WEBVTT - How are movies stored on DVD discs?

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<v Speaker 1>Welcome to Brainstoff front House, stuff works dot com where

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<v Speaker 1>smart happens. Hi. I'm Marshall Brain with today's question, how

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<v Speaker 1>are movies stored on DVD discs? Even though the storage

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<v Speaker 1>capacity of a DVD is huge and it can hold

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<v Speaker 1>several gigabytes of information, the uncompressed video data of a

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<v Speaker 1>full length movie would never fit on a DVD. In

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<v Speaker 1>order to fit a movie onto a DVD, you need

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<v Speaker 1>video compression. A group called the Moving Picture Experts Group,

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<v Speaker 1>or MPEG, establishes the standards for compressing moving pictures. When

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<v Speaker 1>movies are put onto DVDs, they're encoded in MPEG two

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<v Speaker 1>format and then stored onto the disk. This compression format

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<v Speaker 1>is a widely accepted international standard. Your DVD player contains

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<v Speaker 1>an MPEG two dacoder, which can uncompress the data as

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<v Speaker 1>quickly as you can watch it. A movie is usually

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<v Speaker 1>filmed at a rate of twenty four frames per second.

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<v Speaker 1>This means that every second there are twenty four complete

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<v Speaker 1>images displayed on the movie screen. American and Japanese televisions

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<v Speaker 1>use a format called the National Television Standards Committee or NTSC.

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<v Speaker 1>NTSC displays a total of thirty frames per second, but

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<v Speaker 1>it does this in a sequence of sixty fields, each

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<v Speaker 1>of which contains alternating lines of the picture. Other countries

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<v Speaker 1>use phase alternating line or the PAL format, which displays

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<v Speaker 1>fifty fields per second, but at a higher resolution. Because

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<v Speaker 1>of the differences in frame rate and resolution, an MPEG

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<v Speaker 1>movie needs to be formatted for either NTSC or the

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<v Speaker 1>PAL system. The MPEG encoder that creates the compressed movie

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<v Speaker 1>file analyzes each frame and decides how to encode it.

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<v Speaker 1>The compression uses some of the same technology as still

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<v Speaker 1>image impression to eliminate redundant and irrelevant data in the frames.

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<v Speaker 1>It also uses information from other frames to reduce the

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<v Speaker 1>overall size of the file. Each frame could be encoded

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<v Speaker 1>in one of three different ways, as an intro frame,

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<v Speaker 1>which contains the complete image data for that frame. This

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<v Speaker 1>method of encoding provides the least compression as a predicted frame,

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<v Speaker 1>which contains just enough information to tell the DVD player

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<v Speaker 1>how to display the frame based on the most recently

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<v Speaker 1>displayed intra frame or predicted frame. This means that the

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<v Speaker 1>frame contains only the data that relates to how the

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<v Speaker 1>picture is changed from the previous frame, or as a

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<v Speaker 1>bi directional frame. In order to display this type of frame,

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<v Speaker 1>the player must have the information from the surrounding intraframe

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<v Speaker 1>or predicted frames using data from the closest surrounding frames.

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<v Speaker 1>It uses interpolation, which is sort of like averaging, to

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<v Speaker 1>calculate the position it in color of each pixel. Depending

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<v Speaker 1>on the type of scene being converted, the encoder will

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<v Speaker 1>decide which types of frames the use. If a newscast

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<v Speaker 1>were being converted, a lot more predicted frames could be

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<v Speaker 1>used because most of the scene is unaltered from one

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<v Speaker 1>frame to the next. On the other hand, if a

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<v Speaker 1>very fast action scene were being converted, in which things

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<v Speaker 1>changed very quickly from one frame to the next, more

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<v Speaker 1>intra frames would have to be encoded. The newscast would

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<v Speaker 1>compress to a much smaller size than the action sequence.

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<v Speaker 1>This is why the storage capacity of digital video recorders,

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<v Speaker 1>which store video on a hard drive using the MPEG

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<v Speaker 1>format can vary depending on what type of show you're recording.

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<v Speaker 1>If all of this sounds complicated, then you're starting to

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<v Speaker 1>get a feeling for how much work your DVD player

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<v Speaker 1>does to decode and MPEG two movie. A lot of

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<v Speaker 1>processing power is required. Even some computers with DVD players

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<v Speaker 1>can't keep up with the processing required to play a

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<v Speaker 1>DVD movie. For more on this and thousands of other topics,

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<v Speaker 1>visit how stuff Works dot com, m