Dot crawl

Dot crawl
Enlarged detail from a video source exhibiting dot crawl. Note the distinctive checkerboard pattern on the vertical edges between yellow and blue areas.

Dot crawl is the popular name for a visual defect of color analog video standards when signals are transmitted as composite video, as in terrestrial broadcast television. 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.

This takes two forms: chroma interference in luma (chroma being interpreted as luma), and luma interference in chroma.

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.

Some (mostly older) video game consoles and computers use nonstandard color burst phases and may produce dot crawl quite different from that seen in broadcast NTSC or PAL.

The opposite problem, luminance interference in chroma, 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, known as color bleed. Bleed can also make narrowly-spaced text difficult to read. Some computers, such as the Apple II, utilized this to generate color.

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. When the NTSC standard was adopted in the 1950s, TV engineers realized that it should theoretically be possible to design a filter to properly separate the luminance and chroma signals. However, the vacuum tube-based electronics of the time did not permit any cost-effective method of implementing a comb filter. Thus, the early color TVs used only notch filters, which cut the luminance off at 3.5 MHz. This effectively reduced the luminance bandwidth (normally 4 MHz) to that of the chroma, causing considerable color bleed. By the 1970s, TVs had begun using solid-state electronics and the first comb filters appeared. However, they were expensive and only high-end models used them, while most color sets continued to use notch filters.

By the 1990s, a further development took place with the event of three-line digital comb filters. This type of filter uses a computer to analyze the NTSC signal three scan lines at a time and determine the best place to put the chroma and luminance. During this period, digital filters became standard in high-end TVs while the older analog filter began appearing in cheaper models (although notch filters were still widely used).

However, no comb filter can totally eliminate NTSC artifacts and the only complete solutions to dot crawl are not to use NTSC or PAL composite video, maintaining the signals separately by using S-Video or component video connections instead, or encoding the chrominance signal differently as in SECAM or any modern digital video standard as long as the source video has never been processed using any video system vulnerable to dot crawl.

Monochrome film recordings of colour television programs may exhibit dot crawl, and starting in 2008 it has been used to recover the original colour information in a process called colour recovery.

See also

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Star Wars opening crawl — The three components of the opening sequence: the phrase A long time ago in a galaxy far, far away.... , the Star Wars logo and the opening crawl, from the beginning of Star Wars Episode IV: A New Hope. Each film in the Star Wars series opens… …   Wikipedia

  • SECAM — SECAM, also written SÉCAM (Séquentiel couleur à mémoire,[1] French for Sequential Color with Memory ), is an analog color television system first used in France. A team led by Henri de France working at Compagnie Française de Télévision (later… …   Wikipedia

  • Composite video — Not to be confused with Component video. Composite video On consumer products a yellow RCA connector is typically used for composite video. Type Analog video connector …   Wikipedia

  • Chroma dots — are visual artefacts caused by displaying an unfiltered analogue colour video signal on a black and white television or monitor. They are commonly found on black and white recordings of television programmes originally made in colour. Chroma dots …   Wikipedia

  • Multiple sub-Nyquist sampling encoding — MUSE (Multiple sub Nyquist sampling encoding), was a dot interlaced digital video compression system that used analog modulation for transmission to deliver 1125 line high definition video signals to the home. Japan had the earliest working HDTV… …   Wikipedia

  • Color television — Title card for NBC, promoting their broadcast in RCA color . Color television is part of the history of television, the technology of television and practices associated with television s transmission of moving images in color video. In its most… …   Wikipedia

  • Filtro comb — Saltar a navegación, búsqueda Diagrama y análisis espectral de un filtro comb (IIR+FIR) aplicado a ruido blanco. En el procesamiento de señales, un filtro comb (o peine) se produce al sumarle a la señal original una versión retrasada en el tiempo …   Wikipedia Español

  • YPrPb — Соединительный кабель YPbPr, компонентный разъём  разъем для передачи аналогового видеосигнала. Название «компонентный» происходит от того, что компоненты видеосигнала передают по нескольким каналам. YPbPr  аналоговая версия YСbСr; эти… …   Википедия

  • Colour recovery — This article is about film colour recovery. For film colorization, see film colorization. For the Hewlett Packard technique, see HP Color recovery. Colour recovery (or colour restoration) is a process which can restore lost colour, specifically… …   Wikipedia

  • YPbPr — Соединительный кабель YPbPr, компонентное видео трёхмерное цветовое пространство, применяемое для кодирования аналогового цветного видеосигнала и передачи его по …   Википедия