BUP (backup) files are exact redundant copies of IFO files, supplied to help in case of corruption. 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. VOB files may be accompanied with IFO and BUP files. Each VOB file must be less than or equal to 1 GB. On the DVD, all the content for one title set is contiguous, but broken up into 1 GB VOB files in order to be compatible with all operating systems, as some cannot read files larger than that size. VOB files cannot contain AAC audio (MPEG-2 Part 7), MPEG-4 compression formats and others, which are allowed in the MPEG program stream standard. In addition, VOB can contain linear PCM, AC-3 or DTS audio and subpictures (subtitles). Analogous to the MPEG program stream, a 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. While all VOB files are MPEG program streams, not all MPEG program streams comply with the definition for a VOB file. VOB files are a very strict subset of the MPEG program stream standard. The MPEG program stream has provisions for non-standard data (as used in VOB files in the form of so-called private streams. The VOB format is based on the MPEG program stream format, but with additional limitations and specifications in the private streams. vob filename extension and are typically stored in the VIDEO_TS directory at the root of a DVD. VOB can contain digital video, digital audio, subtitles, DVD menus and navigation contents multiplexed together into a stream form. VOB - (DVD-Video Object or Versioned Object Base) - is the container format in DVD-Video media.
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