 On consumer products a yellowRCA connector is typically used for composite video. | |||
| Type | Analog video connector | ||
|---|---|---|---|
| Production history | |||
| Designed | 1954–1956[1][2] | ||
| Superseded by | Analog:S-Video &Component &SCART Digital:SDI &HDMI | ||
| General specifications | |||
| Length | Max. 50 m (160 ft)[citation needed] | ||
| External | Yes | ||
| Video signal | NTSC, PAL or SECAM video | ||
| Cable | Coaxial | ||
| Pins | 1 plus grounding shield | ||
| Connector | RCA,BNC, others | ||
| Electrical | |||
| Signal | 1 volt[3] | ||
| Pinout | |||
| Pin 1 | center | video | |
| Pin 2 | sheath | ground | |
Composite video, also known asCVBS (composite video baseband signal orcolor, video, blanking and sync),[4][5] is ananalog video format that combines image information—such as brightness (luminance), color (chrominance), and synchronization, into a single signal transmitted over one channel. It is most commonly used forstandard-definition television, and is sometimes referred to asSD video.
The signal is typically carried on a yellowRCA connector, with separate connectors used for left and right audio channels. In professional equipment, aBNC connector is often used instead. Other connector types may appear in compact consumer devices like digital cameras.
Composite video supports several line resolutions, including405-line,525-line, and625-lineinterlaced formats. It exists in three major regional variants based on analog color encoding standards:NTSC,PAL, andSECAM. The same format can also be used to transmit monochrome (black-and-white) video.
A composite video signal combines, on one wire, the video information required to recreate a color picture, as well as line andframe synchronization pulses. The color video signal is a linear combination of theluminance (Y) of the picture and achrominance subcarrier which carries the color information (C), a combination ofhue andsaturation. Details of the combining process vary between the NTSC, PAL and SECAM systems.[6]
Thefrequency 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 nearharmonics 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 basebandluma signal, rather than both being in separate continuous frequency bands alongside each other in the frequency domain. The signals may be separated using acomb filter.[7] In other words, the combination of luma and chrominance is indeed a frequency-division technique, but it is much more complex than typicalfrequency-division multiplexing systems like the one used to multiplex analog radio stations on both the AM and FM bands.
A gated and filtered signal derived from the colorsubcarrier, called the burst orcolorburst, is added to thehorizontal blanking interval of each line (excluding lines in thevertical sync interval) as a synchronizing signal and amplitude reference for the chrominance signals. In NTSC composite video, the3.58 MHz burst signal is inverted in phase (180° out of phase) from the reference subcarrier.[8] In PAL, the phase of the4.43 MHz color subcarrier alternates on successive lines. In SECAM, no colorburst is used since phase information is irrelevant.
The combining of component signals to form the composite signal does the same, causing a checkerboard video artifact known asdot crawl. Dot crawl is a 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 high bandwidth, and its spectrum reaches into the band of the luminance frequencies.Comb filters are commonly used to separate signals and eliminate these artifacts from composite sources.S-Video andcomponent video avoid this problem as they maintain the component signals physically separate.
Most homeanalog video equipment record a signal in (roughly) composite format:LaserDiscs andtype C videotape for example store a true composite signal modulated, while consumer videotape formats (includingVHS andBetamax) and commercial and industrial tape formats (includingU-matic) use modified composite signalsFM encoded (generally known ascolor-under).[9] The professionalD-2 videocassette formatdigitally stores a sampledanalog composite video signal onmagnetic tape. With the advent of affordable higher sampling speed analog to digital converters, realtime composite to YUV sampled digital sampling has been possible since the 1980s and raw waveform sampling and software decoding since the 2010s.[10]
A number of so-calledextensions to the visible TV image can be transmitted using composite video. Since TV screens hide thevertical blanking interval of a composite video signal, these take advantage of the unseen parts of the signal. Examples of extensions includeteletext,closed captioning, information regarding the show title, a set of reference colors that allows TV sets to automatically correct NTSC hue maladjustments,widescreen signaling (WSS) for switching between4:3 and16:9 display formats, etc.
In home applications, the composite video signal is typically connected using an RCA connector, normally yellow. It is often accompanied with red and white connectors for right and left audio channels respectively.BNC connectors and higher qualitycoaxial cable are often used in professionaltelevision studios andpost-production applications. BNC connectors were also used for composite video connections on early homeVCRs, often accompanied by either RCA connector or a 5-pinDIN connector for audio. The BNC connector, in turn, post dated thePL-259 connector featured on first-generation VCRs.
Video cables are 75 ohm impedance, low in capacitance. Typical values run from 52 pF/m for anHDPE-foamed dielectric precision video cable to 69 pF/m for a solid PE dielectric cable.[12]
The active image area of composite and S-Video signals are digitally stored at720 × 576 i25 PAL and720 × 480 i29.7 (or720 × 488) pixels. This does not represent the whole signal. Hardware typically samples at four times the color subcarrier frequency (4fsc) that includes thevertical blanking interval (VBI). Only commercial video capture devices used in broadcast output images with the extra VBI space. Direct sampling with high-speedADCs and softwaretime base correction has allowed projects like theopen-source CVBS-Decode[10] to create aD-2 like a 4fsc stream that preserves and allows full presentation and inspection of the entire composite signal. This can then be chroma-decoded to a color image on a standard computer or viaDAC played back to a TV.
Due to the development of digital video technologies, composite video is no longer a universal feature on consumer video products. AnalogCRT displays have been displaced by digital displays, and virtually all newer consumer video devices instead useHDMI. Despite this, modified versions of composite video, such as 960H (960 × 576), remain in wide consumer use forCCTV systems andFPV drones.[citation needed]
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Some devices, such asvideocassette recorders (VCRs),video game consoles, andhome computers output composite video. This may then be converted to FM RF with anRF modulator that generates the proper carrier (often for channel 3 or 4 inNorth America, channel 36 inEurope). Sometimes this modulator is built into the product (such as video game consoles, VCRs, or theAtari,Commodore 64, orTRS-80 CoCo home-computers), is an external unit powered by the computer (TI-99/4A), or with an independent power supply.[a]
Because of thedigital television transition most television sets no longer have analog television tuners but DVB-T and ATSC digital ones. They therefore cannot accept a signal from an analog modulator. However, composite video has an established market for both devices that convert it tochannel 3/4 outputs, as well as devices that convert standards likeVGA to composite, therefore it has offered opportunities torepurpose oldercomposite monitors for newer devices.
The process of modulating RF with the original video signal, and then demodulating the original signal again in the TV, introduces losses including added noise or interference. For these reasons, it is best to use composite connections instead of RF connections if possible for live signals and sample the source FM RF signal for recorded formats. Some video equipment and modern televisions have only RF input.