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