Composite video
 On consumer products a yellowRCA connectoris typically used for composite video. | |||
| Type | Analog video connector | ||
|---|---|---|---|
| Production history | |||
| Designed | 1954[1]–1956[2] | ||
| General specifications | |||
| Length | Maximum of50 m[citation needed] | ||
| External | Yes | ||
| Video signal | NTSC, PAL or SECAM video | ||
| Pins | 1 plus grounding shield | ||
| Connector | RCA connector | ||
| Electrical | |||
| Signal | 1 volt[3] | ||
| Pinout | |||
| Pin 1 | center | video | |
| Pin 2 | sheath | ground | |
Composite videois anbaseband analog videoformat that typically carries a405,525or625 lineinterlacedblack and white or color signal, on a single channel, unlike the higher-qualityS-Video(two channels) and the even higher-qualitycomponent video(three or more channels).
A yellowRCA connectoris typically used for composite video, with the audio being carried on separate additional L/R RCA connectors. In professional settings, or on devices that are too small for an RCA connector, such as a digital camera, other types of connectors can be used.
Composite video is also known by the initialsCVBSforComposite Video Baseband SignalorColor, Video, Blanking and Sync,[4][5]or is simply referred to asSD videofor the standard-definition television signal it conveys.
There are three dominant variants of composite video signals, corresponding to the analog color system used (NTSC,PAL,andSECAM), but purely monochrome signals can also be used.
Signal components
[edit]
A composite video signal combines, on one wire, the video information required to recreate a color picture, as well as line andframe synchronizationpulses. The color video signal is a linear combination of theluminance(Y) of the picture and achrominance subcarrierwhich carries the color information (C), a combination ofhueandsaturation.Details of the combining process vary between the NTSC, PAL and SECAM systems.
Thefrequency spectrumof 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 nearharmonicsof 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 basebandlumasignal, rather than both being in separate continuous frequency bands alongside each other in the frequency domain. The signals may be separated using acomb filter.[6]In other words, the combination of luma and chrominance is indeed a frequency-division technique, but it is much more complex than typicalfrequency-division multiplexingsystems 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 intervalof 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 MHzburst signal is inverted in phase (180° out of phase) from the reference subcarrier.[7]In PAL, the phase of the4.43 MHzcolor subcarrier alternates on successive lines. In SECAM, no colorburst is used since phase information is irrelevant.
Composite artifacts
[edit]
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 filtersare commonly used to separate signals and eliminate these artifacts from composite sources.S-Videoandcomponent videoavoid this problem as they maintain the component signals physically separate.
Recording
[edit]Most homeanalog videoequipment record a signal in (roughly) composite format:LaserDiscsandtype C videotapefor example store a true composite signal modulated, while consumer videotape formats (includingVHSandBetamax) and commercial and industrial tape formats (includingU-matic) use modified composite signalsFM encoded(generally known ascolor-under).[8]The professionalD-2videocassette formatdigitally storinga sampled signal and losslessly reproduces composite video signals usingPCMencoding of theanalog signalon themagnetic 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.[9]
Extensions
[edit]A number of so-calledextensionsto the visible TV image can be transmitted using composite video. Since TV screens hide thevertical blanking intervalof 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 between4:3and16:9display formats, etc.
Connectors and cable
[edit]
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 connectorsand higher qualitycoaxial cableare often used in professionaltelevision studiosandpost-productionapplications. BNC connectors were also used for composite video connections on early homeVCRs,often accompanied by either RCA connector or a 5-pinDIN connectorfor audio. The BNC connector, in turn, post dated thePL-259connector 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.[11]
Digital sampling and modern usage
[edit]The active image area of composite and s-video signals are digitally stored at 720x576i25 PAL and 720x480i29.7 (or 720x488) 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-speedADCsand softwaretime base correctionhas allowed projects like theopen-sourceCVBS-Decode[9]to create aD-2like 4fsc stream that preserves and allows full presentation and inspection of the entire composite signal. This can then be comb-filtered or chroma-decoded to a color image on a standard computer or viaDACplayed back to a TV.
Composite is no longer the universal standard it once was for consumers after the digital era began phasing out analogCRT displaysand virtually all consumer devices moved to usingHDMI.Modified versions of composite such as 960H (960x576) are still in wide use forCCTVsystems today in consumer use alongsidefpv drones.[citation needed]
Modulators
[edit] |
Some devices, such asvideocassette recorders(VCRs),video game consoles,andhome computersoutput composite video. This may then be converted to FM RF with anRF modulatorthat 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 CoCohome-computers), is an external unit powered by the computer (TI-99/4A), or with an independent power supply.[a]
Because of thedigital television transitionmost 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 likeVGAto composite, therefore it has offered opportunities torepurposeoldercomposite monitorsfor newer devices.
Demodulation loss
[edit]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.
See also
[edit]Notes
[edit]- ^In the United States, 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, andTexas Instrumentsto 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.
References
[edit]- ^"Definition: composite video".computer language.Retrieved23 May2019.
- ^"the cable bible".Retrieved23 May2019.
- ^"Understanding Analog Video Signals".Analog Devices.
- ^"TUTORIAL 734 Video Basics".Maxim Integrated.Maxim Integrated.2002. Archived fromthe originalon 14 July 2018.Retrieved14 July2018.
- ^Silva, Robert (11 September 2020)."Composite Video Connections Explained - Many home theater devices still support composite video inputs".Lifewire Tech for Humans.Lifewire.
- ^"Understanding Video Comb Filters"(PDF).Sencore Tech Tips.No. 201.
- ^SMPTE STANDARD for Television – Composite Analog Video Signal – NTSC for Studio Applications.2004. pp. 1–21.doi:10.5594/SMPTE.ST170.2004.ISBN978-1-61482-335-3.Archived fromthe originalon 8 June 2018.
{{cite book}}:|journal=ignored (help) - ^"US Patent 4323915".US Patent and Trademark Office.Archivedfrom the original on 20 December 2016.Retrieved12 May2014.
- ^abMunday, Harry (2021)."CVBS-Decode - Software Defined Composite Video Decoder".GitHub.
- ^"Elwro 800 Junior - MCbx".oldcomputer.info.Archivedfrom the original on 18 March 2017.Retrieved5 May2018.
- ^"LC-1 Audio Cable Design Notes".Blue Jeans Cable.Archivedfrom the original on 28 November 2011.Retrieved21 January2012.
External links
[edit]- Maxim - Apr 8, 2002 - Video BasicsTutorial covering CVBS format structure.
- Marshall Brain (26 November 2006)."Composite Video Signal".Retrieved1 May2020.