An explanation of why most consumer set-top DVD players have problems with FULL-mode lock starts with a brief description of how raw digital DVD images are formatted in the players before being sent to the display. A raw DVD image consists of a 720x480 grid of pixels, as in the following sample image (16x9-enhanced 2.35:1 letterboxed):
|Viva Las Vegas appears courtesy of MGM|
|Raw 16x9-enhanced DVD letterboxed image (1.50:1)|
The DVD format specifies that each individual pixel be displayed not as a square, however, but as a 72:79 rectangle, which when interpreted by the DVD player reshapes the image from a 1.5:1 rectangle into one of roughly 1.37:1, as illustrated below. This shape is what the display will receive. Note that the image is 16x9-enhanced, and thus is distorted vertically, being 33% taller than it should be to appear in its proper proportions.
|Typical DVD player converts image to ~4:3 before sending it to display|
When the image is received by a widescreen display in FULL mode, it is "stretched" horizontally by 33% so that it fits the screen dimensions of 1.78:1. Because this image is 16x9-enhanced and already has its proper height, the horizontal "stretch" leaves it properly proportioned.
|Display stretches image to 1.78:1, restoring its proper proportions|
If, however, the image is already in its proper proportions prior to being sent to the display, as is the case with a non-enhanced letterboxed transfer, the horizontal "stretch" process will not work properly.
|Non-enhanced letterboxed image prior to sending to display|
After being received by the display and "stretched," the image is distorted.
|Non-enhanced letterboxed stretched to 1.78:1 by widescreen display|
These examples illustrate how the problem with FULL-mode lock is not due to design flaws or oversights in the displays themselves, but rather, DVD players formatting images for 4:3 instead of 1.78:1. Why do DVD players operate this way? Perhaps the player manufacturers are optimizing their products for use with 4:3 displays, which still command most of the consumer market, and don't want the extra expense and complexity of having to provide a second set of formatting hardware. Regardless of the reasons for it, the problem exists, and it is now apparent that the solution is to use a DVD player that formats images in the native proportions of the target display type.