023), you may well find that the ratio is not exactly 1.25. It is
> the difference between the two that is of interest.
>
> On a TV screen it could be almost anything. On an old Microvitec Cub
> monitor, I found the ratio between the two values to be about 1.175. A
> more recent Philips is just about perfect.
> --
> Ben