Causes of Audio Popping Between Split VOB Files

Audio crackling, clicking, or popping at the seam between two split VOB (Video Object) files typically occurs when continuous audio streams are abruptly severed across file boundaries. Because DVD VOB files are split based on strict file-size limits rather than media frames, playing or concatenating these files improperly cuts audio packets mid-frame, drops essential sync words, and disrupts playback timestamps. Understanding how DVD multiplexing works reveals why these audio anomalies happen and how they can be prevented.

Arbitrary Binary Splitting

DVD-Video specifications limit individual VOB files to a maximum size of 1 GB (1,048,576 KB) to ensure compatibility with older file systems like FAT32 and standard UDF formats. When authoring a DVD, the multiplexer writes a continuous MPEG-2 Program Stream and slices it into sequential files (VTS_01_1.VOB, VTS_01_2.VOB, etc.) purely based on byte count. The split does not account for the structural boundaries of video keyframes or audio packets.

Severed Audio Frames

Most DVD audio is encoded in formats such as AC-3 (Dolby Digital), DTS, or uncompressed LPCM. These formats divide audio into discrete frames:

• AC-3 Frames: An AC-3 stream consists of synchronized frames containing 1,536 audio samples (roughly 32 milliseconds of audio) along with header data and a sync word (0x0B77).
• Frame Fragmentation: When a file boundary arbitrarily cuts through an audio packet, the first half of the audio frame remains at the end of the first VOB file, while the remainder is placed at the beginning of the next VOB file.

If software processes or plays these files as independent entities, the audio decoder encounters a truncated frame at the end of the first file, resulting in missing data. When the decoder fails to parse the damaged frame or encounters an unexpected start in the next file, it generates a momentary digital discontinuity—heard as a loud pop, click, or crackle.

Loss of Presentation Timestamps (PTS)

Audio and video synchronization on DVDs relies on Presentation Time Stamps (PTS) embedded in the Packetized Elementary Stream (PES) headers. When audio is cut across files without the overarching navigation data provided by the DVD's IFO files:

• Decoders may reset their internal clock between files, leading to a buffer underflow or buffer flush.
• A micro-gap or micro-overlap of several milliseconds can occur during playback transition.
• The sudden jump between audio waveform amplitudes without a smooth crossfade causes a sharp transient spike, perceived as a pop.

Padding and Private Stream Headers

DVD multiplexers insert zero-byte padding, navigation packs (NV_PCK), and private stream identification headers throughout the VOB container to maintain constant bitrate and stream alignment. If split VOBs are merged using rudimentary concatenation tools, these non-audio bytes can be accidentally pushed into the audio decoder. The decoder attempts to process garbage data as audio samples, leading to harsh static or pops at the exact transition point.

How to Prevent Audio Seam Pops

To eliminate audio pops between split VOB files:

• Parse via IFO Files: Always open the associated .IFO file rather than individual .VOB files in ripping or playback software. The IFO file contains the playback chain (PGC) instructions, telling the splitter exactly how to seamlessly buffer the streams across file boundaries.
• Demux to Elementary Streams: If converting or editing, use a DVD-aware demultiplexer (such as DGIndex or MakeMKV) to extract the audio stream as a single, continuous, unbroken file before re-encoding or remuxing into modern containers like MKV or MP4.