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Composite video
From Encyclopedia PRO
Composite Video is the standard that can connect almost all consumer video equipment (television sets, DVD players, VCRs, camcorders) and contain the lowest fidelity audio and video. Normally it is transmitted over basic composite video cables with male RCA plugs on each end. Sometimes you will see these as dual RCA cables, which are used for stereo audio cables, and also a triple RCA cables for combined composite video and stereo audio cables.
Composite video is often designated by the CVBS acronym, meaning either "Color, Video, Blank and Sync", "Composite Video Baseband Signal", "Composite Video Burst Signal", or "Composite Video with Burst and Sync".
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Composite video format
Composite video is the format of an analog television (picture only) signal before it is combined with a sound signal and modulated onto an RF carrier. It is usually in a standard format such as NTSC, PAL, or SECAM. It is a composite of three source signals called Y, U and V (together referred to as YUV) with sync pulses. Y represents the brightness or luminance of the picture and includes synchronizing pulses, so that by itself it could be displayed as a monochrome picture. U and V between them carry the colour information. They are first mixed with two orthogonal phases of a colour carrier signal to form a signal called the chrominance. Y and UV are then added together. Since Y is a baseband signal and UV has been mixed with a carrier, this addition is equivalent to frequency-division multiplexing.
Broadcasting and reception
Composite video can easily be directed to any broadcast channel simply by modulating the proper RF carrier frequency with it. Most analogue home video equipment records a signal in (roughly) composite format: LaserDiscs store a true composite signal, while VHS tapes use a slightly modified composite signal. These devices then give the user the option of outputting the raw signal, or modulating it on to a VHF or UHF frequency to appear on a selected TV channel.
Modulation and demodulation
Some devices that connect to a TV, such as videogame consoles (and the ubiquitous home computers of the 1980s), naturally 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). The RF modulator is preferably left outside the console so the RF doesn't interfere with the components inside the machine. VCRs and similar devices already have to deal with RF signals in their tuners, so the modulator is located inside the box. Also, most early home computers usually employed an internal RF modulator.
The process of modulating RF with the original video signal, and then demodulating the original signal again in the TV, introduces several losses into the signal. 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 composite connectors, so this typically isn't a problem.
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. This has led to a proliferation of systems such as S-Video and component video to separate out one or more of the mixed signals.
Composite signal
Composite video combines the three basic elements of a video picture (color, brightness, and synchronization data) into a single combined "composite signal". While audio is carried on separate left and right channels, the brightness and color information of the video signal is placed in a single channel. A standard RCA connector or RF/coaxial connector is required to transmit composite video.
Composite cables
- Main article: Composite cables
In typical home applications, the composite video signal is typically connected using an RCA jack, normally yellow (often accompanied with red and white for right and left audio channels respectively). BNC connectors and higher quality co-axial cable are often used in more professional applications.
In Europe, SCART connections are often used instead of RCA jacks — though SCART can also carry far superior RGB component video signals (and to a lesser extent, S-Video), so where available, RGB is used instead of composite video with computers, video game consoles, and DVD players.