Date   : Thu, 01 Apr 2004 10:06:13 GMT 
From   : "Thomas Harte " <thomasharte@...>
Subject: Re: Aspect ratio question

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.
 
Pete Turnbull:
> OK, so you want the image on some screen to have the same aspect ratio
> as the image on a real monitor, which we assume is reasonably correctly
> adjusted, connected to a real BBC Micro.  That ratio is 5:4.  Not just
> in theory, but when I measure the image on the screen beside me, which
> is a 12" Zenith monitor, correctly adjusted, made in 1984, connected to
> a BBC B in Mode 0, it is 213mm wide x 171mm high (measured across line
> 16 and down column 40).  That's 1.246, or as close to 5:4 as matters.

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

> Hmm, where have I seen that before, oh yes, your Mode 7 display :-)

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.

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

> This is the calculation where you said "each scanline is 52 µs of
> visible time, of which the Acorn machines use 40 µs"?

By which I meant the defined PAL visible area, not necessarily the area actually
visible on an actual screen, 
which was taken account for later in the calculations. To sum up, my calculations
were predicated on the 
following belief only:

"The total PAL area is defined to be 4:3, and a 4:3 ratio image is defined
by 52 µs sections of scanlines for 288 
scanlines."

That TVs usually don't display it all makes absolutely no difference. In
any case, Tom Seddon's practical 
experiment seems to back up the calculations.

> > 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.

> Pete      Peter Turnbull
>      Network Manager
>      University of York

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.

-Thomas

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