How DVD Authoring Tools Create VOB, IFO, and BUP Files

DVD authoring is the technical process of converting raw digital video, audio, and subtitle streams into a strictly standardized file system that standalone DVD players can parse and play. This article explains how authoring software processes these elementary assets, multiplexes them into sector-aligned Video Object (VOB) containers, and calculates the navigational logic required to generate Information (IFO) and Backup (BUP) files inside the final VIDEO_TS directory.

1. Asset Validation and Demuxing

Before authoring tools compile data, all raw media assets must strictly comply with the DVD-Video standard (DVD Forum specifications). If the source files are not already compliant, the software transcodes them:

• Video: Must be MPEG-1 or MPEG-2, strictly capped at specific resolutions (such as 720×480 for NTSC or 720×576 for PAL) with a maximum bitrate of 9.8 Mbps. The GOP (Group of Pictures) structure must be closed and bounded (typically 15 frames for NTSC, 12 for PAL) to allow seamless seeking.
• Audio: Formatted as Linear PCM, Dolby Digital (AC-3), or DTS, sampled strictly at 48 kHz.
• Subtitles and Menus: Rendered into 4-color Run-Length Encoded (RLE) bitmapped subpicture streams rather than text-based formats.

The tool parses these individual inputs as elementary streams, indexing their duration, frame timestamps, and sync points.

2. Multiplexing into VOB Files

Once the elementary streams are prepared, the authoring engine runs a multiplexer (muxer). This is where raw assets transform into .VOB files:

• Sector Packaging: The DVD physical specification mandates a 2,048-byte sector size. The muxer breaks the video, audio, and subpicture streams into packets and packs them into standard 2 KB MPEG-2 Program Stream (PS) packs.
• Time-Division Interleaving: Audio, video, and subtitle packs are finely interleaved based on their Presentation Time Stamps (PTS) and Decoding Time Stamps (DTS) to guarantee buffer health in hardware players without underflows or overflows.
• Navigation Packs (NV_PCKs): At the start of every Video Object Unit (VOBU)—roughly every 0.4 to 1.0 seconds—the authoring engine inserts a specialized Navigation Pack. These packets contain Real-Time Information (PCI) and Data Search Information (DSI), which provide forward and backward playback pointers, chapter entry markers, and button highlights for menus.
• File Splitting: The muxer continuously writes these sectors to disk, splitting the continuous stream into maximum file chunks of 1 GB (1,048,576 KB) labeled sequentially (VTS_01_1.VOB, VTS_01_2.VOB, etc.) to maintain compatibility with the UDF 1.02 filesystem.

3. Generating Navigation Logic: IFO Files

While VOB files hold the actual audio-visual payload, they cannot be navigated without Information (.IFO) files. The authoring tool generates IFO files by acting as a compiler for the DVD Virtual Machine (VM):

• VIDEO_TS.IFO (Video Manager): This is the master index. The compiler writes the global region code, parental management settings, aspect ratios, and pointers to the first play program chain (First-Play PGC) and the Video Manager Menu (VIDEO_TS.VOB).
• VTS_xx_0.IFO (Video Title Set): Each title set gets its own IFO file. The authoring software maps out the exact sector addresses for every chapter, cell, and audio/subtitle track within that title set.
• Command Tables: Any interactive behavior programmed in the authoring GUI—such as jumping to a specific title after a video ends, routing audio streams, or executing conditional menu branching—is compiled into low-level DVD VM bytecode (Pre-Commands, Post-Commands, and Cell Commands using 16 general-purpose registers).

4. Creating Redundancy: BUP Files

Because optical media is susceptible to physical damage like scratches and read errors, optical drives require redundancy for critical playback data.

For every .IFO file generated, the authoring tool writes an identical mirror file designated with the .BUP extension (e.g., VIDEO_TS.BUP and VTS_01_0.BUP). The authoring engine ensures that the BUP file is written to a physically separate set of sectors on the disc layout, usually placed after the corresponding VOB files. If the DVD player's optical pickup encounters corrupted sectors when reading an IFO file, it seeks to the backup sectors containing the BUP file to preserve navigation integrity.

5. Final Output Compilation

The final compilation stage organizes the files into a root directory containing an empty AUDIO_TS folder and a fully populated VIDEO_TS folder. When the authoring tool outputs an ISO disc image, it formats the sector allocation using the UDF/ISO 9660 Bridge file system, aligning each file's starting location precisely to standard logical block addresses for physical disc pressing or burning.