Dot crawl

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 horizontal color transitions (vertical edges). 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, thereby producing dot crawl that appears 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 advent of three-line digital ("3D") 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 color television programs may exhibit dot crawl, and starting in 2008 it has been used to recover the original color information in a process called color recovery.

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

See also

Blanking level

In video technology, blanking level is the level of the composite video signal during the front and back porches of the video signal.

The composite video signal is actually the video information superimposed on blanking. The total level of the composite video signal (blanking + video) is 1000 mV. This level can also be given in IRE units such that the level difference reserved for video information is 100 IRE units. So white corresponds to 100 IRE units and blanking level corresponds to 0 IRE units. The level of black is 0 IRE units in the case of CCIR System B and CCIR System G (European systems) and 7.5 IRE units in the case of CCIR System M (American system). So, while there is no difference between the black and the blanking levels in most systems they differ by 50 mV. in system M. (When defined in terms of voltage difference, 7.5 IRE units is almost equal to 50 mV.)

CCIR System G

CCIR System G is an analog broadcast television system used in many countries. There are several systems in use and letter G is assigned for the European UHF system which is also used in the majority of Asian and African countries. (However some countries in Europe use different systems.)

CCIR System H

CCIR System H is an analog broadcast television system primarily used in Belgium, the Balkans and Malta on the UHF bands.

Chrominance

Chrominance (chroma or C for short) is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal (or Y for short). Chrominance is usually represented as two color-difference components: U = B′ − Y′ (blue − luma) and V = R′ − Y′ (red − luma). Each of these difference components may have scale factors and offsets applied to it, as specified by the applicable video standard.

In composite video signals, the U and V signals modulate a color subcarrier signal, and the result is referred to as the chrominance signal; the phase and amplitude of this modulated chrominance signal correspond approximately to the hue and saturation of the color. In digital-video and still-image color spaces such as Y′CbCr, the luma and chrominance components are digital sample values.

Separating RGB color signals into luma and chrominance allows the bandwidth of each to be determined separately. Typically, the chrominance bandwidth is reduced in analog composite video by reducing the bandwidth of a modulated color subcarrier, and in digital systems by chroma subsampling.

Composite video

Composite video is an analog video transmission that carries standard definition video typically at 480i or 576i resolution as a single channel. Video information is encoded on one channel, unlike the higher-quality S-video (two channels) and the even higher-quality component video (three or more channels). In all of these video formats, audio is carried on a separate connection.

Composite video is also known by the initials CVBS for composite video baseband signal or color, video, blanking and sync, or is simply referred to as SD video for the standard-definition television signal it conveys.

There are three dominant variants of composite video: NTSC, PAL, and SECAM.

Dipole field strength in free space

Dipole field strength in free space, in telecommunications, is the electric field strength caused by a half wave dipole under ideal conditions. The actual field strength in terrestrial environments is calculated by empirical formulas based on this field strength.

Distortionmeter

Distortionmeter (or more precisely distortion factor meter) is an electronic measuring instrument which displays the amount of distortion added to the original signal by an electronic circuit.

Field strength meter

In telecommunications, a field strength meter is an instrument that measures the electric field strength emanating from a transmitter.

Frequency offset

In radio engineering, a frequency offset is an intentional slight shift of broadcast radio frequency (RF), to reduce interference with other transmitters.

Hanover bars

Hanover bars, in one of the PAL television video formats, are an undesirable visual artifact in the reception of a television image. The name Hanover refers to the city (Hannover) in which the PAL system developer Telefunken Fernseh und Rundfunk GmbH was located.

Two signals, B-Y (U) and R-Y (V) are used in the PAL system to carry the color information for a picture, with the phase of the V signal (hue) reversed (i.e. shifted through 180 degrees) on alternate lines (hence the name PAL, or phase alternate line). This is done to cancel minor phase errors in the reception process. However, if gross errors occur, complementary errors from the V signal carry into the U signal, and thus visible stripes occur.

Later PAL systems introduced alterations to ensure that Hanover bars do not occur, introducing a "swinging burst" to the color synchronization. Other PAL systems may handle this problem differently.

PAL-S

PAL-S is the system of television receiver sets in the early days of PAL system. Here PAL stands for Phase alternating at line rate and S stands for simple.

Residual carrier

In analogue TV technology, residual carrier is the ratio of carrier level which is modulated by the maximum video signal to the unmodulated carrier level.

SECAM

SECAM, also written SÉCAM (French pronunciation: ​[sekam], Séquentiel couleur à mémoire, French for "Sequential colour with memory"), is an analogue color television system first used in France. It was one of three major colour television standards, the others being the European PAL and North American NTSC.

Development of SECAM began in 1956 by a team led by Henri de France working at Compagnie Française de Télévision (later bought by Thomson, now Technicolor). The first SECAM broadcast was made in France in 1967. The system was also selected as the standard for colour in the Soviet Union, who began broadcasts shortly after the French. The standard spread from these two countries to many client states and former colonies.

SECAM remained a major standard into the 2000s. It is in the process of being phased out and replaced by DVB, the new pan-European standard for digital television.

Skew (antenna)

Skew is a term used in antenna engineering. It is a technique to improve the horizontal radiation pattern of a high power transmitter station.

In a high power VHF or UHF station, usually the antenna system is constructed to broadcast to four directions each separated 90° from each other. So the directivity of the antenna system resembles a four leaf clover. While settlements within the main lobe receive enough energy, the energy received by the settlements between the main lobes may be 6 dB less.

One popular method to solve the problem is to skew the antenna panels symmetrically around the central axis of the mast. Usually a skew of λ/4 gives the desired almost-uniform horizontal radiation pattern. But in cases where more than one RF signal is applied to antenna system (via combiner), the improvement in the horizontal radiation pattern may be inadequate for some signals.

Split sound system

Split sound is an old system in analog television transmitters. It has long been superseded, but transmitters working on this principle are still in use. In this system there are two almost independent transmitters, one for sound (aural) and one for picture (visual). The system requires more energy input relative to broadcast energy than the alternative system known as intercarrier system.

Television lines

Television lines (TVL) is a specification of an analog camera's or monitors's horizontal resolution power. It is alternatively known as Lines of Horizontal Resolution (LoHR) or lines of resolution. The TVL is one of the most important resolution measures in a video system. The TVL can be measured with the standard EIA-1956 resolution chart.

Transmitter station

A transmitter station or transmission facility is an installation used for transmitting radio frequency signals for wireless communication, broadcasting, microwave link, mobile telephone or other purposes.

VIT signals

In television broadcasting, VIT signals (vertical interval test signals) are a group of test signals inserted in the composite video signal. These signals are used to weight the transmission characteristics of the system between the test generator and the output of the demodulator, where the system includes the microwave links, or TVROs as well as the TV transmitters and the transposers. There are both ATSC and EBU standards for VIT. (Because analogue television is being phased out globally, VIT standards are considered superseded.)

Zero reference pulse

Zero reference pulse or Zero pulse is an artificially produced pulse in a professional television receiver imitating no radio frequency case for modulation index measurements in analogue TV transmitters.

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