GUIDE : Virtual Commands in DVD
Posted on September 2, 2009
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The DVD specifications provide a set of machine instructions which tell the DVD player how to play back the video and audio contents of the DVD. The complete set of instructions is also known as the Virtual Machine (VM) DVD command set. To have absolute control over your DVD authoring project without any compromise, you will need to understand what these instructions can do. The informational guides and real-world examples presented in this site will help you become a proficient DVD author at the VM command level.
The VM command set is designed to direct the flow of playback and the majority of its instructions are navigation commands. These commands can be placed at the beginning of the PGC (pre-commands), at the end of the PGC (post-commands), at the end of a cell (cell commands), or in the navigation pack of a menu button (button commands). Navigation commands direct playback or alter the order of playback to jump from one video title to another. With navigation commands, the DVD author has complete control and can design custom levels of navigation and interactivity to instruct the DVD player to change and adapt the playback flow and characteristics.
The VM command set provides 16 general parameter registers (GPRM) to hold temporary values and 24 system parameters (SPRM) to hold information specific to the DVD player model such as region, default language settings, parental level, etc.
Some typical usage of navigation commands:
Automatic repetition of playback of a movie title.
Change playback sequence of audio and video titles depending upon the selected audience.
Select the appropriate video contents and audio language based upon the settings users set into their DVD players.
Interactive video quizz and games.
Select fixed branching (the playback flow is not changed).
Select conditional branching (the playback flow changes depending upon the system parameter values or user input settings).
The DVD specification has certain restrictions on playback path.
From VMG domain to a VTS domain via a title or a VTS menu.
From a title to a chapter (through an entry PGC in the VMG)
From a VTS menu or a PGC in VTSM domain to a chapter.
Playback cannot flow directly from one VTS to another but must be routed through an entry PGC in the VMG.
There are six groups of navigation commands,
consisting of a total of 36 instructions,
as shown in the following table.
| Group | Functions |
| Link | Start video within the current domain. Can be combined with SetGPRM, Compare, SetSystem commands. LinkPGCN |
| Jump | Start video in a specified domain. Can be combined with Compare commands. Compare operands can be a GPRM or SPRM only. No immediate values are allowed. JumpTT |
| GoTo | Go to another navigation command. Can be combined with Compare commands. |
| Compare | Compare registers or values. Can be combined with Link, SetGPRM, SetSystem, GoTo, and Jumpcommands. |
| SetSystem | Set System parameters. Can be combined with Link, Compare commands. SetSTN |
| SetGPRM | Compute GPRM values. Can be combined with Link, Compare commands. Add |
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