[MPlayer-users] Question about aspect ratios

[Lasse Kärkkäinen / Tronic]

WARNING: Message coming from me - it's off-topic, as usual.

Might contain some interesting information though.

DAN> square pixels, but this is a fairly recent trend (not by computer history
DAN> standards, of course :-). Anyone remembers 'EGA' with its weird 640x350
DAN> resolution, or CGA with 640x200?

I remember that odd resolution, which people often used on their
VGA-displays ... The resolution has aspect-ratio of 1.25:1, while the
display it was used on always had 1.33:1. Yes, the resolution we are
talking about is 1280x1024 (aka SXGA). Since everything in PC-world is
designed for 1:1-pixels (meaning that the display-mode aspect-ratio
should be equal to monitor aspect-ratio), one should use 1280x960,
1320x990 or something else instead.

DAN> Of course, the civilized world uses either PAL or SECAM as a TV standard,

True :)

Actually there are even better standards, but unfortunately those are
not widely used.

DAN> 50Hz, respectively). Unfortunately, the majority of the movies come from
DAN> less civilized countries, which stick to an obsolete and techincally
DAN> inferior standard called NTSC, which uses lower line frequency (about

Fortunately real movies are not for NTSC, but for film. Since film is
24 frames per second and has "analog" resolution, it is perfect for
PAL. We only need to speed it up by 1/24, so that it runs at 25 frames
per second. Then we scan those frames in 720x576 (DVD-resolution, with
non-1:1-pixels). These frames are cut in two "fields", so that we get
720x288-sized field 50 times per second (this is called interlace).

What is nice with this is that when you watch that DVD on PC, those
fields are combined (yes, it's 720x576 at 25Hz again) and you have every
original frame of the movie in 720x576-resolution.

With NTSC the speed-difference to original film is too big (24 vs.
30), so they use very dirty tricks to make it work. You don't want to
watch NTSC-movies even with NTSC-equipment and you definately don't
want to watch those on PC (proper deinterlacing-software helps a bit,
but the quality is still far behind from PAL).

DAN> border area proportions, one gets 640x480 (interlaced) in the civilized
DAN> world, and 640x400 (interlaced) in the movie-making countries. The shape of

Nope. PAL is 768x576 and NTSC is 640x480 (of course the x-resolution
may vary, but those resolutions give 1:1-pixels for 1.33:1-picture).

For anamorhic 1.78:1-picture corresponding resolutions would be
1024x576 and 854x480.

In DVD the horizontal resolution is always 720 - that is more than
enough for TV, since even high-quality TVs are limited to something
around 400-500 pixels on a line, no matter how many pixels the
source-signal has.

One more thing - AVI only supports 1:1-pixels, so you should never
compress AVI using different aspect-ratio. It kinda works and it can
be played back with MPlayer (if you know the aspect-ratio to supply
MPlayer from commandline), but most players can't do it and it is very
painful with those few players which "support" it.

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Lasse Kärkkäinen aka Tronic      http://snull.cjb.net
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