All DVD Video discs include a VIDEO_TS folder, which contains all the data for the DVD. In a VIDEO_TS folder, the data for a DVD is organized into three different file types - information (.IFO), backup (.BUP), and video object (.VOB) files. The file names in a DVD are dictated by the DVD Specification, and always follow the same conventions. Take a look at any DVD's VIDEO_TS folder in Windows Explorer or the Apple Finder to help you understand the following explanations.
The IFO files contain most of the information you will see in HDAfterEdit's "right pane", including pre and post-commands, which control the player's navigation through the DVD, for example. They also contain other information that the player uses. HDAfterEdit doesn't necessarily show all this information, but takes it into account, and recalculates it as needed when changes are made.
The BUP files are exact copies of the IFO files, which are used if the DVD can't read an IFO file because of a scratch on the disc or other error.
The VOB files contain all the video information of the DVD's menus and features, plus chapter-makers, button commands etc. There is more detailed information here.
The files are always arranged in a certain physical order on the disk, and each relates to a separate part of the logical structure of the DVD.
The first files on the disc are always the always the VIDEO_TS.IFO and VIDEO_TS.VOB files. (We'll ignore the BUP files from here on). These files define the Video Manager (VMG). The VMG contains the Title Menu, which is often the main menu of the DVD, and other global settings for the disc, like Region Code information, whether the disc is PAL or NTSC etc. It also defines the Title Play Map.
The VIDEO_TS.VOB file ( which contains any video for the Title Menu ) is optional, though leaving it out is rare.
The remainder of the files in the DVD contain the Video Title Sets (VTS). Usually the main features of the DVD are contained in a VTS, however exactly how they are organised doesn't have to follow any set rules and so varies widely, being determined by the original author or authoring application. Common methods are to either give each major feature of a DVD ( main title, bonus features etc. ) it's own VTS, or to include ALL the assets for a DVD together in one VTS per aspect ratio.
Each VTS contains an IFO file and one or more VOB files. The IFO file is always named VTS_xx_0.IFO, where xx is the VTS number, from 1 to 99. The VOB files are of two types, menu VOBs and title VOBs. The menu VOB is always named VTS_xx_0.VOB, where xx is the VTS number. This file is optional, since a VTS can have no menus if the author likes.
The remaining files are named VTS_xx_1.VOB, VTS_xx_2.VOB etc, and make up the complete title Video Object when laid end-to-end. (The reason they are split up in this way is that DVD started in the days when file systems weren't capable of representing large files, so each VOB file is limited to 1 GB.) In HDAfterEdit, Video Objects are always handled as one logical unit, so you don't have to worry about the individual 1 GB files.
The file order described above actually relates to the physical arrangement of the files on the disc, and this has an effect on navigation speeds. Navigation between files within the same VTS is quick, which is why it is common to put the chapter select menu for a VTS in the actual menu VOB of that VTS, rather than in the VMG VOB, for example.
All the visual material in an SD DVD, even still images and menus, is stored as MPEG video, and is contained in Video Object or VOB files on the disc. If you renamed one of these VOB files with a ".mpg" extension and opened it in the Quicktime Player, you would see all the video contained in the VOB play continuously - including menus. In the case of static menus, they will flash by, since they are only one nav-pack long, whereas motion menus contain longer sequences of video. VOBs also contain buttons, copy protection flags, ISRC codes and a host of other information.
Each VOB can be subdivided into cells. A cell is just a handy chunk of video defined by the author - they can be invisible to the user, in some instances. All chapters are cells ( see below ). Menus are typically made from one or two cells. Simple looping motion menus have one cell, whereas menus with transitions can have 2 or more cells.
When a cell is referenced by an IFO file, it is assigned a Program Number. As far as the player is concerned, one or more Programs are "contained" in a Program Chain or PGC. The Program Number is generally invisible to the user.
Most video with a running-time of more than a few minutes is sub-divided into chapters. For example, episodes of a TV show, songs in a live music concert, or important scenes in a film. A chapter, or PTT (Part Of Title) is in fact a special type of cell. It is assigned a number by the author, and this chapter number can be displayed by the player ( in one_sequential PGCs ) and navigation can jump easily to chapter-points.
There are several different types of VOB file relating to different parts of the DVD's structure, distinguishable by their names:
- where x denotes the VTS number, and y is a number keeping track of the different sections of the VOB. Because of UDF file-system limitations in the early days of DVD, VOB files are segmented into 1 GB pieces. These chunks however are invisible to the DVD player and user.