All VOB files are essentially MPEG2 Program streams with audio, video, sub-picture and navigation data multiplexed. A VOB file is organized as a set of cells; a cell is a basic unit of play data. Each cell consists of a sequence of units called VOBUs. Each VOBU is a sequence of
packs. The first pack in a VOBU is a navigation pack and contains Program Control Information (PCI) packet and Data Search Information (DSI) packet. The remaining packs contain audio,
video and sub-picture data multiplexed together. Each pack has a fixed size of 2048 bytes. A pack typically contains only one data packet and may be stuffed with dummy bytes or a packet
called ‘padding’ bytes/packet to make it a fixed size.
DVD allows easy navigation in its audio and video data. Information for navigation across different VTS is contained in the VMGM. Within a title, the play order of different cells (from one
or more VOBs in the title) is described in a Program Chain (PGC). A PGC is a logical unit to present a part of or the entire Title or Menu. A PGC is further divided into programs. Each program contains integral number of cells. A Title may have one or more PGCs. However, a Title
that has parental guidance levels, will have more than one PGC. Depending on the parental level selected by the DVD disk viewer, the PGCs are selected for being played. PGC contains PGCI
which gives the order of presentation of cells within that PGC.
The information for presentation of a cell, such as the angle information for seamless and nonseamless play and highlight information is contained in the Navigation packs occurring within the
cell.
When playing non-seamlessly, the cells within a logical block are placed contiguously. Therefore, during cell presentation, intermittent blocks may have to be skipped depending on the angle information selected by the DVD disk viewer.
An angle block is a logical block containing cells for different angle presentations. During presentation, not all the cells within the block are played. The different angle cells are of almost
the same play time and since they are placed adjacent to each other, the DVD disk viewer can seamlessly change from one angle to another.
When playing a parental level seamlessly, cells from different VOBs may be interleaved in a logical block. Such a block is called an interleaved block. Each unit of VOB that lies in an interleaved block is called an ILVU of that VOB. This means that the cells in a VOB may not be placed contiguously over the physical address space and may be interleaved with ILVUs from other VOBs.