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 3.579545 MHz 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.
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 analog 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:
Applications of NTSC subcarrier-frequency crystals, with f = 39375000/11 Hz, approx. 3.580 MHz, have included:
| Component | Frequency | Relationship |
|---|---|---|
| Mattel Intellivision CPU | 0.895 MHz | f/4 |
| Apple II CPU | 1.02 MHz | 2f/7 |
| Commodore VIC-20 CPU | 1.02 MHz | 2f/7 |
| Commodore 64 CPU | 1.02 MHz | 2f/7 |
| Atari 2600 CPU | 1.19 MHz | f/3 |
| Intel 8253 interval timer in IBM PC (remains in use today) | 1.19 MHz | f/3 |
| Fairchild Video Entertainment System CPU | 1.79 MHz | f/2 |
| Odyssey 2 CPU | 1.79 MHz | f/2 |
| Atari 8-bit family CPU | 1.79 MHz | f/2 |
| Nintendo Entertainment System CPU | 1.79 MHz | f/2 |
| TRS-80 Color Computer CPU | 0.895 MHz/1.79 MHz | f/4 or f/2 |
| Sega Master System CPU | 3.58 MHz | f |
| Super Nintendo Entertainment System CPU | 3.58 MHz | f |
| Amateur radio Tx/Rx crystal for 80m band | 3.58 MHz | f |
| ColecoVision CPU | 3.58 MHz | f |
| CPU of IBM Personal Computer 5150 | 4.77 MHz | 4f/3 |
| Commodore Amiga CPU | 7.16 MHz | 2f |
| NEC TurboGrafx-16 CPU | 7.16 MHz | 2f |
[edit] See also
- Color framing
- Color subcarrier
