Aspect Ratios, Letterbox, Panscan & Overscan

A commercial DVD most times includes video streams in a size of 720x576(480).
Now we heard something about 16:9, 4:3, Letterbox & Panscan ... this will be exlplained here.

Often its mentioned on the back of the cover that the movie inside is anamorph.
This means the movie is encoded in a squeezed way just to obtain max. videoinformation in the high of
its size. But if we play back the DVD we won't see a squeezed movie cause the player does a Letterboxing, that
means he resizes the video so it will be shown right.

Here you can see a DVD movie in its original anamorphic proportion, not resized and not letterboxed



The stand alone player now reads the aspect ratio information of the videostream header and performs
for example a letterboxing or a Pan-Scan. By letterboxing a video he just resizes down the high of the videostream and adds black borders on the top and the bottom. By pan-scanning a movie he resizes up the width of the video and crops the sides.





















We also could perform a larger pan-scanning to a full screen 4:3 view but this will bring us to much effective movie-pixel we will have to encode and beside this you will lost an extreme amount of movie-information on the sides.




As already mentioned above we have to know that there is an area in the videopicture we will not see on TV.
It's called overscan area or TV-Cache.
We use the overscan to safe some CQ or Quality by changing this area into a black border around the video picture, like here shown as a green border for explanation.

We have 2 opportunities do do that: First we can just overlap, that means a black border around the picture for example 16px (one macroblock) will be overlayed on the picture. The second one is to resize the video so it fits the
TV-safe area and adding black borders to the sides around to fill out the screen again to its necessary size.





















By doing a resized overscan we maintain more movie-information and also we will obtain a better CQ and Quality cause less effective videoinformation has to be handled by the mpeg-encoder.