Speaker 1: Welcome to Brainstoff front House, stuff works dot com where
Speaker 1: smart happens. Hi. I'm Marshall Brain with today's question, how
Speaker 1: are movies stored on DVD discs? Even though the storage
Speaker 1: capacity of a DVD is huge and it can hold
Speaker 1: several gigabytes of information, the uncompressed video data of a
Speaker 1: full length movie would never fit on a DVD. In
Speaker 1: order to fit a movie onto a DVD, you need
Speaker 1: video compression. A group called the Moving Picture Experts Group,
Speaker 1: or MPEG, establishes the standards for compressing moving pictures. When
Speaker 1: movies are put onto DVDs, they're encoded in MPEG two
Speaker 1: format and then stored onto the disk. This compression format
Speaker 1: is a widely accepted international standard. Your DVD player contains
Speaker 1: an MPEG two dacoder, which can uncompress the data as
Speaker 1: quickly as you can watch it. A movie is usually
Speaker 1: filmed at a rate of twenty four frames per second.
Speaker 1: This means that every second there are twenty four complete
Speaker 1: images displayed on the movie screen. American and Japanese televisions
Speaker 1: use a format called the National Television Standards Committee or NTSC.
Speaker 1: NTSC displays a total of thirty frames per second, but
Speaker 1: it does this in a sequence of sixty fields, each
Speaker 1: of which contains alternating lines of the picture. Other countries
Speaker 1: use phase alternating line or the PAL format, which displays
Speaker 1: fifty fields per second, but at a higher resolution. Because
Speaker 1: of the differences in frame rate and resolution, an MPEG
Speaker 1: movie needs to be formatted for either NTSC or the
Speaker 1: PAL system. The MPEG encoder that creates the compressed movie
Speaker 1: file analyzes each frame and decides how to encode it.
Speaker 1: The compression uses some of the same technology as still
Speaker 1: image impression to eliminate redundant and irrelevant data in the frames.
Speaker 1: It also uses information from other frames to reduce the
Speaker 1: overall size of the file. Each frame could be encoded
Speaker 1: in one of three different ways, as an intro frame,
Speaker 1: which contains the complete image data for that frame. This
Speaker 1: method of encoding provides the least compression as a predicted frame,
Speaker 1: which contains just enough information to tell the DVD player
Speaker 1: how to display the frame based on the most recently
Speaker 1: displayed intra frame or predicted frame. This means that the
Speaker 1: frame contains only the data that relates to how the
Speaker 1: picture is changed from the previous frame, or as a
Speaker 1: bi directional frame. In order to display this type of frame,
Speaker 1: the player must have the information from the surrounding intraframe
Speaker 1: or predicted frames using data from the closest surrounding frames.
Speaker 1: It uses interpolation, which is sort of like averaging, to
Speaker 1: calculate the position it in color of each pixel. Depending
Speaker 1: on the type of scene being converted, the encoder will
Speaker 1: decide which types of frames the use. If a newscast
Speaker 1: were being converted, a lot more predicted frames could be
Speaker 1: used because most of the scene is unaltered from one
Speaker 1: frame to the next. On the other hand, if a
Speaker 1: very fast action scene were being converted, in which things
Speaker 1: changed very quickly from one frame to the next, more
Speaker 1: intra frames would have to be encoded. The newscast would
Speaker 1: compress to a much smaller size than the action sequence.
Speaker 1: This is why the storage capacity of digital video recorders,
Speaker 1: which store video on a hard drive using the MPEG
Speaker 1: format can vary depending on what type of show you're recording.
Speaker 1: If all of this sounds complicated, then you're starting to
Speaker 1: get a feeling for how much work your DVD player
Speaker 1: does to decode and MPEG two movie. A lot of
Speaker 1: processing power is required. Even some computers with DVD players
Speaker 1: can't keep up with the processing required to play a
Speaker 1: DVD movie. For more on this and thousands of other topics,
Speaker 1: visit how stuff Works dot com, m