3.5 What's widescreen? How do the aspect ratios work?
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This article is from the DVD Formats FAQ, by jtfrog@usa.net (Jim Taylor) with numerous contributions by others.
3.5 What's widescreen? How do the aspect ratios work?
Video can be stored on a DVD in 4:3 format (standard TV shape) or 16:9
(widescreen). The width-to-height ratio of standard televisions is 4 to 3;
in other words, 1.33 times wider than high. New widescreen televisions,
specifically those designed for HDTV, have a ratio of 16 to 9; that is,
1.78 times wider than high.
DVD is specially designed to support widescreen displays. Widescreen 16:9
video, such as from a 16:9 video camera, can be stored on the disc in
anamorphic form, meaning the picture is squeezed horizontally to fit the
standard 4:3 rectangle, then unsqueezed during playback.
Things get more complicated when film is transferred to video, since most
movies today have an aspect ratio of 1.66, 1.85 ("flat"), or 2.40
("scope"). Since these don't match 1.33 or 1.78 TV shapes, two processes
are employed to make various movie pegs fit TV holes:
Letterbox (often abbreviated to LBX) means the video is presented in its
theatrical aspect ratio, which is wider than standard or widescreen TV.
Black bars, called mattes, are used to cover the gaps at the top and
bottom. A 1.85 movie that has been letterboxed for 1.33 display has thinner
mattes than a 2.4 movie letterboxed to 1.33 (28% of display height vs.
44%), although the former are about the same thickness as those of a 2.4
movie letterboxed to 1.78 (26% of display height). The mattes used to
letterbox a 1.85 movie for 1.78 display are so thin (2%) that they're
hidden by the overscan of most widescreen TVs. Some movies, especially
animated features and European films, have an aspect ratio of 1.66, which
can be letterboxed for 1.33 display or sideboxed for 1.78 display.
Pan & scan means the thinner TV "window" is panned and zoomed across the
wider movie picture, chopping off the sides. However, most movies today are
shot soft matte, which means a full 1.33 aspect film frame is used. (The
cinematographer has two sets of frame marks in her viewfinder, one for 1.33
and one for 1.85, so she can allow for both formats.) The top and bottom
are masked off in the theater, but when the film is transferred to video
the full 1.33 frame can be used in the pan & scan process. Pan & scan is
primarily used for 1.33 formatting, not for 1.78 formatting, since
widescreen fans prefer that letterboxing be used to preserve the theatrical
effect.
For more details and nice visual aids see Leopold's How Film Is Transferred
to Video page. A list of movie aspect ratios is at
<http://cheezmo.com/wsmc>.
Once the video is formatted to full-frame or widescreen form, it's encoded
and stored on DVD discs. DVD players have four playback modes, one for 4:3
video and three for 16:9 video:
* full frame (4:3 video for 4:3 display)
* auto letterbox (16:9 video for 4:3 display)
* auto pan & scan (16:9 video for 4:3 display)
* widescreen (16:9 video for 16:9 display)
Video stored in 4:3 format is not changed by the player. It will appear
normally on a standard 4:3 display. Widescreen systems will either enlarge
it or add black bars to the sides. 4:3 video may have been formatted with
letterboxing or pan & scan before being transferred to DVD. All formatting
done to the video prior to it being stored on the disc is transparent to
the player. It merely reproduces it as a standard 4:3 TV picture. Video
that is letterboxed before being encoded can be flagged so that the player
will tell a widescreen TV to automatically expand the picture.
Unfortunately, some discs (such as Fargo) do not flag the video properly.
And worse, some players ignore the flags.
The beauty of anamorphosis is that less of the picture is wasted on
letterbox mattes. DVD has a frame size designed for 1.33 display, so the
video still has to be made to fit, but because it's only squeezed
horizontally, 33% more pixels (25% of the total pixels in a video frame)
can be used to store active picture instead of black. Anamorphic video is
best displayed on widescreen equipment, which stretches the video back out
to its original width. Alternatively, many new European 4:3 TV's can reduce
the vertical scan area to restore the proper aspect ratio without losing
resolution (an automatic trigger signal is sent on SCART pin 8). Even
though almost all computers have 4:3 monitors, they have higher resolution
than TVs so they can display the full widescreen picture in a window
(854x480 pixels or bigger for NTSC; 1024x576 or bigger for PAL).
Anamorphic video can be converted by the player for display on standard 4:3
TVs in letterbox or pan & scan form. If anamorphic video is shown unchanged
on a standard 4:3 display, people will look tall and skinny as if they have
been on a crash diet. The setup options of DVD players allow the viewer to
indicate whether they have a 16:9 or 4:3 TV. In the case of a 4:3 TV, a
second option lets the viewer indicate a preference for how the player will
reformat anamorphic video. The two options are detailed below.
For automatic letterbox mode, the player generates black bars at the top
and the bottom of the picture (60 lines each for NTSC, 72 for PAL). This
leaves 3/4 of the height remaining, creating a shorter but wider rectangle
(1.78:1). In order to fit this shorter rectangle, the anamorphic picture is
squeezed vertically using a letterbox filter that combines every 4 lines
into 3, reducing the vertical resolution from 480 scan lines to 360. (If
the video was already letterboxed to fit the 1.78 aspect, then the mattes
generated by the player will extend the mattes in the video.) The vertical
squeezing exactly compensates for the original horizontal squeezing so that
the movie is shown in its full width. Some players have better letterbox
filters than others, using weighted averaging to combine lines (scaling 4
lines into 3 or merging the boundary lines) rather than simply dropping one
out of every four lines. Widescreen video can be letterboxed to 4:3 on
expensive studio equipment before it's stored on the disc, or it can be
stored in anamorphic form and letterboxed to 4:3 in the player. If you
compare the two, the letterbox mattes will be identical but the picture
quality of the studio version will be slightly better.
For automatic pan & scan mode, the anamorphic video is unsqueezed to 16:9
and the sides are cropped off so that a portion of the image is shown at
full height on a 4:3 screen by following a center of interest offset that's
encoded in the video stream according to the preferences of the people who
transferred the film to video. The pan & scan "window" is 75% of the full
width, which reduces the horizontal pixels from 720 to 540. The pan & scan
window can only travel laterally. This does not duplicate a true pan & scan
process in which the window can also travel up and down and zoom in and
out. Auto pan & scan has three strikes against it: 1) it doesn't provide
the same artistic control as studio pan & scan, 2) there is a loss of
detail when the picture is scaled up, and 3) equipment for recording
picture shift information is not widely available. Therefore, no anamorphic
movies have been released with auto pan & scan enabled, although a few
discs use the pan & scan feature in menus so that the same menu video can
be used in both widescreen and 4:3 mode. In order to present a quality
full-screen picture to the vast majority of TV viewers, yet still provide
the best experience for widescreen owners, some DVD producers choose to put
two versions on a single disc: 4:3 pan & scan and 16:9 anamorphic.
Playback of widescreen material can be restricted by the producer of the
disc. Programs can be marked for the following display modes:
- 4:3 full frame
- 4:3 LB (for sending letterbox expand signal to widescreen TV)
- 16:9 LB only (player not allowed to pan & scan on 4:3 TV)
- 16:9 PS only (player not allowed to letterbox on 4:3 TV)
- 16:9 LB or PS (viewer can select pan & scan or letterbox on 4:3 TV)
You can usually tell if a disc contains anamorphic video if the packaging
says "enhanced for 16:9 widescreen" or something similar. If all it says is
"widescreen," it may be letterboxed to 4:3, not 16:9. The Laserviews Web
site has a list of anamorphic DVD titles. Additional explanations of how
anamorphic video works can be found at Greg Lovern's What's an Anamorphic
DVD? page, Bill Hunt's The Big Squeeze: The ABCs of Anamorphic DVD article,
and Dan Ramer's What the Heck Is Anamorphic? article. There are excellent
animated illustrations at DVD Web (requires Flash). More information can be
found at the Anamorphic Widescreen Support Page.
Anamorphosis causes no problems with line doublers and other video scalers,
which simply duplicate the scan lines before they are stretched out by the
widescreen display.
For anamorphic video, the pixels are fatter. Different pixel aspect ratios
(none of them square) are used for each aspect ratio and resolution.
720-pixel and 704-pixel sizes have the same aspect ratio because the first
includes overscan. Note that "conventional" values of 1.0950 and 0.9157 are
for height/width (and are tweaked to match scanning rates). The table below
uses less-confusing width/height values (y/x * h/w).
720x480 720x576
704x480 704x576 352x480 352x576
4:3 0.909 1.091 1.818 2.182
16:9 1.212 1.455 2.424 2.909
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