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A DVD-Video disc is a DVD with specific constraints on its file structure and file contents. It consists mostly of MPEG-2 video files (though apparently MPEG-1 is also allowed), plus additional data defining the navigation menus and possibly other optional extra features (like Karaoke) allowed by the DVD-Video spec.

At the top level of a DVD-Video disc, there are two directories: "AUDIO_TS" and "VIDEO_TS", where the "TS" stands for "titleset". A title set is a set of titles, where a "title" might be the entire movie, or an extra like a documentary about the making of the movie, or an episode of a TV series. The AUDIO_TS directory is only used on DVD-Audio discs, which are quite rare, nevertheless the DVD-Video spec requires it to be present, even though it is empty.

The VIDEO_TS directory is where all the real content of a DVD-Video disc is kept. It contains files with names ending in .IFO, .BUP and .VOB. The .IFO (info) files contain overall information about the titles, including the entry into the menu structure. The .BUP files are backup copies of the .IFO files, supposed to be kept on a physically separated area of the disc to increase reliability. And the .VOB (video object) files are MPEG files containing the actual titles and menus--menus are also MPEG movies, not that different from titles (apart from usually being much shorter).

Each MPEG file is allowed to contain just one video stream, up to eight audio streams (for soundtracks in different languages), and up to 32 "subpicture" streams.

A subpicture is a generalization of the concept of a subtitle. On a DVD-Video disc, subtitles are not kept in text form: instead, they are stored as a picture, allowing for use of arbitrary fonts, language scripts, writing directions etc. Definition of transparent pixels allows the creation of graphics that overlay any part of the screen, and since the subpicture is stored in a stream just like the video and audio streams, it can animate as the title is playing.

See Also