VOB
All DVD movies are stored in on a DVD
video disc in so-called VOB files. VOB files usually contain
multiplexed Dolby
Digital audio and MPEG-2
video. VOB files on a DVD are numbered as follows: vts_XX_y.vob
where XX represents the title and Y the part of the title. There
can be 99 titles and 10 parts, although vts_XX_0.vob does not
contain any video, usually just menu or navigational information.
You can find them on a DVD video disc in a subdirectory labelled
VIDEO_TS (all upper case).
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.
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