What Causes Interlacing Combing in VOB Files?

When playing a DVD VOB file on a modern computer monitor, viewers often see jagged, horizontal lines known as combing artifacts. This article explains the technical reasons behind this phenomenon, focusing on how legacy interlaced video standards conflict with modern progressive displays, how temporal mismatches between video fields cause combing, and how deinterlacing software resolves the problem.

VOB (Video Object) files are the core container format used on standard DVD-Video discs, typically holding MPEG-2 encoded video. The DVD standard was developed primarily for older cathode-ray tube (CRT) televisions, which utilized interlaced scanning (such as 480i in NTSC regions or 576i in PAL regions).

In an interlaced video stream, each complete video frame is split into two separate halves called "fields." One field contains only the odd-numbered horizontal scan lines, while the other contains the even-numbered scan lines. Crucially, these fields are not captured at the exact same instant. A standard 60i video camera captures 60 half-resolution fields per second rather than 30 full frames, creating a small time gap (roughly 1/60th of a second) between the capture of the odd field and the even field.

Combing artifacts occur because modern LCD, OLED, and LED computer monitors are inherently progressive displays. Unlike CRT displays that scan alternating lines sequentially, progressive monitors display all horizontal lines simultaneously to draw an entire frame at once.

When a media player renders an interlaced VOB file on a progressive display without processing, it simply weaves the two consecutive fields together into a single frame. If the scene is completely static, the image appears normal. However, if there is any movement—such as a moving subject or a camera pan—the object occupies one position during the odd field and a slightly shifted position by the time the even field is captured.

When these two temporally mismatched fields are drawn together on a progressive screen, the alternating lines do not align. The resulting horizontal misalignment produces a serrated, comb-like pattern along the edges of moving objects, commonly referred to as "combing" or "mouse teeth."

Eliminating these artifacts requires deinterlacing. Modern video playback software or hardware scalers can apply deinterlacing algorithms—such as Yadif, blend, or motion-adaptive interpolation—which reconstruct or smooth the missing temporal data between fields, rendering a clean image suitable for progressive screens.