Composite video

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Composite video
Composite-video-cable.jpg
On consumer products a yellow RCA connector is typically used for composite video.
Type Analog video connector
Designed 1954 [1] –1956 [2]
External Yes
Video signal NTSC, PAL or SECAM video
Pins 1 plus shield
Connector RCA connector, 1/8 inch minijack plug, etc.
Pin 1 video

Composite video is an analog video signal format 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.

Contents

Composite video is also known by the initials CVBS for composite video baseband signal or color, video, blanking and sync, [3] [4] 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.

Signal components

Composite Video.svg

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 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. [5] 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 composite video, the burst signal is inverted in phase (180° out of phase) from the reference subcarrier. [6] In PAL, the phase of the color subcarrier alternates on successive lines. In SECAM, no colorburst is used since phase information is irrelevant.

Signal representation

Most home analog video equipment record a signal in (roughly) composite format: LaserDiscs store a true composite signal, while consumer videotape formats (including VHS and Betamax) and commercial and industrial tape formats (including U-Matic) use modified composite signals (generally known as color-under). [7] The professional D-2 videocassette format digitally records and reproduces composite video signals using PCM encoding of the analog signal on the magnetic tape.

Connectors and cable

Rear of the Polish Elwro 800 Junior computer. DIN output carries a composite video signal to an external monitor. Elwro 800 Junior jednostka centralna tyl.jpg
Rear of the Polish Elwro 800 Junior computer. DIN output carries a composite video signal to an external monitor.
Intergraph Intense3D Voodoo Rush with TV-out; S-video (topmost connector) and composite video (yellow RCA connector below) IntergraphVoodooRush.jpg
Intergraph Intense3D Voodoo Rush with TV-out; S-video (topmost connector) and composite video (yellow RCA connector below)

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. [9]

Modulators

Some devices that connect to a TV, such as VCRs, older video game consoles and home computers, output a composite signal. This may then be converted to RF with an external box known as an RF modulator that generates the proper carrier (often for channel 3 or 4 in North America, channel 36 in Europe). Sometimes this modulator was built into the product (such as video game consoles, VCRs, or the Atari, Commodore 64, or TRS-80 CoCo home-computers) and sometimes it was an external unit powered by the computer (in the case of the TI-99/4A or some Apple modulators) or with an independent power supply. [lower-alpha 1]

Modern-day devices with analog outputs have typically omitted the modulator option in favor of composite and S-video outputs (or have switched to using HDMI or other digital video formats) as composite and S-video have become more common as inputs for TVs. In addition, many TV sets sold these days no longer have analog television tuners and cannot accept channel 3/4 from a modulator. But because composite video has a well-established market for both devices that convert it to channel 3/4 outputs, as well as devices that convert things like VGA to composite, it has offered opportunities to repurpose older composite monitors for newer devices.

Demodulation loss

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. 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 available to translate composite signals for older equipment.

Just as the modulation and demodulation of RF degrades quality, 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 separately.

Extensions

Since TV screens hide the vertical blanking interval of a composite video signal and even crop the edges of the picture, extensions have been implemented by taking advantage of these unseen parts of the signal. Examples of these extensions include teletext, closed captioning, digital information regarding the show title, transmitting a set of reference colors that allows TV sets to automatically correct the hue maladjustments common with the NTSC color encoding system, etc.

When used for connecting a video source to a video display that supports both 4:3 and 16:9 display formats, the PAL and SECAM television standards provide for signaling pulses that will automatically switch the display from one format to the other. This is called widescreen signaling (WSS). This signaling is not included in the NTSC specification.

Other extensions to the standard include S-video; S-video is an extension to the standard because it uses parallel signal paths for luminance and for chrominance (color), both are compatible with a composite video input but produce either monochrome (luma), or uniform-luma color (chroma) when connected.

See also

Notes

  1. 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.

Related Research Articles

Analog television Television that uses analog signals

Analog television or analogue television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of the analog signal.

Chrominance signal used in video systems

Chrominance 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′ (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.

NTSC Analog color television system developed in the United States

NTSC, named after the National Television System Committee, is the analog television color system that was introduced in North America in 1954 and stayed in use until digital conversion. It was one of three major analog color television standards, the others being PAL and SECAM.

PAL Colour encoding system for analogue television

Phase Alternating Line (PAL) is a colour encoding system for analogue television used in broadcast television systems in most countries broadcasting at 625-line / 50 field per second (576i). It was one of three major analogue colour television standards, the others being NTSC and SECAM.

SECAM French analog color television system

SECAM, also written SÉCAM, is an analog color television system first used in France. It was one of three major color television standards, the others being PAL and NTSC.

Video Electronic moving image

Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode ray tube (CRT) systems which were later replaced by flat panel displays of several types.

Colorburst

Colorburst is an analog video, composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal. By synchronizing an oscillator with the colorburst at the back porch (beginning) of each scan line, a television receiver is able to restore the suppressed carrier of the chrominance (color) signals, and in turn decode the color information. The most common use of colorburst is to genlock equipment together as a common reference with a vision mixer in a television studio using a multi-camera setup.

S-Video signaling standard for SD video

S-Video is a signaling standard for standard definition video, typically 480i or 576i. By separating the black-and-white and coloring signals, it achieves better image quality than composite video, but has lower color resolution than component video.

S-VHS Improved version of VHS

S-VHS (スーパー・ヴィエイチエス), the common initialism for Super VHS, is an improved version of the VHS standard for consumer-level video recording. Victor Company of Japan introduced S-VHS in Japan in April 1987 with their JVC-branded HR-S7000 VCR, and in certain overseas markets soon afterward.

Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance.

A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broadcast. There is no physical difference between a carrier and a subcarrier; the "sub" implies that it has been derived from a carrier, which has been amplitude modulated by a steady signal and has a constant frequency relation to it.

Component video Video signal that has been split into component channels

Component 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 (CAV) information that is transmitted or stored as three separate signals. Component video can be contrasted with composite video in which all the video information is combined into a single line level signal that is used in analog television. Like composite, component-video cables do not carry audio and are often paired with audio cables.

In telecommunications, a pilot signal is a signal, usually a single frequency, transmitted over a communications system for supervisory, control, equalization, continuity, synchronization, or reference purposes.

Vectorscope

A vectorscope is a special type of oscilloscope used in both audio and video applications. Whereas an oscilloscope or waveform monitor normally displays a plot of signal vs. time, a vectorscope displays an X-Y plot of two signals, which can reveal details about the relationship between these two signals. Vectorscopes are highly similar in operation to oscilloscopes operated in X-Y mode; however those used in video applications have specialized graticules, and accept standard television or video signals as input.

576i Standard-definition video mode

576i is a standard-definition video mode originally used for terrestrial television in most countries of the world where the utility frequency for electric power distribution is 50 Hz. Because of its close association with the color encoding system, it is often referred to as simply PAL, PAL/SECAM or SECAM when compared to its 60 Hz NTSC-color-encoded counterpart, 480i. In digital applications it is usually referred to as "576i"; in analogue contexts it is often called "625 lines", and the aspect ratio is usually 4:3 in analogue transmission and 16:9 in digital transmission.

Dot crawl

Dot crawl is a visual defect of color analog video standards when signals are transmitted as composite video, as in terrestrial broadcast television. It consists of moving checkerboard patterns which appear along horizontal color transitions. It results from intermodulation or crosstalk between chrominance and luminance components of the signal, which are imperfectly multiplexed in the frequency domain.

B-MAC form of analog video encoding

B-MAC is a form of analog video encoding, specifically a type of Multiplexed Analogue Components (MAC) encoding. MAC encoding was designed in the mid 80s for use with Direct Broadcast Satellite systems. Other analog video encoding systems include NTSC, PAL and SECAM. Unlike the FDM method used in those, MAC encoding uses a TDM method. B-MAC was a proprietary MAC encoding used by Scientific-Atlanta for encrypting broadcast video services; the full name was "Multiple Analogue Component, Type B".

A white clipper is a circuit in professional video products that limits the maximum amplitude of the luminance part of the analogue video signal to 1 volt. It is essential for both analogue recording and transmission of video material.

CCIR System A was the 405-line analog broadcast television system broadcast in the UK and Ireland. CCIR service was discontinued in 1985.

CCIR System I is an analog broadcast television system. It was first used in the Republic of Ireland starting in 1962 as the 625-line broadcasting standard to be used on VHF Band I and Band III, sharing Band III with 405-line System A signals radiated in the north of the country. The UK started its own 625-line television service in 1964 also using System I, but on UHF only – the UK has never used VHF for 625-line television except for some cable relay distribution systems.

References

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  3. "Video Basics". Maxim Integrated . Retrieved 2018-07-14.
  4. Robert Silva (November 8, 2017). "Composite Video - The Basics". Lifewire . Retrieved 2018-07-14.
  5. Understanding Video Comb Filters (PDF), retrieved 2020-05-29
  6. SMPTE STANDARD for Television - Composite Analog Video Signal - NTSC for Studio Applications. 2004.
  7. "US Patent 4323915". US Patent and Trademark Office. Archived from the original on 20 December 2016. Retrieved 12 May 2014.
  8. "Elwro 800 Junior - MCbx". oldcomputer.info. Archived from the original on 18 March 2017. Retrieved 5 May 2018.
  9. "LC-1 Audio Cable Design Notes". Blue Jeans Cable. Archived from the original on 2011-11-28. Retrieved 2012-01-21.