VOB Sequence Header Aspect Ratio Flags Explained

VOB (Video Object) files, the core container format used in DVD-Video media, encapsulate MPEG-2 video streams that define display geometry at the bitstream level. This article explains how aspect ratio flags are embedded within the MPEG-2 sequence headers of a VOB file, detailing the specific 4-bit values used to signal display formats such as standard 4:3 and widescreen 16:9.

The MPEG-2 Sequence Header Structure

Inside a VOB container, the video stream is structured as an MPEG-2 Program Stream. At regular intervals—typically preceding every Group of Pictures (GOP)—the stream contains a sequence_header identified by the 32-bit start code 0x000001B3.

Following the horizontal and vertical frame dimensions within this header, the stream embeds a 4-bit field designated as aspect_ratio_information. This nibble defines the intended display aspect ratio (DAR) or sample aspect ratio (SAR) for the decoded frames.

Aspect Ratio Flag Values

The MPEG-2 standard specifies the following definitions for the 4-bit aspect_ratio_information flag:

Valid Flags for DVD-Video VOB Streams

While the MPEG-2 specification defines multiple aspect ratios, the DVD-Video standard strictly restricts compliant VOB files to two specific flags:

  1. Value 2 (0010): Indicates a 4:3 display aspect ratio. This is standard for full-screen playback on legacy CRT displays and letterboxed widescreen transfers.
  2. Value 3 (0011): Indicates an anamorphic 16:9 display aspect ratio. The standard storage resolution (such as 720x480 for NTSC or 720x576 for PAL) is horizontally expanded upon playback to fill a widescreen display.

Values 1 and 4 are defined within the broader MPEG-2 standard, but they are out-of-spec for commercial DVD-Video authoring and will typically cause playback failure or display errors on standalone DVD players.

Container Flags vs. Stream Flags

While the sequence header holds the baseline aspect_ratio_information flag inside the VOB file, DVD navigation files (.IFO) also contain separate video attribute tables. DVD players typically prioritize the IFO table settings to determine display behavior—such as handling automatic pan-and-scan or letterboxing options—while decoders directly reading raw VOB or MPEG-2 elementary streams rely entirely on the sequence header's embedded 4-bit flag.