VOB (for video object) is the container format in DVD-Video media. VOB can contain digital video, digital audio, subtitles, DVD menus and navigation contents multiplexed together into a stream form. Files in VOB format may be encrypted. File format Files in VOB format have a .vob filename extension and are typically stored in the VIDEO_TS directory at the root of a DVD. The VOB format 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, 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. In addition, a VOB file can contain linear PCM, AC-3 or DTS audio and subpictures (subtitles). 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. On the DVD, all of the content for one title set is contiguous, but if necessary is 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. Companion files VOB files may be accompanied with IFO and BUP files. These files respectively have .ifo and .bup filename extensions. 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 redundant 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. Copy protection 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