Composite video

Source: Wikipedia, the free encyclopedia.
Not to be confused with Component video.

Composite Video Baseband Signal (CVBS)
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Superseded by

Analog: S-Video & Component

Digital: SDI & 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

interlaced black and white or color signal, on a single channel, unlike the higher-quality S-Video (two channels) and the even higher-quality YPbPr
(three channels).

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, 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 acronym CVBS, for composite video baseband signal or color, video, blanking and sync,[4][5] or is simply referred to as 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.

Signal components

Composite video signal graphic
DSO
.

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.[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

vertical sync interval) as a synchronizing signal and amplitude reference for the chrominance signals. In NTSC composite video, the 3.58 MHz burst signal is inverted in phase (180° out of phase) from the reference subcarrier.[7]
In PAL, the phase of the 4.43 MHz color subcarrier alternates on successive lines. In SECAM, no colorburst is used since phase information is irrelevant.

Composite artifacts

Enlarged detail from a video source exhibiting dot crawl. Note the distinctive checkerboard pattern on the vertical edges between yellow and blue areas.

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

PCM encoding of the analog signal on the 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.[9]

Extensions

A number of so-called extensions to the visible TV image can be transmitted using composite video. Since TV screens hide the

16:9
display formats, etc.

Connectors and cable

Rear of the Polish Elwro 800 Junior computer. DIN output carries a composite video signal to an external monitor.[10]
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.

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 and modern usage

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 the

open-source CVBS-Decode[9] to create a D-2 like 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 via DAC
played back to a TV.

Composite is no longer the only universal standard it once was for consumers after the digital era began phasing out analog

CCTV systems today in consumer use alongside fpv drones.[citation needed
]

Modulators

Some devices, such as

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

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

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.

References

  1. ^ "Definition: composite video". computer language. Retrieved 23 May 2019.
  2. ^ "the cable bible". Retrieved 23 May 2019.
  3. ^ "Understanding composite video". ClearView Communications Ltd. 21 May 2023.
  4. ^ "TUTORIAL 734 Video Basics". Maxim Integrated. Maxim Integrated. 2002. Archived from the original on 14 July 2018. Retrieved 14 July 2018.
  5. ^ Silva, Robert (11 September 2020). "Composite Video Connections Explained - Many home theater devices still support composite video inputs". Lifewire Tech for Humans. Lifewire.
  6. ^ "Understanding Video Comb Filters" (PDF). Sencore Tech Tips. No. 201.
  7. ISBN 978-1-61482-335-3. Archived from the original on 8 June 2018. {{cite book}}: |journal= ignored (help
    )
  8. ^ "US Patent 4323915". US Patent and Trademark Office. Archived from the original on 20 December 2016. Retrieved 12 May 2014.
  9. ^ a b Munday, Harry (2021). "CVBS-Decode - Software Defined Composite Video Decoder". GitHub.
  10. ^ "Elwro 800 Junior - MCbx". oldcomputer.info. Archived from the original on 18 March 2017. Retrieved 5 May 2018.
  11. ^ "LC-1 Audio Cable Design Notes". Blue Jeans Cable. Archived from the original on 28 November 2011. Retrieved 21 January 2012.
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