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 back porch (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.

Contents

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.

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 brightness (luminance) 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 color (chrominance) 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]

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