Date   : Thu, 01 Apr 2004 10:43:25 +0100
From   : Richard_Talbot-Watkins@...
Subject: Re: Aspect ratio question

Pete Turnbull wrote:

> This is the calculation where you said "each scanline is 52 µs of
> visible time, of which the Acorn machines use 40 µs"?
>
> Firstly: that's not strictly true.  That 52µs is the non-blanked
> interval, not the visible part, because of overscan.  Or to put it
> another way, the other 12µs is only the horizontal blanking
> interval, without any consideration of non-visibility because of
> overscan.  The visible part is less than 50µs.  That makes the
> Beeb usage significantly more than 80% of the total, not 77%
> (10/13).

No, Thomas is right.  The PAL horizontal period is 64us.  12us of this
contains the horizontal flyback (non-visible).  The remaining 52us is the
visible time, of which 40us is the actual memory-mapped screen, and the
rest is the left and right borders.

There's a bit more on this at:
http://www.howell1964.freeserve.co.uk/logic/video_clone.htm

These calculations are starting to do my head in, so I have no idea whether
they're on the right track or not :)

(An aside: The Beeb *appears* to use a sync pulse width of 4us and a front
porch of 2us, rather than the PAL standard values of 4.5us and 1.5us - at
least, this is the only way I can see that it can set the horizontal sync
position to 49 and still have equal left and right borders... unless anyone
can explain otherwise?)

> Why can't they be square?
>
> They are *designed* to be square, so that if you draw a circle with
> a diameter of, say, 1000 OS units, it comes out visibly as wide as
> it is high, or if you draw a "square" which is 1000 OS units high
> and 1000 OS units wide, it actually is square.  This is the same
> reason that pixels in 1024 x 768 or 1280 x 1024 screen modes on
> modern machines are designed to be square.

I would say they are designed to be roughly square, but probably do not
display quite as such on some screens.  For example, on my TV set, a 24x24
pixel block (a size I became very used to plotting from my games writing
days) looks slightly taller than it is wide.  I'd say calculations are all
very well, but there's no substitute for getting an appropriate image on
screen and then measuring its dimensions with a ruler!

Rich



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 SCEE 2004