DVD Hierarchy: Title, Part of Title, and VOB Files

Understanding the architecture of a DVD-Video requires distinguishing between logical playback structures and physical data storage. This article breaks down the hierarchy between a Title, a Part of Title (PTT), and a VOB file, illustrating how user-facing playback navigation maps directly to the underlying media files on the disc.

Logical Structure vs. Physical Storage

The DVD-Video specification divides disc architecture into two distinct layers:

1. Logical Layer: The navigation hierarchy recognized by the DVD player and the user (Titles, Chapters/PTTs, Programs, and Cells).
2. Physical/File Layer: The actual container files stored in the standard VIDEO_TS directory (IFO, BUP, and VOB files).

A Title and a Part of Title exist entirely in the logical layer, while a VOB file belongs to the physical storage layer.

1. Title (Top Logical Unit)

A Title is the highest-level presentation unit within a DVD. A single DVD can contain up to 99 Titles.

• Function: A Title represents a complete, distinct video feature. For example, a commercial DVD typically assigns the main feature film as Title 1, while individual bonus features, deleted scenes, or trailers are assigned as separate subsequent Titles (Title 2, Title 3, etc.).
• Grouping: Titles are organized into Video Title Sets (VTS). All Titles within the same VTS share the same video attributes (such as resolution, aspect ratio, and compression standards) and audio/subtitle configurations.

2. Part of Title / PTT (Sub-Logical Unit)

A Part of Title (PTT), universally referred to as a Chapter, is a logical division within a specific Title.

• Function: PTTs allow users to jump to specific scenes or reference points within a Title using the "Next" or "Previous" buttons on a remote, or via a scene selection menu.
• Hierarchy: A Title can contain up to 99 PTTs (Chapters). A PTT cannot exist independently; it is always subordinate to a parent Title.
• Granularity: Under the DVD specification, each PTT points to a specific Program or sequence of Cells (the smallest addressable playback units) governed by timecodes defined in the disc's .IFO (information) files.

3. VOB File (Physical Container Layer)

A VOB (Video Object) file is the actual file stored on the disc file system that contains the multiplexed MPEG-2 video, audio tracks (AC-3, DTS, or PCM), and subtitle streams.

• Function: VOB files do not represent navigational items; they are raw physical containers for the streaming data.
• Size Limitations: Due to file system constraints (specifically UDF/ISO 9660 requirements), a single VOB file cannot exceed 1 GiB (1,048,576 KB).
• Splitting: Because a full-length movie exceeds 1 GiB, the continuous video stream of a Video Title Set is split sequentially across multiple physical files: VTS_01_1.VOB, VTS_01_2.VOB, VTS_01_3.VOB, and so on.

How They Relate in the Hierarchy

The structural hierarchy operates from abstract navigation down to physical data:

\[\text{Title (Logical Feature)} \longrightarrow \text{Part of Title / Chapter (Logical Point)} \longrightarrow \text{VOB Files (Physical Data Stream)}\]

• One Title can span multiple VOB files: A two-hour movie (Title 1) may continuously stretch across five or six separate 1 GiB VOB files. The DVD player reads through the sequence seamlessly using address tables stored in the corresponding .IFO file.
• One VOB file can contain multiple Titles: If multiple Titles belong to the same Video Title Set and are short (such as several short featurettes), they can reside inside the same 1 GiB VOB file.
• A Part of Title (PTT) does not map 1:1 with a VOB file: A Chapter mark simply tells the player to seek to a specific sector address inside the VOB stream. A single VOB file typically contains multiple Chapters, and a single Chapter can span across the boundary between two sequentially split VOB files without interruption.