Date : Tue, 30 May 2000 19:48:03 +0100
From : Stuart William McConnachie <stuart@...>
Subject: Re: Screen proportions?
In message <Pine.LNX.3.96.1000530135757.20291D-100000@...
l.ac.uk>, jim <jim@...> writes
>> does anyone happen to know the correct proportions of the pixel area of the
>> screen? Are pixels square at 320x256?
>
>Computer monitors have the same aspect ratio as televisions i.e. 4:3 so
>pixels are square at 320x240 and so on up. Beeb screens are 5:4 and have a
>virtual area of 1280x1024 pixels, which is coincidentally the only
>non-square video mode in common use on modern PCs.
With respect, you haven't taken into account the fact that, on the BBC
there is a substantial unused border to the left and right of the
screen, but virtually none top or bottom.
My calculations (see below if you're interested) would indicate that the
BBC pixels are actually slightly taller than they are wide which is the
opposite of what you might expect. i.e. a 1280x1024 matrix, if
displayed with no border on a 4:3 ratio screen, should give pixels which
are slightly wider than they are tall.
I've just drawn a square box (1024x1024 pixels) on may Master connected
to a CUB monitor. The resulting rectangle is 180mm wide by 187mm high,
which gives a pixel aspect ration of 0.963. This is slightly closer to
square than calculations would predict. Anyone care to do the same for
a TV screen to compare results?
For practical purposes I'd consider that the BBC pixels are indeed
"square". Anyway they're certainly more square than the pixels a PC
displays in a 1280x1024 mode.
CALCULATIONS:
I selected MODE 1/4 for the following example because the pixels have
the same horizontal and vertical size (4 virtual co-ordinates each),
which theoretically should imply square pixels.
The PAL field rate is 50Hz = 20ms per field
During this time 312.5 lines are displayed = 64us per line
The 6545 displays 128 characters per line (see AUG) = 500ns per
character
Each character consists of 4 pixels = 125ns per
pixel
Alternatively, the clock speed, as supplied to the Video ULA, is 16MHz
62.5ns
But this is divided by 2 in mode 1 (pixels are twice the width of MODE
0) = 125ns per pixel
The PAL standard defines 52us (out of the total 64us) of picture
information per horizontal line
Therefore 1 pixel occupies 125ns/52us of the entire width of the screen
= 0.240%
There are 290 usable lines in each PAL field (the rest are used for
vertical retrace), therefore each line is 1/290 of the entire vertical
area = 0.345%
A PAL screen aspect ratio is 4:3
Therefore each pixel is 4 * (125 / 52000) units wide = 0.009615
And 3 * (1 / 290) units high = 0.01034
Consequently the ratio of width to height of each pixel is 0.009615 /
0.01034 = 0.930
PAL timings from http://godot.tuniv.szczecin.pl/~mjaskula/tomi/video.htm
l
--
Stuart McConnachie (stuart@... )
43 The Hollows, Long Eaton, Nottingham, NG10 2ES, UK