Dot crawl

Dot crawl is the popular name for a visual defect of color analog video standards when signals are transmitted as composite video. It consists of animated checkerboard patterns which appear along vertical color transitions. It results from intermodulation or crosstalk between chrominance and luminance components of the signal, which are imperfectly multiplexed in the frequency domain.

Dot crawl is most visible when the chrominance is transmitted with a high bandwidth, so that its spectrum reaches well into the band of frequencies used by the luminance signal in the composite video signal. This causes high-frequency chrominance detail at color transitions to be interpreted as luminance detail.

Another, similar problem is the appearance of a colored noise in image areas with high levels of detail. This results from high-frequency luminance detail crossing into the frequencies used by the chrominance channel and producing false coloration. Because of dot crawl, television studio guests usually avoid clothing with fine patterns. Dot crawl can also make narrowly-spaced text difficult to read.

Dot crawl has long been recognized as a problem by professionals since the creation of composite video, but was first widely noticed by the general public with the advent of Laserdiscs.

Dot crawl can be greatly reduced by using a good comb filter in the receiver to separate the encoded chrominance signal from the luminance signal. However, the only complete solution to dot crawl is to not use composite video, and to use S-Video or component video processing instead.