What Is an Interleaved Video Unit (ILVU) in VOB?
An Interleaved Video Unit (ILVU) is a foundational structural element in DVD-Video authoring designed to support multi-angle scenes, parental levels, and branching story paths without playback interruption. This article explains what an ILVU is, why it was developed for optical disc technology, and how it physically organizes multi-angle video streams within a DVD Video Object (VOB) file.
Understanding the Interleaved Video Unit (ILVU)
On a DVD-Video disc, video, audio, and subtitle streams are multiplexed into Video Object (VOB) files. Standard playback reads these files sequentially. However, features such as multi-angle playback—which allows a viewer to switch between different camera views in real time—present a physical challenge for optical drives.
Because optical drive pick-up heads take a noticeable amount of time to physically move (seek) across disc tracks, storing each camera angle as a separate, continuous file would cause the video to freeze or stutter whenever the viewer changed angles. To solve this, the DVD specification introduced the Interleaved Video Unit (ILVU). An ILVU is a contiguous segment of video containing one or more Video Object Units (VOBUs) belonging to a specific angle, typically lasting between 0.5 and 2 seconds.
How ILVU Organizes Multi-Angle Streams
Instead of placing entire video tracks one after the other, the DVD authoring process chops each angle into small ILVU blocks and alternates them sequentially inside the same VOB file.
If a scene has three camera angles (Angle 1, Angle 2, and Angle 3), the VOB organizes the physical data in an interleaved pattern:
- ILVU 1: Angle 1 (Time 00:00 – 00:01)
- ILVU 2: Angle 2 (Time 00:00 – 00:01)
- ILVU 3: Angle 3 (Time 00:00 – 00:01)
- ILVU 4: Angle 1 (Time 00:01 – 00:02)
- ILVU 5: Angle 2 (Time 00:01 – 00:02)
- ILVU 6: Angle 3 (Time 00:01 – 00:02)
Playback Mechanics and Buffer Management
To play a single angle smoothly from an interleaved VOB, the DVD player relies on a track buffer and navigation metadata:
- Buffer Filling: When playing Angle 1, the laser reads ILVU 1 at higher than playback speed and deposits the data into an internal track buffer.
- Laser Jump: While the player plays the video out of the buffer, the physical laser quickly skips over ILVU 2 and ILVU 3 to locate the start of ILVU 4.
- Buffer Continuity: Because the jump occurs within milliseconds, the track buffer never empties (underflows), producing seamless playback.
- Angle Switching: If the viewer selects Angle 2 while Angle 1 is playing, the player finishes reading the current Angle 1 ILVU, checks the navigation tables, and directs the laser head to jump immediately to the next sequential ILVU assigned to Angle 2.
Navigation via NV_PCK and DSI
This skipping and switching process is coordinated entirely by
metadata stored inside each VOBU's Navigation Pack
(NV_PCK). Within this pack, the Data Search Information
(DSI) sub-stream contains dedicated tables:
- Seamless Multi-Angle Information
(
SML_AGLI): Explicitly stores the relative sector addresses and sizes of the next ILVUs for all available angles. - Seamless Angle Information
(
SML_AA_IT): Details the jump destinations required for parental management and branching points.
These pointers allow the player's firmware to know the exact sector to seek to before the optical head ever lifts, eliminating calculation latency.
Technical Constraints of ILVU Multi-Angle Setup
For ILVU interleaving to work seamlessly, strict encoding rules must be followed during DVD authoring:
- Closed GOPs (Group of Pictures): Each ILVU must begin with an I-frame (Intra-coded frame) and use a closed GOP so it can be decoded independently of adjacent units.
- Matching Timestamps: Presentation Time Stamps (PTS) and audio synchronization must align perfectly across all parallel angles.
- Bitrate Limitations: The aggregate size of the interleaved blocks and the distance between identical angles must not exceed the read transfer rate of the optical drive or the capacity of the player's track buffer.