Colorburst

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In composite video, colorburst is a signal used to keep the chrominance subcarrier synchronized in a color television signal. By synchronizing an oscillator with the colorburst at the beginning of each scan line, a television receiver is able to restore the suppressed carrier of the chrominance signals, and in turn decode the color information.

[edit] Explanation

In NTSC, its frequency is 39375/11 kHz or 3579545 Hz with a phase of 180°, whereas PAL uses a frequency of 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, making synchronization irrelevant.

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.

[edit] 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 are so common, economies of scale have driven down the cost of colorburst crystals, which often find uses in various other applications:

Applications of NTSC subcarrier-frequency crystals, with f = 39375000/11 Hz, approx. 3.580 MHz, have included:

Component	Frequency	Relationship
Intel 8253 interval timer in IBM PC	1.19 MHz	f/3
Atari 2600 CPU	1.19 MHz	f/3
Nintendo Entertainment System CPU	1.79 MHz	f/2
Sega Master System CPU	3.58 MHz	f
Super Nintendo Entertainment System CPU	3.58 MHz	f
IBM Personal Computer CPU	4.77 MHz	4f/3
Commodore Amiga CPU	7.16 MHz	2f