CVBS

Composite video, also known as CVBS (composite video baseband signal or color, video, blanking and sync), is an analog 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 for standard-definition television, and is sometimes referred to as ''SD video''.

The signal is typically carried on a yellow RCA connector, with separate connectors used for left and right audio channels. In professional equipment, a BNC connector is often used instead. Other connector types may appear in compact consumer devices like digital cameras.

Composite video supports several line resolutions, including 405-line, 525-line, and 625-line interlaced formats. It exists in three major regional variants based on analog color encoding standards: NTSC, PAL, and SECAM. The same format can also be used to transmit monochrome (black-and-white) video.

Signal components

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'' (Y) of the picture and a chrominance subcarrier which carries the color information (C), 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. 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. In PAL, the phase of the color subcarrier alternates on successive lines. In SECAM, no colorburst is used since phase information is irrelevant.

Composite artifacts

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

Most home analog video equipment record a signal in (roughly) composite format: LaserDiscs and type C videotape 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''). The professional D-2 videocassette format digitally stores a sampled analog composite video signal on magnetic 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.

Extensions

A 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

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.

Digital sampling and modern usage

The active image area of composite and S-Video signals are digitally stored at i25 PAL and i29.7 (or ) pixels. This does not represent the whole signal. Hardware typically samples at four times the color subcarrier frequency (4fsc) that includes the vertical blanking interval (VBI). Only commercial video capture devices used in broadcast output images with the extra VBI space. Direct sampling with high-speed ADCs and software time base correction has allowed projects like the open-source CVBS-Decode to create a D-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 via DAC 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. Analog CRT displays have been displaced by digital displays, and virtually all newer consumer video devices instead use HDMI. Despite this, modified versions of composite video, such as 960H (), remain in wide consumer use for CCTV systems and FPV drones.

Modulators

Some devices, such as videocassette recorders (VCRs), video game consoles, and home computers output composite video. 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.

Because of the digital 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 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

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

* List of video connectors
* NTSC color encoding
* PAL color encoding

Notes

References

External links

Maxim - Apr 8, 2002 - Video Basics
Tutorial covering CVBS format structure.
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{{Audio_and_video_interfaces_and connectors

Analog video connectors
Television technology
Repurposing