Date   : Thu, 01 Apr 2004 18:59:07 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: Aspect ratio question

On Apr 1, 10:06, Thomas Harte  wrote:
>
> tom@...
> > Mode 4 on a BBC gives a 9.25" x 7.75" display on my TV, give
> > or take
> > a few mm, which is 1.193:1, so it sounds like you are close.
>
> Tom Seddon's practical
> experiment seems to back up the calculations.

But my measurements, posted last night, disagree.  As I mentioned in my
reply to Richard a few moments ago, a small error in either setup or
measurement will account for much of that difference, and TVs are
notorious for being badly set up.  I once had one which some "engineer"
had set up using a Teletext news page to centre the image.  Needless to
say, he's not realised that he's centred the rightmost 38 columns (he
didn't know that most pages with coloured backgrounds have two
invisible control characters at the start).

> So, in summary this does very much appear to be a TV versus monitor
thing.

There would be no difference at all between a correctly-adjusted TV and
a correctly adjusted PAL monitor.

> My Mode 7 display was, as I explained, scaled by the emulator to
640x512 in order to fill the same graphics area
> as the normal modes despite the source image being just 480 pixels
across. The point it made was that scaling
> images to the correct aspect ratio does not, as you suggest, lead
always to gross ugliness.

I didn't say it was ugly, because it's not.  Nevertheless, when I scale
it up to fill the screen -- exactly x2 -- I can see the aliasing, so
it's not *accurate*.  Mode 7 doesn't contain any fine lines because its
basic resolution is only 240 x 250 (inside the SAA5050, which then does
some character rounding).  The lack of fine detail means aliasing is
not really a problem, but it could be if you did the same thing to Mode
0.  Not likely for a digitised picture, which the smoothing might
actually improve, but for some fine detail it could.

> Are we talking about the same image?  I'm talking about the exact
area
> you can plot pixels in, ie not including the borders.

Yup.

> > > I'm just saying: I don't see how they can be square from the
clock
> > > rates, and they never looked square to me.
> >
> > Why can't they be square?
>
> 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 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) and an
aspect ratio of 5:4, since I and many others, including Acorn,
Acornsoft, Oak, etc, have been using them for over 20 years to draw
squares and circles based on that premise.

> I just realised (I usually skip signatures): I graduated from your
> university last year! I would ask how the old
> place is, but as I'm still in York I could very easily check for
> myself.

You could :-)   Warn me the day before and I'll bring the Beeb and a
monitor in ;-)  Sadly, we no longer have any active Beebs in the
Computing Service!

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York