Composite video is the format of an
analog televisionAnalog television is the analog transmission that involves the broadcasting of encoded analog audio and analog video signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal...
(picture only) signal before it is combined with a sound signal and
modulatedIn electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
onto an
RFRadio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
carrierIn telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
. In contrast to
component videoComponent video is a video signal that has been split into two or more component channels. In popular use, it refers to a type of component analog video information that is transmitted or stored as three separate signals...
(
YPbPr' is a color space used in video electronics, in particular in reference to component video cables. is the analog version of the YCBCR color space; the two are numerically equivalent, but YPBPR is designed for use in analog systems whereas YCBCR is intended for digital video. cables are also...
) it contains all required video information, including colors in a single line-level signal. Like component video, composite-video cables do not carry audio and are often paired with audio cables (see
RCA connectorAn RCA connector, sometimes called a phono connector or cinch connector, is a type of electrical connector commonly used to carry audio and video signals...
).
Composite video is often designated by the
CVBS initialism, meaning "Composite Video, Blanking, and Sync."
It is usually in standard formats such as
NTSCNTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
,
PALPAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
, and
SECAMSECAM, also written SÉCAM , is an analog color television system first used in France....
.
Signal components
A composite video signal combines on one wire the video information required to recreate a color picture, as well as line and frame synchronization pulses. The color video signal is a linear combination of the
luminanceLuminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
of the picture, and a modulated subcarrier carries the
chrominanceChrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
or color information, a combination of
hueHue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...
and
saturationIn colorimetry and color theory, colorfulness, chroma, and saturation are related but distinct concepts referring to the perceived intensity of a specific color. Colorfulness is the degree of difference between a color and gray. Chroma is the colorfulness relative to the brightness of another color...
. Details of the encoding process vary between the NTSC, PAL and SECAM systems.
The frequency spectrum of the modulated color signal overlaps that of the baseband signal, and separation relies on the fact that frequency components of the baseband signal tend to be near harmonics of the horizontal scanning rate, while the color carrier is selected to be an odd multiple of half the horizontal scanning rate; this produces a modulated color signal that consists mainly of harmonic frequencies that fall between the harmonics in the baseband luma signal, rather than both being in separate continuous frequency bands alongside each other in the frequency domain. In other words, the combination of luma and chroma is indeed a frequency-division technique, but it is much more complex than typical frequency division multiplexing systems like the one used to multiplex analog radio stations on both the AM and FM bands.
ColorburstColorburst is a analog video, composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal...
is a composite
analog videoAnalog video is a video signal transferred by an analog signal. An analog color video signal contains luminance, brightness and chrominance of an analog television image...
signal generated by a video-signal generator used to
genlockGenlock is a common technique where the video output of one source, or a specific reference signal from a signal generator, is used to synchronize other television picture sources together. The aim in video and digital audio applications is to ensure the coincidence of signals in time at a...
, keep the
chrominanceChrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
subcarrierA subcarrier is a separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth...
synchronized in
television studioA television studio is an installation in which a video productions take place, either for the recording of live television to video tape, or for the acquisition of raw footage for post-production. The design of a studio is similar to, and derived from, movie studios, with a few amendments for the...
s for
color televisionColor 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....
.
Signal modulation
Composite video can easily be directed to any broadcast channel simply by modulating the proper RF carrier frequency with it. Most home
analog videoAnalog video is a video signal transferred by an analog signal. An analog color video signal contains luminance, brightness and chrominance of an analog television image...
equipment record a signal in (roughly) composite format:
LaserDiscLaserDisc was a home video format and the first commercial optical disc storage medium. Initially licensed, sold, and marketed as MCA DiscoVision in North America in 1978, the technology was previously referred to interally as Optical Videodisc System, Reflective Optical Videodisc, Laser Optical...
s store a true composite signal, while consumer videotape formats (including
VHSThe Video Home System is a consumer-level analog recording videocassette standard developed by Victor Company of Japan ....
and
BetamaxBetamax was a consumer-level analog videocassette magnetic tape recording format developed by Sony, released on May 10, 1975. The cassettes contain -wide videotape in a design similar to the earlier, professional wide, U-matic format...
) and lesser commercial and industrial tape formats (including
U-MaticU-matic is an analog recording videocassette format first shown by Sony in prototype in October 1969, and introduced to the market in September 1971. It was among the first video formats to contain the videotape inside a cassette, as opposed to the various Reel-to-Reel or open-reel formats of the...
) use modified composite signals (generally known as "color-under"). On playback, these devices often give the user the option to outputting the baseband signal or to modulating it onto a
VHFVery high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...
or
UHFUltra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
frequency compatible with a TV tuner (i.e. appearing on a selected TV channel). The professional television production uncompressed
digital videoDigital video is a type of digital recording system that works by using a digital rather than an analog video signal.The terms camera, video camera, and camcorder are used interchangeably in this article.- History :...
videocassette format known as
D-2 (video)D-2 is a professional digital recording videocassette format created by Ampex and other manufacturers through a standards group of the Society of Motion Picture and Television Engineers and introduced at the 1988 NAB convention as a lower-cost alternative to the D-1 format...
, directly
recordedIn digital recording, digital audio and digital video is directly recorded to a storage device as a stream of discrete numbers, representing the changes in air pressure for audio and chroma and luminance values for video through time, thus making an abstract template for the original sound or...
and reproduced standard
NTSCNTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
composite video signals, using PCM encoding of the
analog signalAn analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
on the
magnetic tapeMagnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
.
Standard connectors
In home applications, the composite video signal is typically connected using an RCA jack (phono plug), normally yellow. It is often accompanied with red and white connectors for right and left audio channels respectively.
BNC connectorThe BNC connector ' is a common type of RF connector used for coaxial cable. It is used with radio, television, and other radio-frequency electronic equipment, test instruments, video signals, and was once a popular computer network connector. BNC connectors are made to match the characteristic...
s and higher quality
coaxial cableCoaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
are often used in professional
television studioA television studio is an installation in which a video productions take place, either for the recording of live television to video tape, or for the acquisition of raw footage for post-production. The design of a studio is similar to, and derived from, movie studios, with a few amendments for the...
s and
post-productionPost-production is part of filmmaking and the video production process. It occurs in the making of motion pictures, television programs, radio programs, advertising, audio recordings, photography, and digital art...
applications.
BNC ConnectorThe BNC connector ' is a common type of RF connector used for coaxial cable. It is used with radio, television, and other radio-frequency electronic equipment, test instruments, video signals, and was once a popular computer network connector. BNC connectors are made to match the characteristic...
s were also used for composite video connections on early home VCRs, often accompanied by either phono connector(s) or a 5-pin
DIN connectorA DIN connector is a connector that was originally standardized by the , the German national standards organization. There are DIN standards for a large number of different connectors, therefore the term "DIN connector" alone does not unambiguously identify any particular type of connector unless...
for audio.
In Europe,
SCARTSCART is a French-originated standard and associated 21-pin connector for connecting audio-visual equipment together...
connections are often used instead of RCA jacks (and to a lesser extent,
S-VideoSeparate Video, more commonly known as S-Video and Y/C, is often referred to by JVC as both an S-VHS connector and as Super Video. It is an analog video transmission scheme, in which video information is encoded on two channels: luma and chroma...
), so where available, RGB is used instead of composite video with computers, video game consoles, and
DVDA DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
players.
Modulators
Some devices that connect to a TV, such as VCRs, older
video game consoleA video game console is an interactive entertainment computer or customized computer system that produces a video display signal which can be used with a display device to display a video game...
s and
home computerHome computers were a class of microcomputers entering the market in 1977, and becoming increasingly common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a single nontechnical user...
s of the 1980s, output a composite signal. This may then be converted to RF with an external box known as an
RF modulatorAn RF modulator is a device that takes a baseband input signal and outputs a radio frequency-modulated signal....
that generates the proper carrier (often for channel 3 or 4 in
North AmericaNorth America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, channel 36 in
EuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
). Sometimes this modulator was built into the product (such as video game consoles, VCRs, or the
AtariAtari is a corporate and brand name owned by several entities since its inception in 1972. It is currently owned by Atari Interactive, a wholly owned subsidiary of the French publisher Atari, SA . The original Atari, Inc. was founded in 1972 by Nolan Bushnell and Ted Dabney. It was a pioneer in...
,
Commodore 64The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...
, or
TRS-80TRS-80 was Tandy Corporation's desktop microcomputer model line, sold through Tandy's Radio Shack stores in the late 1970s and early 1980s. The first units, ordered unseen, were delivered in November 1977, and rolled out to the stores the third week of December. The line won popularity with...
CoCo home-computers) and sometimes it was an external unit powered by the computer (in the case of the TI-99 or some Apple modulators) or with an independent power supply. In the USA, using an external RF modulator frees the manufacturer from obtaining FCC approval for each variation of a device. Through the early-1980s, electronics that output a television channel signal were required to meet the same shielding requirements as broadcast television equipment, thus forcing manufacturers such as Apple to omit an RF modulator, and Texas Instruments to have their RF modulator as an external unit, which they had certified by the FCC without mentioning they were planning to sell it with a computer. In Europe, while most countries used the same broadcast standard, there were different modulation standards (PAL-G versus PAL-I, for example), and using an external modulator allowed manufacturers to make a single product and easily sell it to different countries by changing the modulator.
Demodulation loss
The process of modulating RF with the original video signal, and then demodulating the original signal again in the TV, introduces several losses. RF is also "noisy" because of all of the video and radio signals already being broadcast, so this conversion also typically adds noise or interference to the signal as well. For these reasons, it is typically best to use composite connections instead of RF connections if possible. Almost all modern video equipment has at least composite connectors, so this typically isn't a problem; however, older video equipment and some very low-end modern televisions have only RF input (essentially the antenna jack); while RF modulators are no longer common, they are still widely available to translate baseband signals for older equipment.
However, just as the modulation and demodulation of RF loses quality, the mixing of the various signals into the original composite signal does the same, causing a checkerboard video artifact known as
dot crawlDot 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...
. Dot crawl is an infamous defect that results from crosstalk due to the intermodulation of the chrominance and luminance components of the signal. This is usually seen when chrominance is transmitted with a high bandwidth, and its spectrum reaches into the band of the luminance frequencies. This has led to a proliferation of systems such as
S-VideoSeparate Video, more commonly known as S-Video and Y/C, is often referred to by JVC as both an S-VHS connector and as Super Video. It is an analog video transmission scheme, in which video information is encoded on two channels: luma and chroma...
and
component videoComponent video is a video signal that has been split into two or more component channels. In popular use, it refers to a type of component analog video information that is transmitted or stored as three separate signals...
to maintain the signals separately.
Comb filterIn signal processing, a comb filter adds a delayed version of a signal to itself, causing constructive and destructive interference. The frequency response of a comb filter consists of a series of regularly spaced spikes, giving the appearance of a comb....
s are also commonly used to separate signals, and eliminate artifacts, from composite sources.
Aspect ratio in composite signal
When used for connecting a video source to a video display that supports both 4:3 and 16:9 display formats, the PAL television standard provides for signalling pulses that will automatically switch the display from one format to the other. The Composite video connection supports this operation. However the NTSC television standard has no such provision, and thus the display must be manually switched.
Extensions to the composite video standard
Since TV screens hide the
vertical blanking intervalThe vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...
of a composite video signal and even
crop the edges of the pictureOverscan is extra image area around the four edges of a video image that may not be seen reliably by the viewer. It exists because television sets in the 1930s through 1970s were highly variable in how the video image was framed within the cathode ray tube .-Origins of overscan:Early televisions...
, extensions have been implemented by taking advantage of these unseen parts of the signal.
Examples of these extensions include
teletextTeletext is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules...
,
closed captioningClosed captioning is the process of displaying text on a television, video screen or other visual display to provide additional or interpretive information to individuals who wish to access it...
, digital information regarding the show title, transmitting a set of reference colors that allows TV sets to automatically correct the hue mal-adjustments common with the NTSC color encoding system, etc.
Other extensions to the standard include
S-videoSeparate Video, more commonly known as S-Video and Y/C, is often referred to by JVC as both an S-VHS connector and as Super Video. It is an analog video transmission scheme, in which video information is encoded on two channels: luma and chroma...
; S-video is an extension to the standard because it uses 2 parallel signals, one for
luminanceLuminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
and one for
chrominanceChrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
(color).
See also
- Composite monitor
A composite monitor is any analog video display that receives input in the form of an analog composite video signal through a single cable — in contrast to multiple-cable or multiple-wire video sources such as VGA cable...
- List of display interfaces
- NTSC color encoding
External links