VOB (Video Object) is the container format used in DVD-Video media. VOB can contain digital video, digital audio, subtitles (including closed captioning), DVD menus and navigation contents multiplexed together into a stream form. Files in VOB format may be encrypted. Files in VOB format have .vob filename extension.
The VOB format is based on the MPEG program stream format. The MPEG program stream has provisions for nonstandard data in the form of private streams. VOB files are a subset of the MPEG program stream standard. While all VOB files are MPEG program streams, not all MPEG program streams comply with the definition for a VOB file. On the DVD, all the content for one title set is contiguous, but broken up into 1 GB VOB files in the computer compatible file systems for the convenience of the various operating systems.
Each VOB file must be less than or equal to 1 GB. VOB files may be accompanied with IFO and BUP files. These files respectively have .ifo and .bup filename extensions. Images, video and audio used in DVD menus are stored in VOB files. IFO (information) files contain all the information a DVD player needs to know about a DVD so that the user can navigate and play all DVD content properly, such as where a chapter starts, where a certain audio or subtitle stream is located, information about menu functions and navigation. BUP (backup) files are exact copies of IFO files, supplied to help in case of corruption. Video players may not allow DVD navigation when IFO or BUP files are absent.
Almost all commercially produced DVD-Video titles use some restriction or copy protection method, which also affects VOB files. Copy protection is usually used for copyrighted content. Many DVD-Video titles are encrypted with CSS (Content Scramble System). This is a data encryption and communications authentication method designed to prevent copying video and audio data directly from DVD-Video discs.
Decryption and authentication keys needed for playing back encrypted VOB files are stored in the normally inaccessible lead-in area of the DVD and are used only by CSS decryption software in a DVD player or software player. If someone is trying to copy the VOB files of an encrypted DVD-Video to a hard drive, an error can occur, because the DVD was not authenticated in the drive by CSS decryption software. This makes VOB inconvenient to use for purposes of file-sharing for uses such as reference, archiving filmed material, editing and transcription.
A player of generic MPEG-2 files can usually play unencrypted VOB files, which contain MPEG-1 Audio Layer II audio. Mplayer, VLC media player, GOM player, Windows Media Player Classic and more platform-specific players like ALLPlayer play VOB files.