Both DVD's and digital television use a technique called "data compression" to squeeze more data into less space. Although the actual methods used are sophisticated, the principles are simple.
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Frame 2 |
The parts that change |
One technique uses the fact that the difference between one frame and the next in a movie will be small. An old movie film had an entire picture for every frame. However, a DVD stores frame 1 entirely, and then stores only the parts that move to get frame 2. To store just the horse's legs obviously takes up much less space than the whole horse. How well this works depends on the film - newsreaders don't move much, so compress very well, whereas action films with lots of movement won't compress much.
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