A VOB file (Video Object) is a container format in DVD-Video media. VOB can contain video, audio, subtitle and menu contents multiplexed together into a stream form. VOB is based on the MPEG program stream format, but with additional limitations and specifications in the private streams.
The MPEG program stream has provisions for non-standard data (as used in VOB files) in the form of so-called private streams. VOB files are a very strict 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.
Analogous to the MPEG program stream, VOB file can contain H.262/MPEG-2 Part 2 or MPEG-1 Part 2 video, MPEG-1 Audio Layer II or MPEG-2 Audio Layer II audio, but usage of these compression formats in a VOB file has some restrictions in comparison to the MPEG program stream. In addition, VOB can contain Linear PCM, AC-3 or DTS audio and subpictures (subtitles). VOB file cannot contain AAC audio (MPEG-2 Part 7), MPEG-4 compression formats and others, which are allowed in MPEG program stream standard.
On the DVD, all the content for one title set (VTS) is contiguous, but broken up into 1 GiB 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 one GiB.
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 Content Scramble System (CSS). This is a data encryption and communications authentication method designed to prevent copying video and audio data directly from the 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 disc and are used only by CSS decryption software.
If someone is trying to copy the contents of an encrypted DVD-Video to a hard drive, an error can occur, because the DVD disc was not authenticated in the drive by CSS decryption software. Authentication of the disc allows the copying of individual VOB files without error, but the encryption keys will not be copied. If the copied undecrypted VOB files are opened in a player, they will request the keys from the DVD-ROM drive and will fail.
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