8. Cell Architecture.
Modern DVD players use advanced firmware and control software to provide smooth playback of high-quality video and audio contents present on today DVD-Video discs. DVD-Video contents are organized as contiguous addressable chunks of data, known as program stream. In order to play back the contents recorded on a DVD disc, the DVD player firmware includes two major components: a presentation engine and a navigation engine. The presentation engine uses information in the presentation data stream from the disc to know how to display its contents. The navigation engine uses the information in the navigation data stream from the disc to provide interactivity for the user interface, display menus, and to support random access and conditional branching, etc. via a standard set of player instructions known as DVD virtual commands.

A typical DVD program stream contains four packetized elementary streams (commonly abbreviated or known as PES): video, audio, sub-picture, and navigation. The navigation data stream contains the presentation control information (PCI), and data search information (DSI). A data stream can be as short as a few thousand bytes as in the case of a sub-picture stream, or as long as many gigabytes as in the case of a long movie. Similar to CD-ROM media, data stream is stored in individual segment on a DVD disc called sector.

Each sector contains a total of 2064 bytes of raw data including a header area, user data area, and error detection code. The header area contains manufacturing and encryption information (used by the Contents Scrambling System, CSS). The usable 2048-byte data size is fixed and located between the sector header and the Error Detection Code (EDC). The EDC area contains information to help the DVD player make its best guess to correct or read from the data area if its contents are damaged. The data area holds the packetized elementary streams which make up the DVD contents. This area is known as a logical sector or a logical block address (LBA).

Logical sectors are recorded continuously on the DVD disc. A typical cell can span from one to many logical sectors. Data stored in a video, audio, sub-picture, or navigation stream in a sector have to be packetized. It is a standard process in the MPEG world to slice the elementary stream to be Packetized Elementary Stream (PES). However, this process creates a bit of complexity in the DVD format.

A cell is comprised of one or more Video Object Units (VOBU). Each VOBU consists of 0.4 second to 1 second of playback time. All VOBUs belong to the same cell have the same VOB Id (from 1 to 65,535) and Cell Id (from 1 to 255). Each VOBU begins with a navigation pack (N) and is followed by one or more structures which contain video (V), audio (A), sub-picture (S), and other data in a packetized, time-division multiplexed fashion. The order of video, audio, and sub-picture stream in a VOBU is arbitrary. However, a VOBU is not required to contain any data other than the navigation pack (N), and thus the content within a VOBU may be shorter than the playback time of the VOBU itself. It is possible to include still frames displayed for an arbitrary length of time at arbitrary points within the video playback. Details on the data structures of PCI and DSI are discussed in section 15 and 16 of the Unofficial DVD specifications 2.2 ebook.