Sequential transmission of images 1. Pixel A black and white still image is composed of many small units, and the small units that make up the image are called pixels (Figure 07-02-3). This is how the fax pictures in the newspaper are constructed. For the observer, each pixel has its optical characteristics and spatial position, and changes with time. Any pixel P in the scene image can be expressed by eight physical quantities, namely P = f (x, y, z, L, H, S, R, t) Among them, (x, y, z) represents the spatial position of the pixel, L, H, S represent the brightness, hue and saturation of the pixel, R is the resolution of the image (each pixel area accounts for Ratio), and t is the time when the pixel produces the above physical quantity. For the current color TV system, since it can only express the two-dimensional plane information of the image instead of the three-dimensional stereoscopic information, it is impossible to reflect all eight physical quantities. Obviously, the more pixels that an image decomposes, the clearer the image. Usually, an image of a 35mm film film has about one million pixels, and a TV image has about 400,000 pixels. For a color image on a fluorescent screen, it is composed of many color pixels. For a color TV, a color pixel is composed of red, green, and blue dots. Figure 07-02-3 An image is composed of many pixels In a television system, the image is first decomposed into pixels at the sending end, and then the brightness information (or color information) of these pixels is converted into electrical information for transmission, and then the pixels are combined into an image at the receiving end. 2. Sequential transmission of still images How is the pixel information of the image transmitted? Obviously, the simultaneous transmission system is unrealistic. The so-called simultaneous system is to transmit all the pixel information of an image to the receiving end through their respective signal channels at the same time. This method is not easy to implement because there are too many signal channels required for the simultaneous transmission mode. In fact, the sequential transmission system is often used in television technology, that is, the brightness information of each pixel on the image is converted into a corresponding electrical signal one by one in a certain time sequence at the sending end, and then transmitted through the same channel in turn, at the receiving end In the same order, the electrical signals of each pixel are converted into light spots of different brightness at the corresponding positions of the TV screen. As long as the speed of this sequential transmission is fast enough, then due to the visual inertia of the human eye and the afterglow characteristics of the luminescent material, the entire image will feel illuminated at the same time. A television system formed by this method of sequentially transmitting image pixels is called a sequential transmission system, as shown in Figure 07-02-4. Figure 07-02-4 Schematic diagram of sequential transmission television system It can be seen from Figure 07-02-4, assuming that the sending end is a photoelectric panel composed of many photocells, and the receiving end is a display panel composed of many light bulbs, and the two ends are connected by a signal channel to form a TV transmission system. If the image of "middle" is transmitted, the electronic pen S starts to scan the photoelectric panel in order from the upper left corner of the image, decomposes the pixels while completing the photo-electric conversion, and sequentially transmits the electrical signals of each pixel to the electronic pen S ', S through a channel 'Sweep the display panel in the same order, complete the electro-optical conversion while recombining pixels, and reproduce the image. The electronic pens S and S 'at the sending end and the receiving end must work synchronously, that is, the scanning positions of the two electronic pens S and S' correspond to each other, and the scanning speed is the same, so that the reproduced image is accurate. Of course, the conversion speed of the electronic pen should be fast enough to use the visual inertia of the human eye to see a complete image. In fact, in the TV system, the conversion function of the electronic pens S and S 'is completed by electron beam scanning. 3. Sequential transmission of moving images Most of the images transmitted by modern TV systems are moving images. How to transmit moving images has become an important issue in sequential image transmission systems. The way the movie is shown inspires people. When showing a movie, as long as 24 frames per second, time-related still pictures with similar contents are displayed. Due to the visual inertia of the human eye, the feeling of the previous image has not disappeared, the latter image has arrived, and the result makes people feel The image is continuously moving. In the same way, TV can also use a method similar to that of movies to decompose moving images into still images one after another. When the transmission speed is fast enough, human eyes will feel continuous moving images. China stipulates that the transmission speed of television images is 25 frames per second. It can be known from the film practice that although images can be transmitted continuously up to 24 frames per second, continuous moving images can be obtained, but there is a sense of light flicker that causes fatigue to human eyes. In order to overcome this phenomenon, the film machine needs to block the light once when projecting an image, so that each image is projected twice in succession. In this way, there are actually 48 images per second for the human eye. The TV system is also facing the same problem. It uses interlaced scanning technology to transmit each image in two fields, so that the TV can actually transmit 50 times per second, thereby effectively overcoming the light flicker phenomenon.
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