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Picture Compression
Picture Compression on the DVD disc is made possible by a process known as MPEG. DVDs use the the second version of this compression process know as MPEG2.
MPEG video compression is quite popular in the computer world as a means of storing video without the burden of taking up vast amounts of space, thus it has been around long enough to have the bugs worked out. It was developed by those whom it is named after, the Motion Picture Expert Group (MPEG for short), a group founded to do what it has done, develop quality video compression.
How MPEG works is this; video that you watch is generally shot with 20-30 frames per second, which means a lot of frames in a two hour movie. If you were to go through the movie frame by frame, you would find that many elements in the picture, such as the background, do not change from frame to frame. MPEG takes advantage of this; remembering the last frame, and only storing the information that is different from the previous frame. What this means is a lot of compression can be done on still shots, while the compression is limited on fast moving action shots.
Compression of the video is measured in bit rates, and generally ranges from 3-10 megabits per second, the higher being for the more detailed fast moving shots. Although this compression rate can be slightly adjusted when necessary, such as turning it slightly up to allow for more video on the disk, or down to get maximum picture quality. Bit rate however does not determine by itself the quality of the image, most movies only use about a 4 bit rate, but when the disk is well mastered, it can look just as good as if using a 10 bit rate.
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