DVD-Video Disc Structure

DVD-Video titles consist of a hierarchical structure as illustrated below. A DVD-Video disc is divided into Video Title Sets (or titles). As a minimum there will be one Video Manager (VMG) and one VTS. A disc containing a movie plus documentary about the making of the movie is likely to be divided into two titles. A disc containing six episodes of a TV programme would be divided into six titles. However in both cases it is possible to have only one title per disc.

Each Video Title Set Information (VTSI) comprises control data and Video Objects (VOB's) for both menus (if present) and titles (stills and video). Each VOB (the fundamental file element of the disc) comprises video, audio, subpictures and navigation data. When a VOB is played the player not only plays the video sequentially but obeys the navigation instructions for displaying menus, getting user selections etc.

Each VOB comprises individual cells linked together by Program Chains (PGCs), which provide the necessary interactivity using a simple programming language developed for DVD-Video. PGCs are used to control the playing of video, audio and subtitles in VOBs, to display menus and input and obey user commands. There are three types of PGC: sequential play, random play and shuffle play. Individual cells may be used by more than one PGC, which can define different sequences through the video content, for example to implement seamless branching. PGCs allow a command set for simple programming including mathematical and logical operators, conditional branching, countdown timer etc. There are 16 general registers for more complex programming and 24 system registers.

DVD Files and Directories

The VOBs and other data are contained in files in the VIDEO_TS directory. The table below gives an example of these files for a single title set disc.

The audio, video and subpictures for the movie are contained in up to 9 Title VOB files, where each file is not more than 1GB in size. For a DVD-5 there will be no more than 5 Title VOB files, for a DVD-9 a full 9 will be required if the disc is full.

The VTS*.* files can be repeated for each VTS and will be named VTS_02*.*, VTS_03*.* etc. Each VTS will have one each of the .IFO and .BUP files plus one or more .VOB files.

DVD-Video Data Streams

To meet the Hollywood requirements, DVD-Video discs are designed to store audio visual sequences each comprising data streams of four types of data:

The maximum bit rate is 9.8 Mb/s for video, audio and subpictures (the overall maximum, including control information, being 10.08 Mb/s). For a playing time of 133 minutes, the average bit rate is 4.7 Mb/s.

The average video bit rate available depends on the number of audio streams and the encoding used, but should be close to 4 Mb/s for high quality results. Higher bit rates are often used to give even higher video quality.

To protect the copyright of Hollywood studios DVD-Video discs can be copy protected and can be region coded to play only in designated region(s). Copy protection comprises both digital and analogue methods.