Menu VOB vs Title VOB Stream Allocation

In the DVD-Video specification, Menu VOBs (Video Object files for menus) and Title VOBs (files containing the primary feature content) are both MPEG-2 Program Streams, but they differ significantly in their internal stream allocation limits and packet usage. While Title VOBs are engineered to accommodate multiple audio tracks and extensive subtitle options, Menu VOBs are structurally constrained to fewer streams, repurposing specific sub-streams and navigation packets primarily for interactive button overlays and interface navigation.

Audio Stream Allocations

The most prominent allocation difference between the two file types is in audio multiplexing:

• Title VOBs (VTS_xx_1.VOB, etc.): Can allocate up to eight concurrent audio streams (numbered 0 to 7). These correspond to MPEG audio streams 0xC0–0xC7 or private stream 1 sub-stream IDs 0x80–0x87 (used for Linear PCM, AC-3, and DTS). This architecture allows the disc to store alternative language tracks, commentary, and different audio formats (such as stereo and 5.1 surround) within the same title set.
• Menu VOBs (VIDEO_TS.VOB or VTS_xx_0.VOB): Are constrained to one audio stream. The DVD standard permits only a single audio track for any menu domain (VMGM or VTSM). Consequently, menus cannot feature selectable background language tracks multiplexed into the same VOB; authoring different languages for menus requires switching between separate menu cells, programs, or VOB files altogether.

Subpicture Stream Allocations

Subpicture packets carry bitmap graphics and run-length-encoded overlays:

• Title VOBs: Support up to 32 subpicture streams (sub-stream IDs 0x20 to 0x3F within private stream 1). This high limit is designed to accommodate multiple languages for foreign subtitles, closed captions, and forced director commentary notes.
• Menu VOBs: While the stream space technically supports subpicture streams, internal allocation is restricted to interface needs. Rather than multi-language text, menu subpictures store the visual button states (normal, selected, and activated button highlights). Generally, a menu allocates between one and four subpicture streams. These additional streams are not used for different subtitle languages, but to provide distinct highlight graphics mapped directly to different display modes (such as 4:3, 16:9 Letterbox, and 16:9 Pan & Scan).

Navigation Pack (NV_PCK) Payload Differences

Every Video Object Unit (VOBU) in both Menu and Title VOBs begins with a Navigation Pack containing two 980-byte packets: Presentation Control Information (PCI) and Data Search Information (DSI). However, the internal allocation of the PCI data differs:

• Menu VOBs: The PCI packet actively utilizes the Highlight Information (HLI) data structure. This table defines the interactive button areas, color palette translation tables (BTN_COLIT), button coordinate groups (BTNIT), and execution commands triggered by user remote-control events.
• Title VOBs: Title content usually leaves the HLI tables empty or deactivated, as passive movie playback does not require on-screen button routing. Instead, Title NV_PCKs allocate most processing overhead to the DSI packet to manage forward and backward scan pointers, angle transitions, and seamless branching addresses.

Video Stream Behavior

Both Menu and Title VOBs allocate only one elementary video stream (Stream ID 0xE0). However, their stream delivery patterns differ internally:

• Title VOBs: Video is allocated as continuous, high-bitrate motion video structured into consecutive Groups of Pictures (GOPs) running sequentially from start to finish.
• Menu VOBs: Frequently allocate video as a "still picture" VOBU. This consists of a single I-frame without subsequent P- or B-frames, combined with a VOBU pause flag or an indefinite display duration, saving disc bandwidth for title content while retaining an active subpicture overlay for button navigation.