 On consumer products a yellow RCA connector is typically used for composite video. | |||
| Type | Analog video connector | ||
|---|---|---|---|
| Production history | |||
| Designed | 1954[1]–1956[2] | ||
| Superseded by | Digital: Serial digital interface & HDMI | ||
| General specifications | |||
| Length | Maximum of 50 m[citation needed] | ||
| External | Yes | ||
| Video signal | NTSC, PAL or SECAM video | ||
| Cable | Coaxial | ||
| Pins | 1 plus grounding shield | ||
| Connector | RCA connector & BNC connector | ||
| Electrical | |||
| Signal | 1 volt[3] | ||
| Pinout | |||
| Pin 1 | center | video | |
| Pin 2 | sheath | ground | |
Composite video is an baseband analog video format that typically carries a 525-line (29.97i) or 625-line (25i) interlaced video signal on a single channel, unlike the higher signal quality S-Video (two physically separated channels) and the even higher-quality component video (three physically separated channels) that also could carry analog/digital HD video signals.
A yellow RCA connector is typically used for composite video, with the audio being carried on separate additional L/R RCA connectors. In professional settings the BNC connector is used for video and XLR for audio. However on devices that are too small for an RCA connector, such as a digital camera, other types of connectors can be used such as 3.5mm TRRS breakout cables or various DIN connectors.
Composite video is also known by the initials CVBS for Composite Video Baseband Signal or Color, Video, Blanking and Sync,[4][5] or is simply referred to as Baseband or just SD Video for 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, and SECAM), but purely monochrome signals can also be used such as CCIR.
Signal components
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A composite video signal combines, on one signal 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 luminance ( ) of the picture and a modulated subcarrier which carries the chrominance or color information ( ), a combination of hue and saturation. Details of the combining 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. The signals may be separated using a comb filter.[6] In other words, the combination of luma and chrominance 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.
A gated and filtered signal derived from the color subcarrier, called the burst or colorburst, is added to the horizontal blanking interval of each line (excluding lines in the vertical sync interval) as a synchronizing signal and amplitude reference for the chrominance signals. In NTSC (3.58Mhz) composite video, the burst signal is inverted in phase (180° out of phase) from the reference subcarrier.[7] In PAL (4.43Mhz), the phase of the color subcarrier alternates on successive lines. In SECAM, no colorburst is used since phase information is irrelevant.
Composite artifacts
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The combining of component signals to form the composite signal does the same, causing a checkerboard video artifact known as dot 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 and component video avoid this problem as they maintain the component signals physically separate.
Recording
editMost home analog video equipment record a signal in (roughly) composite format: LaserDiscs & SMPTE 1" Type-C for example store a true composite signal modulated, while consumer videotape formats (including VHS and Betamax) and commercial and industrial tape formats (including U-matic) use modified composite signals FM encoded (generally known as color-under).[8] The professional D-2 videocassette format however started digitally storing a 4fsc (four times sub-carrier) sampled PCM encoding of the analog signal on the magnetic tape and losslessly reproduces composite video signals using, with the advent of affordable higher sampling speed analog to digital converters, real-time composite to YUV sampled digital sampling has been possible since the 1980s and raw waveform sampling and software decoding since the 2010s[9].
Extensions
editA number of so-called extensions to the visible TV image can be transmitted using composite video. Since TV screens hide the vertical blanking interval of a composite video signal, these take advantage of the unseen parts of the signal. Examples of extensions include teletext, 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 between 4:3 and 16:9 display formats, etc.
Connectors and cable
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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 quality coaxial cable are often used in professional television studios and post-production applications. BNC connectors were also used for composite video connections on early home VCRs, often accompanied by either RCA connector or a 5-pin DIN connector for audio. The BNC connector, in turn, post dated the PL-259 connector featured on first-generation VCRs.
Video cables are 75 ohm impedance, low in capacitance. Typical values run from 52 pF/m for an HDPE-foamed dielectric precision video cable to 69 pF/m for a solid PE dielectric cable.[11]
Digital Sampling
editDigital sampling in hardware and software uses the 4fsc sampling (four times sub-carrier) [12]
- NTSC 3.58MHz x 4 = 14.3MHz
- PAL 4.43Mhz x 4 = 17.2MHz
However the initial signal sampling or digitization in simple terms ranges from 28-54msps at 8/10-bits in older equipment and 12-bits in modern (2010's) systems.
This visually includes the full signal area alongside any vertical blanking interval data that may be present.
4fsc sampling: (Visually sampled non-square pixels)
- 1135x625 PAL
- 910x525 NTSC
Active image sampling: (In consumer use Composite SD signals are digitally stored only with the active image area.)
- 720x576i25 PAL
- 720x480i29.7 NTSC
- 720x488i29.7 NTSC (with line 21 visible for WSS/Closed Captions)
Commercial systems in broadcast used the full height D10 standard or more commonly called IMX sampling as it was introduced by Sony[13] this allowed for signals such as teletext to be preserved.
- 720x608 PAL
- 720x512 NTSC
In recent years direct sampling with high speed ADC's (28-54msps range) and software time base correction has allowed projects like the open-source CVBS-Decode[9] to create a D-2 like 4fsc file stream that preserves and allows full presentation and inspection of the full composite signal in the digital domain, this can then be comb-filtered or chroma-decoded to a color image on a standard computer, as this is a PCM sampled file like audio, via a DAC can played back to a standard television set or display with analog inputs.
Modern Usage
editConsumer:
editComposite is no longer the defacto universal "plug & play" standard it once was for consumers globallly, shortly after the digital era began phasing out analog CRT displays, consumer devices moved to using the hybrid able digital interfaces like Component and fullly digital standards such as HDMI, composite falling into a secondary and legacy support standard on most products in the late 2010's onwards.
Composite is still found quite widly on affordable video only monitors, optical media players, and via 3.5mm TRRS breakout cables on camcorders untill the later 2010s.
Broadcasting & Video Production:
editIn commercial and broadcast usage physical coaxial cable runs that were used for composite were not replaced, instead it was widly reperposued for new end point devices (cameras, monitors & recorders) the 1990s began a switchover to SDI based equipment this has been in effect since the 1990s in Europe and 2000s in North America[14] as of the 2020s SDI is fully adopted in the dedicated video camara market alongside HDMI.
Maintaing of Legacy Hardware:
editThere is 2 growing markets, retro nastalaga and media migration that sustain the requirements for modern hardware to have CVBS/S-Video/Compoent support.
There is a ever growing amount of composite to HDMI/Compoent converters using all in one IC's from companys like MacroSilicon & analog devices is still found on SBC devices such as the rasberrry pi, replacing and emulating hardware in devices such as arcade units with CRT displays.
CCTV Systems:
editModified versions of composite such as 960H (960x576) are still in wide use for the affordable end of CCTV systems today, alongside composite's usage in fpv drones & low resolution camera applications that still use analog transmission systems for longer range without relying on more complex and expensive digital hardware that may require extra licences for manufactorurers.
Modulators
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Some devices output live or de-modulated composite video, such as Videocassette recorders (VCR), video game consoles, and home computers. This may then be converted to FM RF with an RF modulator that generates the proper carrier (often for channel 3 or 4 in North America, channel 36 in Europe). Sometimes this modulator is built into the product (such as video game consoles, VCRs, or the Atari, Commodore 64, or TRS-80 CoCo home-computers), is an external unit powered by the computer (TI-99/4A), or with an independent power supply.[a]
Because of the digital television transition most television sets no longer have analog television tuners but DVB-T and ATSC digital ones they cannot accept a signal from an analog modulator. However, composite video has an established market for both devices that convert it to channel 3/4 outputs, as well as devices that convert standards like VGA to composite, therefore it has offered opportunities to repurpose older composite monitors for newer devices.
Demodulation loss
editThe 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 alongside digital interfaces such as HDMI.
See also
editNotes
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, 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.
References
edit- ^ "Definition: composite video". computer language. Retrieved 23 May 2019.
- ^ "the cable bible". Retrieved 23 May 2019.
- ^ "Understanding Analog Video Signals". Analog Devices.
- ^ "TUTORIAL 734 Video Basics". Maxim Integrated. Maxim Integrated. 2002. Archived from the original on 14 July 2018. Retrieved 14 July 2018.
- ^ 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. ISBN 978-1-61482-335-3.
{{cite book}}:|journal=ignored (help) - ^ "US Patent 4323915". US Patent and Trademark Office. Archived from the original on 20 December 2016. Retrieved 12 May 2014.
- ^ a b c Munday, Harry (2021). "CVBS-Decode - Software Defined Composite Video Decoder".
{{cite web}}: CS1 maint: url-status (link) - ^ "Elwro 800 Junior - MCbx". oldcomputer.info. Archived from the original on 18 March 2017. Retrieved 5 May 2018.
- ^ "LC-1 Audio Cable Design Notes". Blue Jeans Cable. Archived from the original on 28 November 2011. Retrieved 21 January 2012.
- ^ EBU Rec 601 BBC
- ^ Sony IMX Standards
- ^ Poynton, Charles. "Works of Charles Poynton".
{{cite web}}: CS1 maint: url-status (link)
External links
edit- Maxim - Apr 8, 2002 - Video Basics Tutorial covering CVBS format structure.
- Marshall Brain (26 November 2006). "Composite Video Signal". Retrieved 1 May 2020.