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.
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.
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 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 (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.
Because the color TV 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.
+ Non-television uses of NTSC color burst frequency f = 315/88 = | ||
Component | Frequency | Relationship |
Mattel Intellivision CPU | 0.8949 MHz | |
Apple II CPU (short cycles only, one in 65 cycles is longer) | rowspan=3|rowspan=3>2f/7 | |
Commodore VIC-20 CPU | ||
Commodore 64 CPU | ||
Atari 2600 CPU | rowspan=2|rowspan=2>f/3 | |
Intel 8253 interval timer in IBM PC (remains in use ) | ||
rowspan=5 | |rowspan=5>f/2 | |
Odyssey 2 CPU | ||
Atari 8-bit family and Atari 7800 CPU | ||
Commodore Plus/4 CPU | ||
Nintendo Entertainment System CPU | ||
0.8949 MHz | ||
1.7898 MHz | f/2 | |
Commodore 128 CPU (FAST mode & CP/M mode) | 2.0454 MHz | |
rowspan=2 | Super Nintendo Entertainment System CPU | 2.6847 MHz |
3.5795 MHz | f | |
Sega Master System CPU | rowspan=4|rowspan=4>f | |
MSX CPU | ||
Amateur radio Tx/Rx crystal for 80m band | ||
ColecoVision CPU | ||
CPU of IBM Personal Computer 5150 | 4.7727 MHz | |
Commodore Amiga CPU | rowspan=2|rowspan=2>2f | |
NEC TurboGrafx-16 CPU |
Category:Video signal Category:Television terminology
de:Burst-Signal ko:컬러버스트This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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