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Colorburst is an analog video, composite video signal generated by a
video-signal generator A video signal generator is a type of signal generator which outputs predetermined video and/or television oscillation waveforms, and other signals used in the synchronization of television devices and to stimulate faults in, or aid in parametric m ...
used to keep the
chrominance subcarrier The chrominance subcarrier is a separate subcarrier signal that carries the color information during transmission of a composite video signal. It is modulated and synchronized using the colorburst signal and then attached to the back porch of the s ...
synchronized in a
color television Color television or Colour television is a television transmission technology that includes color information for the picture, so the video image can be displayed in color on the television set. It improves on the monochrome or black-and-white t ...
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 Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture (see YUV color model), separately from the accompanying luma signal (or Y' for short). Chrominance is usually represen ...
(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 A television studio, also called a television production studio, is an installation room in which video productions take place, either for the production of live television and its recording onto video tape or other media such as SSDs, or for ...
using a multi-camera setup.


Explanation

In
NTSC The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
, its frequency is exactly 315/88 = 3.579  MHz with a phase of 180°. PAL uses a frequency of exactly 4.43361875 MHz, with its phase alternating between 135° and 225° from line to line. Since the colorburst signal has a known amplitude, it is sometimes used as a reference level when compensating for amplitude variations in the overall signal.
SECAM SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''color sequential with memory''), is an analog color television system that was used in France, some parts of Europe and Africa, and Russia. It was one of th ...
is unique in not having a colorburst signal, since the chrominance signals are encoded using FM rather than QAM, thus the signal phase is immaterial and no reference point is needed.


Rationale for NTSC Color burst frequency

The original black and white NTSC television standard specified a frame rate of 30 Hz and 525 lines per frame, or 15750 lines per second. The audio was frequency modulated 4.5 MHz above the video signal. Because this was black and white, the video consisted only of luminance (brightness) information. Although all of the space in between was occupied, the line-based nature of the video information meant that the luminance data was not spread uniformly across the frequency domain; it was concentrated at multiples of the line rate. Plotting the video signal on a spectrogram gave a signature that looked like the teeth of a comb or a gear, rather than smooth and uniform. RCA discovered that if the
chrominance Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture (see YUV color model), separately from the accompanying luma signal (or Y' for short). Chrominance is usually represen ...
(color) information, which had a similar spectrum, was modulated on a carrier that was a half-integer multiple of the line rate, its signal peaks would fit neatly between the peaks of the luminance data and interference was minimized. It was not eliminated, but what remained was not readily apparent to human eyes. (Modern televisions attempt to reduce this interference further using a
comb filter In signal processing, a comb filter is a filter implemented by adding a delayed version of a signal to itself, causing constructive and destructive interference. The frequency response of a comb filter consists of a series of regularly space ...
.) To provide sufficient bandwidth for the chrominance signal, yet interfere only with the highest-frequency (and thus least perceptible) portions of the luminance signal, a chrominance subcarrier near 3.6 MHz was desirable. 227.5 = 455/2 times the line rate was close to the right number, and 455's small factors (5 × 7 × 13) make a divider easy to construct. However, additional interference could come from the audio signal. To minimize interference there, it was similarly desirable to make the distance between the chrominance carrier frequency and the audio carrier frequency a half-integer multiple of the line rate. The sum of these two half-integers implies that the distance between the frequency of the luminance carrier and audio carrier must be an integer multiple of the line rate. However, the original NTSC standard, with a 4.5 MHz carrier spacing and a 15750 Hz line rate, did not meet this requirement: the audio was 285.714 times the line rate. While existing black and white receivers could not decode a signal with a different audio carrier frequency, they could easily use the copious timing information in the video signal to decode a slightly slower line rate. Thus, the new color television standard reduced the line rate by a factor of 1.001 to 1/286 of the 4.5 MHz audio subcarrier frequency, or about 15734.2657 Hz. This reduced the frame rate to 30/1.001 ≈ 29.9700 Hz, and placed the color subcarrier at 227.5/286 = 455/572 = 35/44 of the 4.5 MHz audio subcarrier.


Crystals

An NTSC or PAL television's color decoder contains a colorburst crystal oscillator. Because so many analog color TVs were produced from the 1960s to the early 2000s,
economies of scale In microeconomics, economies of scale are the cost advantages that enterprises obtain due to their scale of operation, and are typically measured by the amount of output produced per unit of time. A decrease in cost per unit of output enables ...
drove down the cost of colorburst crystals, so they were often used in various other applications, such as oscillators for
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
s or for
amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency commu ...
: 3.5795 MHz has since become a common QRP calling frequency in the
80-meter band The 80-meter or 3.5 MHz band is a band of radio frequencies allocated for amateur radio use, from 3.5 to 4.0 MHz in IARU Region 2 (consisting mostly of North and South America), and generally 3.5 to 3.8 or 3.9 MHz in Regions 1 and 3 ( ...
, and its doubled frequency of 7.159 MHz is a common calling frequency in the 40-meter band. Tripling this frequency is also how
FM radio FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is cap ...
circuits came to use a nominally 10.7 MHz intermediate frequency in superheterodyne conversion.


See also

* Camera control unit * Color framing *
Color killer The color killer is an electronic stage in color TV receiver sets which acts as a cutting circuit to cut off color processing when the TV set receives a monochrome signal. Monochromatic transmission When a receiver is tuned to a monochrome tran ...
* Sync pulse * Glossary of video terms


References


Notes

{{Analogue TV transmitter topics Video signal Television terminology