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Dot crawl is the popular name for a visual defect of color analog video standards when signals are transmitted as composite video
Composite video

Composite video is the format of an analog television signal before it is combined with a sound signal and modulation onto an Radio Frequency carrier wave....
, as in terrestrial broadcast television
Terrestrial television

Terrestrial television is a term which refers to modes of television broadcasting which do not involve satellite transmission. . The term is uncommon in the United States while more common in Europe....
. It consists of animated checkerboard
Checkerboard

A checkerboard is a board on which English draughts is played. It is an 8×8 board and the 64 squares are of alternating dark and light color, often red and black....
 patterns which appear along horizontal color
Color

Color or colour is the visual perception property corresponding in humans to the categories called red, yellow, blue and others....
 transitions. It results from intermodulation
Intermodulation

Intermodulation or intermodulation distortion , or intermod for short, is the result of two or more Signal of different frequencies being Frequency mixer together, forming additional signals at frequencies that are not, in general, at harmonic frequencies of either....
 or crosstalk between chrominance
Chrominance

Chrominance , is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal....
 and luminance
Luminance (video)

Relative luminance follows the Luminance, but with the values normalized to 1 or 100 for a reference white. Like the photometric definition, it is related to the luminous flux density in a particular direction, which is radiant flux density weighted by the Luminosity_function of the CIE Standard Observer....
 components of the signal, which are imperfectly multiplex
Multiplexing

In telecommunications and computer networks, multiplexing is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium....
ed in the frequency domain
Frequency domain

In electronics and control systems engineering, frequency domain is a term used to describe the analysis of mathematical functions or Signal with respect to frequency, rather than time....
.

Dot crawl is most visible when the chrominance is transmitted with a high bandwidth, so that its spectrum
Spectrum

A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a Continuum . The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a triangular prism ; it has since been applied by analogy to many fields other than op...
 reaches well into the band of frequencies used by the luminance signal in the composite video signal.






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Crawl
Dot crawl is the popular name for a visual defect of color analog video standards when signals are transmitted as composite video
Composite video

Composite video is the format of an analog television signal before it is combined with a sound signal and modulation onto an Radio Frequency carrier wave....
, as in terrestrial broadcast television
Terrestrial television

Terrestrial television is a term which refers to modes of television broadcasting which do not involve satellite transmission. . The term is uncommon in the United States while more common in Europe....
. It consists of animated checkerboard
Checkerboard

A checkerboard is a board on which English draughts is played. It is an 8×8 board and the 64 squares are of alternating dark and light color, often red and black....
 patterns which appear along horizontal color
Color

Color or colour is the visual perception property corresponding in humans to the categories called red, yellow, blue and others....
 transitions. It results from intermodulation
Intermodulation

Intermodulation or intermodulation distortion , or intermod for short, is the result of two or more Signal of different frequencies being Frequency mixer together, forming additional signals at frequencies that are not, in general, at harmonic frequencies of either....
 or crosstalk between chrominance
Chrominance

Chrominance , is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal....
 and luminance
Luminance (video)

Relative luminance follows the Luminance, but with the values normalized to 1 or 100 for a reference white. Like the photometric definition, it is related to the luminous flux density in a particular direction, which is radiant flux density weighted by the Luminosity_function of the CIE Standard Observer....
 components of the signal, which are imperfectly multiplex
Multiplexing

In telecommunications and computer networks, multiplexing is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium....
ed in the frequency domain
Frequency domain

In electronics and control systems engineering, frequency domain is a term used to describe the analysis of mathematical functions or Signal with respect to frequency, rather than time....
.

Dot crawl is most visible when the chrominance is transmitted with a high bandwidth, so that its spectrum
Spectrum

A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a Continuum . The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a triangular prism ; it has since been applied by analogy to many fields other than op...
 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. 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 Laserdisc
Laserdisc

The Laserdisc is an obsolete home video disc format, and was the first commercial optical disc storage medium. Initially marketed as Discovision in 1978, the technology was licensed and sold as Reflective Optical Videodisc, Laser Videodisc, 'Laservision, 'Disco-Vision, 'DiscoVision, and MCA DiscoVision...
s.

Dot crawl can be greatly reduced by using a good comb filter
Comb filter

In signal processing, a comb filter adds a delayed version of a signal processing to itself, causing Destructive interference#Constructive and destructive interference....
 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 maintain the signals separately by using S-Video
S-Video

Separate Video, more commonly known as S-Video, and sometimes incorrectly referred to as "Super Video" and also known as Y/C, is an analog signal video signal that carries the video data as two separate signals, lumen and chroma ....
 or component video
Component video

Component video is a video signal that has been split into two or more components. In popular use, it refers to a type of Analog signal video information that is transmitted or stored as three separate signals....
 connections instead.