Audio-Only VOBU in Interactive Menu VOBs Explained
An audio-only Video Object Unit (VOBU) within an interactive menu Video Object (VOB) delivers continuous background sound while preserving a static visual display and maintaining active menu navigation. Instead of streaming continuous video frames, this specialized VOBU relies on a sequence of audio packets coupled with DVD navigation data to update the audio buffer, leave the video decoder frame intact, and keep interactive button overlays fully responsive.
The Role of the Navigation Pack (NV_PCK)
Every VOBU, regardless of content, must begin with a Navigation Pack (NV_PCK). In an audio-only menu VOBU, the NV_PCK carries two critical sub-structures:
- Presentation Control Information (PCI): Contains the Highlight Information (HLI). The HLI defines the coordinates, color palettes, states (normal, selected, activated), and execution commands for the interactive buttons visible on the screen.
- Data Search Information (DSI): Manages timing, playback continuity, and address offsets to ensure seamless navigation across the menu structure.
Because the NV_PCK is processed at the start of each VOBU (typically every 0.4 to 1.0 seconds), the player continually refreshes navigation states and user-input readiness even when no new video is supplied.
Video Buffer Persistence
In standard VOBUs, Video Packs (V_PCKs) feed compressed MPEG-2 frames to the video decoder. An audio-only VOBU contains zero V_PCKs.
When the DVD player encounters a VOBU without video data, the video decoder does not clear its output. Instead, it holds the last decoded I-frame (intra-coded picture) in its frame buffer. This mechanism allows a single background image to remain visible on the display without consuming the bit rate or storage space required to re-transmit identical frames.
Audio Stream Delivery (A_PCK)
Following the NV_PCK, the remainder of the VOBU payload consists of Audio Packs (A_PCKs), formatted in AC-3, LPCM, or DTS. These packets are continuously delivered to the audio buffer.
- System Clock Synchronization: The System Clock Reference (SCR) and Presentation Time Stamps (PTS) ensure that audio plays back without interruption or buffer underflows.
- Continuous Playback: While the visual state remains stationary, the audio stream proceeds sequentially across successive audio-only VOBUs, creating continuous background music or dialogue for the menu.
Sub-Picture and Interactive Highlight Management
Interactive menus overlay sub-pictures (SP_PCKs) on top of the background image to show button states. In an audio-only VOBU sequence:
- If button highlights are static, the sub-picture decoder maintains the existing overlay generated by preceding units.
- If a button changes state (via user remote interaction), the PCI within the current VOBU’s NV_PCK dynamically alters the display properties of the highlight area without interrupting the audio stream.
Looping and Playback Termination
Audio-only menu VOBUs are organized inside a Program Chain (PGC). Once the final VOBU in the audio stream finishes playing:
- The end of the cell is reached.
- The player executes cell or post-commands defined in the IFO file.
- The command typically loops playback back to the first VOBU of the menu cell to restart the background audio, or automatically executes a default action if a timer expires.