PTS Behavior Across Split DVD VOB Files

When a DVD title spans multiple 1 GB Video Object (VOB) files, Presentation Timestamps (PTS) do not reset or alter at the physical split seam; instead, they remain strictly continuous. The split between files like VTS_01_1.VOB and VTS_01_2.VOB is purely a file-system constraint rather than a container or codec boundary. This article explains how PTS values flow across these seams, how MPEG-2 Program Stream packs interact at the split point, and what distinguishes a physical file boundary from a logical timestamp discontinuity.

The Physical Nature of VOB Splitting

DVD-Video specifications require that no single file exceed 1 GiB (1,073,741,824 bytes) to maintain compatibility with the UDF 1.02 and ISO 9660 file systems. When an authored title exceeds this limit, the multiplexer cuts the resulting MPEG-2 Program Stream into 1 GB segments.

This cut occurs strictly at the boundary of a 2,048-byte DVD sector (pack). It does not represent the end of a video sequence, a Group of Pictures (GOP), or an audio frame. Because the split is an arbitrary physical partition of a single continuous stream, the underlying elementary streams remain uninterrupted.

Timestamp Continuity at the Seam

In a standard MPEG-2 Program Stream, the PTS dictates when a decoded video picture or audio frame must be presented to the viewer. At the exact boundary where VOB_1 ends and VOB_2 begins:

• Monotonic Progression: The PTS values in the elementary stream packets (PES headers) continue incrementally based on the media's nominal frame rate and sample rate. If the last video frame in the first file has a PTS corresponding to 00:14:58.966, the first video frame in the next file will carry a PTS of 00:14:59.000 (assuming a 29.97 fps sequence).
• System Clock Reference (SCR): Like the PTS, the System Clock Reference values embedded in the pack headers continue linearly across the seam. The decoder's System Time Clock (STC) does not reload or reset.
• 跨-Packet Payloads: An elementary stream packet or even a compressed frame can logically span across the split. A PES packet header may reside in the final sector of VTS_01_1.VOB, while the remaining payload carrying the actual picture data resides in the opening sector of VTS_01_2.VOB.

Physical Seams vs. Logical Discontinuities

It is critical to separate the physical file seam from DVD logical markers. A PTS reset or jump on a DVD is governed by the .IFO metadata and DVD Cell properties, not the .VOB file cut:

1. Seamless Splits: If a file boundary falls in the middle of a continuous Cell or between seamlessly linked Cells, there is zero gap, no change in the PTS timebase, and no decoder flush.
2. Discontinuities: A discontinuity in PTS only occurs if the multiplexer deliberately starts a new Cell marked as non-seamless (such as an angle change, layer break, or jump between separate Program Chains). In these cases, the IFO flags a discontinuity, and the PTS jumps to a new baseline. However, these logical boundaries rarely align with the arbitrary 1 GB file cut.

Implications for Demuxing and Playback

DVD-compliant playback hardware reads sequential VOB files as a single contiguous stream by concatenating their sector streams in memory.

Issues only arise when modern media players or demuxing utilities process VOB files as standalone entities rather than parts of a whole:

• Standalone Parsing Failures: If a demuxer inspects VTS_01_2.VOB in isolation, it will encounter frames with PTS values starting far above zero, often without a sequence header or IDR keyframe at byte zero.
• Audio Desynchronization: Slicing or converting split VOBs individually shifts audio and video sync because the audio PES packets and video PES packets are interleaved with different buffer latencies. Concatenating the elementary streams prior to demuxing or using tools that parse the entire set via the .IFO file is required to preserve the continuous flow of PTS.