DVD Demystified

DVD Technology Primer
Aspect Ratios Revisited
127
To review, video comes out of a DVD-Video player in basically four ways:
1. Full frame (4:3 original)
2. Pan and scan (widescreen original)
3. Letterbox (widescreen original)
4. Anamorphic (widescreen original)
All four can be displayed on any TV, but the fourth is specifically
intended for widescreen TVs. This may seem straightforward, but it gets
much more complicated. The problem is that very few movies are in 16:9
(1.78) format. The preceding discussions dealt with 16:9 widescreen in a
general case. Until 16:9 cameras become more widespread, however, there
will be very little video created in 16:9.
Most movies are usually 1.85 or wider, although European movies are often
1.66. DVD only supports aspect ratios of 1.33 (4:3) and 1.78 (16:9) because they
are the two most common television shapes. Movies that are a different shape
must be made to fit, which brings us back to pegs and holes. In this case, the
hole is DVD’s 16:9 shape (which is either shown in full on a widescreen TV or
formatted to letterbox or pan and scan for a regular TV). There are essentially
four ways to fit a 1.85 or wider movie peg into a 1.78 hole:
1. Letterbox to 16:9 When the movie is transferred from film, black
mattes are added to box it into the 16:9 shape. These mattes become a
permanent part of the picture. The position and thickness of the
mattes depend on the shape of the original.
a. For a 1.85 movie, the mattes are very small. On a widescreen TV or
in automatic pan and scan on a regular TV (where the player is
extracting a vertical slice from the letterboxed picture), the mattes
are hidden in the overscan area. 22 On a standard TV in automatic
letterbox mode (where the player is letterboxing an already
letterboxed picture), the thin permanent mattes merge
imperceptibly with the thick player-generated mattes.
b. For a 2.35 movie, the permanent mattes are much thicker. In this
case, the picture has visible mattes no matter how it is displayed.
22 Overscan refers to covering the edges of the picture with a mask around the screen. Overscan
was implemented originally to hide distortion at the edges. Television technology has improved
to the point where overscan is not usually necessary, but it is still used. Most televisions have an
overscan of about 4 to 5 percent. Anyone producing video intended for television display must be
mindful of overscan, making sure that nothing important is at the edge of the picture. It should
be noted that when DVD-Video is displayed on a computer, there is no overscan, and the entire
picture is visible.