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.