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Posted by George Graves on 12/08/05 15:32
I have a technical question about DVD aspect ratios. I'm not sure
exactly how to phrase it so that it's clear, but I've come here because
I can find nothing on the web that covers this.
As you probably know, many HDTVs and HD-ready sets automatically assume
that any time they see a component video input that has a 31.5KHz raster
or higher, it's indicative of a high-definition program. This causes the
TV to automatically apply the anamorphic stretch to the standard NTSC
4:3 frame to make it 16:9. In some sets, this anamorphic stretch is user
defeatable, but in most, it is not.
Most modern DVDs made from old TV shows, older, non-widscreen movies, or
even DVDs that have the widescreen picture letterboxed rather than
anamorphically squeezed, display the progressive picture output fine on
sets where the anamorphic stretch can not be defeated, but other DVDs
will display the progressive output as 16 X 9 when they should display
it as 4:3 - with the inevitable distortion to the picture, of course.
My question is simply this. Why is it that some 4:3 DVDs get stretched
to 16:9 when played on a progressive player (irrespective of what one
does with the "aspect ratio" controls in the set-up menu) while other
4:3 DVDs don't get stretched? Is there a 'bit' that can be set in the
DVD headers at mastering which tells the player that the picture is 4:3,
and when that bit isn't set, the player outputs the picture as 16:9? Or
is the content of the discs that display correctly mastered
anamorphically squeezed complete with side-bars? If the latter, how come
they still look correct when played back on a normal NTSC monitor using
composite or S-VHS inputs?
I need to know this because I review a lot of DVD players for an on-line
video magazine and I've not seen ONE player that will allow some of
these 4:3 DVDs to display properly when output progressively or
up-sampled and I'm at a loss to explain exactly why this happens.
Thank you in advance for any insight into this phenomenon that you can
give me.
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