The DVD is broken into two separate types of information: Navigation Data (control) and Presentation (object) data. Control data acts as pointers (like an operating system's File Allocation Table) to the actual video and audio object data on the disc.
In the DVD reference player model, Presentation and Navigation data packets are separated at the track buffer.
Control data can be expressed as a series of nested layers:
|Title||distinguishes multiple movies or TV episodes on one disc. Each title is one of two types: a single program chain (One_Sequential_PGC_Title) or a collection of different program chains (Multi_PGC_Title).|
|Program Chain||A collection of programs with, e.g., a particular theme in common.|
|Part_of_Title||Links to one or more Program (PG) units on the disc. Like PGC, this mechanism can be used to create different versions (camera angle, ratings, outcomes, etc.) of the same program chain. POTs can also be used to mark scenes.|
|Program||Usually a scene. Consists of multiple cells.|
|Cell||Preceded by a navigation packet, and alternating video and audio packets. A cell is typically all the video and audio data associated with an integer number of a group of pictures.|
|VOBU||Video Object Unit: nominally a group of pictures (GOP)|
1. smallest granularity of random access on disc (Group of pictures being
with a coded Intra frame)|
2. largest interframe dependent coding unit. (Interframe compression is bounded within a GOP)
Usually 15 display frames of data (0.5 seconds duration) for NTSC-rate (525/60) content.
|Packet||DVD packets are 2048 bytes (sector payload size) large. As per MPEG-2 PES/Program streams, they contain data from only one data elementary stream (video, audio, etc.)|
|NAV packet||contains the optional Buttom-Command defining the playback behaviour of the current cell.|
A disc volume may contain up to 99 different titles, each with an initial Navigation Menu allowing the user to select among different versions of the title. The root menu which branches to all titles on the disc is coded within the Video Manager. Each title is organized as a Video Tile Set (VTS).
DVD: [VM][VTS #1][VTS #2] ..... [VTS #n] where n<=99
The VM's VMGI includes: Attributes for the Menu, Tile Search Pointers, and the PCGI for the Menu.
VM: [VMGI][VOBS for Menu][Back up for VMGI]
The Control Data (VTSI) for the title (VTS) includes: attributes for Menu, Attributes for Title, Part of Title Search Pointer, Time Map Table, PGCI for Menu, and PGCI for Title. The Video Objects (VOBS) contain the actual program chains, Part_of_Tiles, programs, and so forth.
VTS: [VTSI][VOBS for Menu][VOBS for Title][Back up for VTSI]
Legend: VM Video Manager: sets up menus for a series of titles (1 through n) VTS Video Tile Set: a collection of video objects. VMGI Video Manager Information: VOBS Video Object Set PGCI Program Chain Information
A title begins with the entry program chain (Entry PGC). It can branch to a single program chain (One_Sequential_PGC_Title) or multiple program chains (Multi_PGC_Title). The location of the branch is determined by the link condition.
The PGCI defines the playback order of Programs by acting as a table of addresses which point to the sector locations of the program cells on the DVD. A program cell is essentially a group of pictures (GOP), spanning multiple sectors, and contains the actual interleaved packets of compressed bits for video and audio data.
The Part_of_Title divides a title in a maximum of 99 different pieces. The intent of the PTT is aid in the construction of multiple versions of the same title.
One_Sequential_PCG_Title: The Part_of_Title and Program numbers are synchronized.
[ PTT #1 | PTT #2 | .... | PTT #n ] Part_of_Title [ [PG #1] | [PG #2] | .... | [PG #n] ] Program Chain (PGC) Multi_PGC_Title: branch PTT #2 --> [PG #1] (PGC1) PTT #3 PTT #m PTT #1 --> [PG #1] [PG #j] ... [PG #k] (PGC2) [PG #1] --> [PG #1] (PGC3)