A lot of misinformation floating around here. Let's clear it up.
The 525 and 480 designations are exactly the same thing. In both
cases, the video signal is comprised of 525 scanlines, but only 480
contain actual picture information. The other 45, which do not contain
any picture information, are provided for timing reasons.
The native format of DVD is 480i, the operational mode of traditional
televisions. 480i is also the native format of traditional video
cameras. Most DVDs, however, are sourced from film, not video. Film is
natively 24 frames per second, and not interlaced. When a film is made
into a DVD, It is converted to 480i through a technique called "3:2
pulldown":
http://www.zerocut.com/tech/pulldown.html
On a "normal" TV, this is how the DVD would be displayed -- as a 480i
conversion of a 24fps source. To our eyes, this looks perfectly
acceptable.
It is not, however, an accurate representation of the film.
Fortunately, it is not difficult to reconstruct the original film
frames via a process known, not surprisingly, as "reverse 3:2
pulldown". This is why "progressive scan" is a buzzword around DVDs --
applying reverse 3:2 pulldown to a film-sourced DVD yields a
progressive image. (Ideally, it would be displayed at 24 frames per
second, or a multiple such as 48 or 72 frames per second.)
Most HDTVs are capable of performing reverse 3:2 pulldown, and perform
it automatically when film-sourced material is detected. This is one
reason why DVDs usually look better on HDTVs.
Another reason is due to the fact that HDTVs are generally far more
capable display devices than tradtional televsions. HDTVs come in many
different varieties (CRT direct-view, CRT rear projection, DLP, LCD,
LCOS, plasma, etc.), and the details vary considerably between them,
but the general idea is that they have a much higher "fill rate" than
typical televisions--more of the image is illuminated at once, at a
higher refresh rate. This provides a richer, more vivid image with
fewer temporal artifacts, such as flicker. In the CRT world, simply
providing more scan lines provides a more pleasing image, even if
there is no additional detail. This is particularly true for larger
screen sizes.
In addition, most widescreen DVDs will actually show higher resolution
on an HDTV, because of anamorphic encoding. Anamorphic encoding
involves stretching the widescreen content vertically to cover the
entire video frame during the encoding process. This allows the full
resolution of the DVD format to be used. All DVD players are capable
of the inverse transformation -- squashing the picture vertically to
the correct aspect ratio for display on a normal 4:3 aspect ratio
televsion (and losing some detail in the process). A properly
configured HDTV setup will perform this transformation such that the
full resolution encoded on the DVD can be used.
If the 480 scanlines of data are "expanded" to 720 or 1080, it is
performed in exactly the same manner as resizing an image on your
computer -- no additional information is added. The increase in image
quality is not a direct result of the image being resized, but rather
because display devices typically produce a more pleasing picture with
more scanlines.
If you have a particular interest in a specific display device, I'd be
happy to go into greater depth about how it is affected by these
issues. |
Clarification of Answer by
alexander-ga
on
17 Feb 2004 19:14 PST
My apologies--we don't get automatically notified when rated, only
when a clarification request is posted.
1) Yes, both 480p and 480i (NTSC) have the same number of lines: 480.
However, 480p has twice as much "information", because the complete
frame is updated every 1/60th of a second. In 480i, only half of the
frame (alternating sets of alternating lines) is updated each 1/60th
second. When creating 480p from a 480i source, as most displays
capable of displaying 480p do, there is obviously no more detail
available, but the "upconversion" still looks better, much as a
smoothly scaled image on a computer looks better then a poorly scaled
image.
2) Progressive scan from a progressive scan DVD player playing a film
source, yes. "Real" 480p/60 frames from video cameras is possible
("FOX Widescreen" DTV may do this, I'm not sure), but you're not going
to get that from a DVD player.
3) There are no "30fps" TVs. (see next answer) On a 480p (60fps) set,
it would be displayed with each frame being accurate to a film frame,
but the timing would be slightly inaccurate--2 copies of one frame,
then 3 copies of the next frame, etc.
4) On a 480i TV, yes, the progressive scan feature is not used.
5) I'm assuming we're talking about a widescreen anamorphic DVD here,
which most film-based DVDs are. Let's use one that has an aspect ratio
of 16:9.
The resolution stored on the DVD is actually 720x480. However, to get
the correct 16:9 aspect ratio, we'll have to stretch it a little, to
853x480.
A 720p 16:9 display has a resolution of 1280x720, clearly more than
the 853x480 the DVD is providing, so yes, it's simply scaled up. The
"full resolution" of the display isn't being used.
A 480i 4:3 display has a resolution (assuming square pixels) of
640x480. However, to display widescreen material, only a portion of
this can be used. For 16:9 material, the usable "window" is only 360
lines high, far less than the 853x480 resolution from the DVD. So it
must be downscaled, and you are actually losing resolution from the
DVD. The resolution is lost *because* of the difference in aspect
ratio.
This may seem counterintuitive, because both the TV and the DVD are
natively 480i, but it occurs because of the anamorphic encoding,
described in my original answer, used to push the DVD format to its
limits, and provide extra resolution to displays capable of using it.
The difference is even more pronounced when viewing a "wider" film
with an aspect ratio greater than 16:9. "Scope" films are also very
common, and have an aspect ratio of 2.35:1. On a 480i set, the usable
window would be only 272 lines high, resulting in use of only 57% of
the resolution available from the DVD.
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