Colorburst


Colorburst
Horizontal sync and color burst of PAL videosignal

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.

Contents

Explanation

In NTSC, 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.

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 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 15734.2657… Hz. This reduced the frame rate to 30/1.001 = 29.97000… Hz, and placed the color subcarrier at 227.5/286 = 455/572 = 35/44 of the 4.5 MHz audio subcarrier.[1]

Crystals

An NTSC or PAL television's color decoder contains a colorburst crystal oscillator. These operate at some multiple of the colorburst frequency, such as 4f = 157.5/11 MHz 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.

Non-television uses of NTSC color burst frequency
f = 315/88 = 3.579545… MHz
Component Frequency Relationship
Mattel Intellivision CPU 0.8949 MHz f/4
Apple II CPU (short cycles only, one in 65 cycles is longer) 1.0227 MHz 2f/7
Commodore VIC-20 CPU
Commodore 64 CPU
Atari 2600 CPU 1.1932 MHz f/3
Intel 8253 interval timer in IBM PC (remains in use today)
Fairchild Video Entertainment System CPU 1.7898 MHz f/2
Odyssey 2 CPU
Atari 8-bit family and Atari 7800 CPU
Commodore Plus/4 CPU
Nintendo Entertainment System CPU
TRS-80 Color Computer CPU 0.8949 MHz f/4
1.7898 MHz f/2
Commodore 128 CPU (FAST mode & CP/M mode) 2.0454 MHz 4f/7
Super Nintendo Entertainment System CPU 2.6847 MHz 3f/4
3.5795 MHz f
Sega Master System CPU 3.5795 MHz f
MSX CPU
Amateur radio Tx/Rx crystal for 80m band
ColecoVision CPU
CPU of IBM Personal Computer 5150 4.7727 MHz 4f/3
Commodore Amiga CPU 7.1591 MHz 2f
NEC TurboGrafx-16 CPU

See also



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