Color TV basics

Color TV basics

According to the color vision characteristics of the human eye, it is important to obtain the same color perception as the original scene during the color reproduction process, and it is not required to restore the spectrum of the original scene. The color TV system is designed and works according to the principle of three primary colors. The principle of three primary colors points out that any one color can be made by mixing three other colors in different proportions. This means that if three standard primary colors are selected, any color can be represented by the number of three primary colors needed to synthesize it. The color TV system is based on the human eye function and the principle of three primary colors, designed a color camera and display.

Figure 07-03-1 illustrates how to decompose the colors of natural scenes into R, G, and B components when capturing scenes with a color camera, and how to reproduce the colors of natural scenes.

Figure 07-03-1 Color image reproduction process

In a typical color TV camera, three camera tubes are used to pick up the red, green, and blue components in the optical image of the scene to imitate the three cone cells in the human eye to form three red, green, and blue in the color TV signal. Primary color component. The additive color mixing rule constitutes the basic principle of CRT display color display. The inner surface of the color fluorescent screen is coated with a large number of phosphor dots composed of a group of three colors of red, green and blue. Phosphor is a compound that emits light when bombarded by electrons, and its luminous intensity depends on the intensity of the electron beam. When the image is reproduced, the red, green, and blue components of the received color TV signal are used to control the three electron guns to bombard the phosphor dots of the corresponding colors. Because the phosphor dots are small, the three primary colors emit light when viewed at a certain distance. The mixing effect of the human eye allows us to see evenly mixed colors.

In the early days of the development of color TV, since there have already been a considerable number of black and white TVs and black and white TV stations, in order to protect the interests of consumers and TV stations and expand the ratings of color TV programs, the design of color TV systems must be considered TV compatible. For this reason, in the color television system, instead of transmitting the three primary color components of red, green, and blue in the color television signal, one luminance component and two color difference components are transmitted. At the sending end, the luminance component and the color difference component are obtained by matrix transformation of the three primary color components of red, green, and blue; the receiving end is then restored to three primary color components by matrix inverse transformation. When a black-and-white TV receives a color TV signal, it only uses its brightness component to display a black-and-white image; when a color TV receives a black-and-white TV signal, it uses the black-and-white TV signal as its brightness signal to also display a black-and-white image, and then realize Color TV is compatible with black and white TV. In the color TV, the proportional relationship of the luminance signals composed of the three primary colors R, G, and B is as follows:

Y = 0.299R + 0.587G + 0.114B

This formula is the brightness equation of the TV system. As for the two color difference signals, the difference signals of the red and blue primary color components and the luminance component, namely U and V, are transmitted respectively.

U = k1 (B – Y)

V = k2 (R – Y)

k1, k2 are weighting coefficients.

Thus for black and white images: R = G = B, then U = 0, V = 0.

For color images: Represents the saturation of the image, U / V represents the hue of the image.

From the perspective of data compression, we also want to transmit Y, U, and V instead of R, G, and B, because Y, U, and V are three quantities that have a certain correlation removed. A major problem in the TV system is how to use one channel to transmit the above three signals Y, U and V. For the different synthesis methods of these three signals, three different color TV systems of PAL system, NTSC system and SECAM system are formed. The difference between these three systems is the different processing methods adopted for the transmission of chrominance signals.

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