How VOB Files Switch Between 16:9 and 4:3 Video

This article explains how DVD-Video structures and VOB (Video Object) containers handle aspect ratio transitions between a 16:9 main feature and 4:3 bonus materials. It details the interaction between MPEG-2 stream metadata, Video Title Set (VTS) architecture, and IFO navigation files that instruct hardware and software players to adjust their display outputs dynamically.

Fixed Resolutions and Non-Square Pixels

DVD video does not store video in native widescreen or standard definition pixel grids. Regardless of the intended display ratio, standard-definition video stored in a VOB container uses a fixed frame buffer:

• NTSC: 720×480 pixels
• PAL: 720×576 pixels

Because both 16:9 and 4:3 share the exact same resolution, the VOB container does not alter its pixel dimensions when switching formats. Instead, it relies on non-square pixels and metadata flags to instruct the display device how to stretch or scale the video buffer—a process known as anamorphic scaling.

Video Title Set (VTS) Isolation

The official DVD-Video specification dictates that all video titles grouped within a single Video Title Set (VTS) must share identical video attributes, including resolution, standard (NTSC/PAL), and aspect ratio.

To present a 16:9 widescreen feature alongside 4:3 bonus features, standard-compliant DVDs isolate these assets into separate Video Title Sets:

1. The 16:9 Feature: Housed in VTS_01_1.VOB, VTS_01_2.VOB, etc.
2. The 4:3 Extras: Housed in an entirely separate set, such as VTS_02_1.VOB.

The aspect ratio information is not strictly governed by the VOB file in isolation during playback on a DVD system. Instead, the corresponding information files (VTS_01_0.IFO and VTS_02_0.IFO) define the global video attributes for each title set. When a user selects a bonus feature from the DVD menu:

• The DVD player reads navigation commands directing it to jump from VTS 1 to VTS 2.
• The player reads VTS_02_0.IFO and reconfigures its video output processing to 4:3 mode.
• The player starts decoding the VOB files within VTS 2.

MPEG-2 Sequence Headers Inside the VOB

While the DVD navigation architecture uses IFO files for global control, raw VOB files are essentially MPEG-2 Program Streams. Within the video stream itself, aspect ratio data is encoded directly into MPEG-2 Sequence Headers.

Every MPEG-2 sequence header contains a 4-bit aspect ratio code:

• Value 2: Specifies a 4:3 aspect ratio.
• Value 3: Specifies a 16:9 aspect ratio.

These headers repeat frequently throughout the stream (typically once every Group of Pictures, or every half-second). Modern software media players that play standalone .VOB files directly—bypassing the IFO navigation structure—rely on these embedded Sequence Headers. When the player encounters a new Sequence Header with an altered aspect ratio flag, it recalculates the Display Aspect Ratio (DAR) on the fly, instantly adjusting the playback window or adding pillarbox/letterbox bars.

Player Display Handling

When an aspect ratio switch occurs—either triggered by an IFO transition or an in-stream Sequence Header—the player applies formatting rules based on the connected display type:

• On a 16:9 Display: The player sends the full 16:9 frame for the main movie, filling the screen. When switching to 4:3 bonus content, the player either adds black bars to the left and right (pillarboxing) or instructs the television to adjust its display ratio to 4:3.
• On a 4:3 Display: The player formats the 16:9 movie using Letterbox (black bars top and bottom) or Pan-and-Scan mode, depending on user settings. When switching to the 4:3 bonus features, it presents the video fullscreen without horizontal or vertical cropping.