Video Object (VOB) DVD Forum Definition

This article provides an overview of the formal definition and technical structure of a Video Object (VOB) under the specifications established by the DVD Forum. It examines the precise role of a VOB within the DVD-Video logical hierarchy, details its composition as a constrained MPEG-2 Program Stream, and explains the critical data packs that govern its playback.

According to the DVD Forum’s official standard—the DVD Specifications for Read-Only Disc, Part 3: Video Specifications—a Video Object (VOB) is formally defined as a contiguous sequence of Video Object Units (VOBUs) that constitute the presentation data for a video title, menu, or interactive segment. Mechanically, a VOB is an application-specific implementation of an MPEG-2 Program Stream (ISO/IEC 13818-1 Systems) constrained to meet the strict physical and logical requirements of DVD media.

Structural Composition of a VOB

A VOB serves as the core multimedia container within DVD-Video. Rather than an arbitrary bitstream, the DVD Forum defines the internal structure of a VOB with rigid parameters:

• Video Object Units (VOBU): The fundamental building blocks of a VOB. Every VOB consists of one or more contiguous VOBUs. A VOBU represents a duration of playback typically spanning 0.4 to 1.0 seconds (up to 1.2 seconds for still frames).
• Fixed Pack Sizing: All data inside a VOB is divided into standardized "Packs" of exactly 2,048 bytes. This aligns perfectly with the physical sector size of a DVD disc, allowing direct mapping between logical stream packets and physical media sectors.

Pack Types Contained in a VOB

The DVD Forum dictates that each VOBU within a VOB must begin with specialized navigation data followed by multiplexed elementary streams:

1. Navigation Pack (NV_PCK): The mandatory first pack of every VOBU. It contains two 980-byte packets: Presentation Control Information (PCI) for display timing, aspect ratios, and angle switches; and Data Search Information (DSI) for forward/backward seeking, fast-forward, and rewind operations.
2. Video Pack (V_PCK): Carries compressed video elementary streams conforming to MPEG-1 or MPEG-2 (ISO/IEC 13818-2) standards.
3. Audio Pack (A_PCK): Carries compressed or uncompressed audio streams, including Linear PCM, Dolby Digital (AC-3), MPEG-1/MPEG-2 Audio, or DTS.
4. Sub-picture Pack (SP_PCK): Encapsulates run-length encoded bitmapped graphics used for subtitles, closed captions, and interactive button overlays in menus.

Hierarchical Context in DVD-Video

Within the DVD Forum's structural hierarchy, the VOB functions as the bridge between raw MPEG data and playback logic:

• Video Title Set (VTS): A collection of files that make up a distinct portion of the disc.
• Program Chain (PGC): The logical sequence that dictates playback order.
• Cells and Programs: A PGC is divided into Programs, which are further divided into Cells. A Cell points to a specific start and end address within a single VOB (or VOB ID).

In summary, the DVD Forum defines a VOB not merely as a file extension, but as a bounded, sector-aligned MPEG-2 Program Stream structured into VOBUs, initiated by navigation packs, and referenced by logical cells to deliver synchronized audio, video, and interactive sub-pictures.