DVD VOB File Numbering Conventions Explained

On standard DVD-Video discs, VOB (Video Object) files follow a strict, standardized naming convention dictated by the DVD Forum's DVD-Video specification. This article explains how files within a Video Title Set (VTS) are indexed, the distinct roles of the two numeric fields in each filename, and the technical limitations—such as file-system constraints and content type separation—that determine this numbering scheme.

The Structure: VTS_XX_Y.VOB

Every standard title file inside a DVD's VIDEO_TS directory follows the VTS_XX_Y.VOB template. The numbering convention is broken down into two distinct numerical components: XX and Y.

 VTS _ XX _ Y .VOB
  │     │   │
  │     │   └── File Sequence / Content Type Index (0 to 9)
  │     └────── Title Set Number (01 to 99)
  └──────────── Video Title Set Prefix

1. The Title Set Number (XX)

The first two digits (XX) represent the specific Video Title Set (VTS) on the disc, ranging from 01 to 99.

• Logical Grouping: During the authoring process, creators group related video, audio, and subtitle streams together into distinct sets. For example, a feature film may be placed in VTS_01, while bonus features, deleted scenes, or behind-the-scenes documentaries are assigned to VTS_02, VTS_03, and so forth.
• Independent Attributes: Each title set possesses uniform technical attributes (such as aspect ratio, audio codecs, and video resolution). Whenever a piece of content requires a different format (e.g., switching from 16:9 widescreen to 4:3 fullscreen), it must be placed into a separate Title Set with its own sequential number.
• Global Menus Exception: The master disc menu resides outside this numbering scheme in the Video Manager, labeled as VIDEO_TS.VOB.

2. The File Sequence Index (Y)

The single digit (Y) following the title set number designates the specific role or sequence of the file, ranging from 0 to 9.

Index 0: Menu Content

A value of 0 (e.g., VTS_01_0.VOB) indicates that the file contains the menu assets for that specific title set. It holds the menu background video, audio loops, and subpicture highlight data for navigation buttons. If a title set has no dedicated menu, this file is omitted.

Indexes 1 through 9: Sequential Title Media

Values from 1 through 9 (e.g., VTS_01_1.VOB, VTS_01_2.VOB) contain the actual continuous video and audio playback stream. Numbering starts at 1 and increments sequentially as content is added.

The Primary Cause of Sequential Splitting: The 1 GB Limit

The need for multiple sequential files (_1, _2, _3, etc.) is determined by file-system specifications:

• UDF 1.02 File Size Constraint: DVDs use the Universal Disk Format (UDF) alongside ISO 9660 (UDF Bridge). The standard mandates that no single file on a DVD-Video disc may exceed 1 GiB (1,073,741,824 bytes, or 1,048,576 KB).
• Seamless Playback: Because a full-length movie exceeds 1 GB, authoring tools automatically slice the continuous multiplexed stream into chunks that sit just under the 1 GB threshold.
• Maximum Size Limit per VTS: Because the sequence digit (Y) only spans from 1 to 9, a single Video Title Set can span a maximum of nine 1 GB media files (approximately 9 GB). This aligns directly with the storage limit of a dual-layer DVD-9 disc.