Colorburst
Encyclopedia
Colorburst is a 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.
, its frequency is exactly 315/88 MHz or approximately 3.579545 MHz with a phase of 180°, whereas PAL
uses a frequency of exactly 4.43361875 MHz, with its phase alternating between 135° and 225° from line to line. SECAM
is unique in not having a colorburst signal, since the chrominance signals are encoded using FM
rather than QAM.
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.
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
(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
.)
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 make a divider easy to construct.
However, additional interference could come from the audio signal
. To minimize interference there, it was similarly desirable for the distance between the chrominance carrier and the audio carrier to be a half-integer multiple of the line rate. The sum of these two half-integers implies that the distance between the luminance carrier and the 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 at 285.714× 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 included in the video signal to decode a slightly slower line rate. Thus, for color television, the line rate was reduced by a factor of 1.001 to 1/286 of the 4.5 MHz audio subcarrier frequency, or . This reduced the frame rate to 30/1.001 = , and placed the color subcarrier at 227.5/286 = 455/572 = 35/44 of the 4.5 MHz audio subcarrier.http://www.videointerchange.com/pal_secam_conversions.htm
.
These operate at some multiple of the colorburst frequency, such as 4f = 157.5/ or 14.318 MHz for NTSC.
Because color TVs were so common between the 1960s and 2000s, economies of scale
have driven down the cost of colorburst
crystals, which often find uses in various other applications.
Analog video
Analog video is a video signal transferred by an analog signal. An analog color video signal contains luminance, brightness and chrominance of an analog television image...
, composite video
Composite video
Composite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
signal generated by a video-signal generator used to keep the chrominance subcarrier
Chrominance subcarrier
Chrominace subcarrier refers to a separate subcarrier signal that carries the color information during transmission of a video signal. It is modulated and synchronized using the colorburst signal and then attached to the back porch of the color composite video signal...
synchronized in a color television
Color television
Color television is part of the history of television, the technology of television and practices associated with television's transmission of moving images in color video....
signal. By synchronizing an oscillator with the colorburst at the back porch (beginning) of each scan line
Scan line
A scan line or scanline is one line, or row, in a raster scanning pattern, such as a line of video on a cathode ray tube display of a television set or computer monitor....
, a television receiver is able to restore the suppressed carrier of the chrominance
Chrominance
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' and V = R' − Y'...
(color) signals, and in turn decode the color information. The most common use of colorburst is to genlock
Genlock
Genlock is a common technique where the video output of one source, or a specific reference signal from a signal generator, is used to synchronize other television picture sources together. The aim in video and digital audio applications is to ensure the coincidence of signals in time at a...
equipment together as a common reference with a vision mixer
Vision mixer
A vision mixer is a device used to select between several different video sources and in some cases Compositing video sources together to create special effects...
in a television studio
Television studio
A television studio is an installation in which a video productions take place, either for the recording of live television to video tape, or for the acquisition of raw footage for post-production. The design of a studio is similar to, and derived from, movie studios, with a few amendments for the...
using a multi-camera setup.
Explanation
In NTSCNTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
, its frequency is exactly 315/88 MHz or approximately 3.579545 MHz with a phase of 180°, whereas PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
uses a frequency of exactly 4.43361875 MHz, with its phase alternating between 135° and 225° from line to line. SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
is unique in not having a colorburst signal, since the chrominance signals are encoded using FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
rather than QAM.
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.
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 encoded 4.5 MHz above the video signal. Because this was black and white, the video consisted only ofluminance (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
Frequency domain
In electronics, control systems engineering, and statistics, frequency domain is a term used to describe the domain for analysis of mathematical functions or signals with respect to frequency, rather than time....
; it was concentrated at multiples of the line rate. Plotting the video signal on a spectrogram
Spectrogram
A spectrogram is a time-varying spectral representation that shows how the spectral density of a signal varies with time. Also known as spectral waterfalls, sonograms, voiceprints, or voicegrams, spectrograms are used to identify phonetic sounds, to analyse the cries of animals; they were also...
gave a signature that looked like the teeth of a comb or a gear, rather than smooth and uniform.
RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
discovered that if the chrominance
Chrominance
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' and V = R' − Y'...
(color) information, which had a similar spectrum, was modulated on a carrier that was a half-integer
Half-integer
In mathematics, a half-integer is a number of the formn + 1/2,where n is an integer. For example,are all half-integers. Note that a half of an integer is not always a half-integer: half of an even integer is an integer but not 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
Comb filter
In signal processing, a comb filter adds 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 spaced spikes, giving the appearance of a comb....
.)
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 make a divider easy to construct.
However, additional interference could come from the audio signal
Audio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
. To minimize interference there, it was similarly desirable for the distance between the chrominance carrier and the audio carrier to be a half-integer multiple of the line rate. The sum of these two half-integers implies that the distance between the luminance carrier and the 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 at 285.714× 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 included in the video signal to decode a slightly slower line rate. Thus, for color television, the line rate was reduced by a factor of 1.001 to 1/286 of the 4.5 MHz audio subcarrier frequency, or . This reduced the frame rate to 30/1.001 = , and placed the color subcarrier at 227.5/286 = 455/572 = 35/44 of the 4.5 MHz audio subcarrier.http://www.videointerchange.com/pal_secam_conversions.htm
Crystals
An NTSC or PAL television's color decoder contains a colorburst crystal oscillatorCrystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...
.
These operate at some multiple of the colorburst frequency, such as 4f = 157.5/ or 14.318 MHz for NTSC.
Because color TVs were so common between the 1960s and 2000s, economies of scale
Economies of scale
Economies of scale, in microeconomics, refers to the cost advantages that an enterprise obtains due to expansion. There are factors that cause a producer’s average cost per unit to fall as the scale of output is increased. "Economies of scale" is a long run concept and refers to reductions in unit...
have driven down the cost of colorburst
Colorburst
Colorburst is a analog video, composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal...
crystals, which often find uses in various other applications.
| Component | Frequency | Relationship |
|---|---|---|
| Mattel Intellivision Intellivision The Intellivision is a video game console released by Mattel in 1979. Development of the console began in 1978, less than a year after the introduction of its main competitor, the Atari 2600. The word intellivision is a portmanteau of "intelligent television"... CPU |
0.8949 MHz | f/4 |
| Apple II Apple II The Apple II is an 8-bit home computer, one of the first highly successful mass-produced microcomputer products, designed primarily by Steve Wozniak, manufactured by Apple Computer and introduced in 1977... CPU (short cycles only, one in 65 cycles is longer) |
1.0227 MHz | 2f/7 |
| Commodore VIC-20 Commodore VIC-20 The VIC-20 is an 8-bit home computer which was sold by Commodore Business Machines. The VIC-20 was announced in 1980, roughly three years after Commodore's first personal computer, the PET... CPU |
||
| Commodore 64 Commodore 64 The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595... CPU |
||
| Atari 2600 Atari 2600 The Atari 2600 is a video game console released in October 1977 by Atari, Inc. It is credited with popularizing the use of microprocessor-based hardware and cartridges containing game code, instead of having non-microprocessor dedicated hardware with all games built in... CPU |
1.1932 MHz | f/3 |
| Intel 8253 Intel 8253 The Intel 8253 and 8254 are Programmable Interval Timers , which perform timing and counting functions. They were primarily designed for the Intel 8080/8085-processors, but later used in x86-systems... interval timer in IBM PC (remains in use ) |
||
| Fairchild Video Entertainment System Fairchild Channel F The Fairchild Channel F is a game console released by Fairchild Semiconductor in August 1976 at the retail price of $169.95. It has the distinction of being the first programmable ROM cartridge-based video game console... CPU |
1.7898 MHz | f/2 |
| Odyssey 2 CPU | ||
| Atari 8-bit family Atari 8-bit family The Atari 8-bit family is a series of 8-bit home computers manufactured from 1979 to 1992. All are based on the MOS Technology 6502 CPU and were the first home computers designed with custom coprocessor chips... and Atari 7800 Atari 7800 The Atari 7800 ProSystem, or simply the Atari 7800, is a video game console re-released by Atari Corporation in January 1986. The original release had occurred two years earlier under Atari Inc. The 7800 had originally been designed to replace Atari Inc.'s Atari 5200 in 1984, but was temporarily... CPU |
||
| Commodore Plus/4 Commodore Plus/4 The Commodore Plus/4 was a home computer released by Commodore International in 1984. The "Plus/4" name refers to the four-application ROM resident office suite ; it was billed as "the productivity computer with software built-in"... CPU |
||
| Nintendo Entertainment System Nintendo Entertainment System The Nintendo Entertainment System is an 8-bit video game console that was released by Nintendo in North America during 1985, in Europe during 1986 and Australia in 1987... CPU |
||
| TRS-80 Color Computer TRS-80 Color Computer The Radio Shack TRS-80 Color Computer was a home computer launched in 1980. It was one of the earliest of the first generation of computers marketed for home use in English-speaking markets... CPU |
0.8949 MHz | f/4 |
| 1.7898 MHz | f/2 | |
| Commodore 128 Commodore 128 The Commodore 128 home/personal computer was the last 8-bit machine commercially released by Commodore Business Machines... CPU (FAST mode & CP/M mode) |
2.0454 MHz | 4f/7 |
| Super Nintendo Entertainment System Super Nintendo Entertainment System The Super Nintendo Entertainment System is a 16-bit video game console that was released by Nintendo in North America, Europe, Australasia , and South America between 1990 and 1993. In Japan and Southeast Asia, the system is called the , or SFC for short... CPU |
2.6847 MHz | 3f/4 |
| 3.5795 MHz | f | |
| Sega Master System Sega Master System The is a third-generation video game console that was manufactured and released by Sega in 1985 in Japan , 1986 in North America and 1987 in Europe.... CPU |
3.5795 MHz | f |
| MSX MSX MSX was the name of a standardized home computer architecture in the 1980s conceived by Kazuhiko Nishi, then Vice-president at Microsoft Japan and Director at ASCII Corporation... CPU |
||
| Amateur radio Amateur radio Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication... Tx/Rx crystal for 80m band |
||
| ColecoVision ColecoVision The ColecoVision is Coleco Industries' second generation home video game console which was released in August 1982. The ColecoVision offered arcade-quality graphics and gaming style, and the means to expand the system's basic hardware... CPU |
||
| CPU of IBM Personal Computer 5150 | 4.7727 MHz | 4f/3 |
| Commodore Amiga Amiga The Amiga is a family of personal computers that was sold by Commodore in the 1980s and 1990s. The first model was launched in 1985 as a high-end home computer and became popular for its graphical, audio and multi-tasking abilities... CPU |
7.1591 MHz | 2f |
| NEC TurboGrafx-16 TurboGrafx-16 TurboGrafx-16, fully titled as TurboGrafx-16 Entertainment SuperSystem and known in Japan as the , is a video game console developed by Hudson Soft and NEC, released in Japan on October 30, 1987, and in North America on August 29, 1989.... CPU |
See also
- Camera control unitCamera control unit- Description :The Camera Control Unit is typically part of a live television broadcast "chain". It is responsible for powering the professional video camera, handling signals sent over the camera cable to and from the camera, and can be used control various camera parameters such as iris...
- Color framingColor framingIn video engineering, color framing refers to the color frame sequence of fields in a composite video signal through which the video frame timing and chrominance subcarrier signal timing—in particular, that of the color burst -- cycle through all possible phase relationships.The exact nature of the...
- Color killerColor killerThe color killer is an electronic stage in color TV receiver sets which acts as a muting circuit to cut off the color amplifiers when the TV receives a monochrome signal.- Monochromatic transmission :...
- Sync pulse