Colorburst
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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.
In NTSC, its frequency is 39375/11 kHz or 3579545 Hz with a phase of 180°, whereas PAL uses a frequency of 4.19 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.
Because color televisions are so common, so are colorburst crystals, and they are used in various other applications. For instance, the time-of-day clock in a PC often runs at four times the NTSC colorburst frequency.