Before the implementation of the stage rental LED display screen project, it is necessary to make a design scheme, especially when designing the screen body, it is necessary to consider important factors such as the display screen model, display content, installation site environment and cost, and ensure that the production process and technical indicators are suitable for the actual indoor application needs, so as to carry out reasonable design.The sense of visual graininess mainly comes from the fact that the human eye has a certain resolution. When the two points are close to a certain extent, the human eye will not be able to distinguish them. In recent two years, with the improvement of LED display manufacturing technology and the continuous improvement of small spacing LED display body resolution, the indoor display design has been improved from the initial selection of the minimum point distance specification scheme to the selection of the appropriate point distance specification scheme.Pixel spacing is the distance between lamp beads, for example, P3 pixel spacing is 3mm; The minimum sight distance is the distance that human eyes can't distinguish pixel particles, but this distance will damage vision if viewed for a long time; The best viewing distance is not only the comfortable distance to watch the screen, but also the clearest distance; The pixel density is the screen definition index. The larger the value, the higher the screen definition; Full HD image and video, screen 16:9 golden ratio, human vision is the best.According to the analysis in the above table, the rental LED display screens of p1.27, p1.54, P1.6, p1.9 and p2.5 are small spacing LED display screens (the point spacing is less than 2.5mm). The minimum viewing distance and the best viewing distance are very close to the viewing requirements of large flat panel TV, and the whole screen resolution is up to 4800 2400, clearer pictures can be displayed on the same screen body, but one HD signal needs to be amplified by 3 4 times on a 12m wide screen, and two HD pictures need to be amplified by 2 times; At the same time, high-resolution display requires high-performance image processing equipment, and the overall cost of the system will be very high.P4 and p4.8 cannot display two high-definition pictures at the same time on a 12m wide screen, which can not meet the display requirements of high-definition conference TV pictures. P3 can meet the requirements of displaying two high-definition pictures at the same time. According to the size of the display screen, P3 with high cost performance is finally selected as the display point distance of the main screen and auxiliary screen of the project, and P4 is selected as the display point distance of the monogram screen.One factor that determines the quality of LED is the main materials, such as chip, bracket, glue (epoxy resin), gold wire, etc. the other factor is the packaging process. Pixel packaging technology is related to the color saturation, viewing angle and other display effects of the whole display screen, production cost and quality stability. It is one of the key indicators for selecting LED display screen.The mainstream indoor LED packaging technologies mainly include in-line dip and SMD mode. The in-line module encapsulates the light-emitting tube into a square shape, and then puts the red, green and blue tubes together to make a pixel module, which is directly inserted and welded on the PCB board. The process is relatively simple, the cost is low and the defect rate is high. It is mainly used in outdoor display screens. Surface pasting refers to pasting the encapsulated light-emitting tube on the circuit board, and then processing the integrated circuit welding process into a screen body. It has good heat dissipation and uniform color, and the whole display screen can be maintained in the front, which greatly reduces the maintenance cost and difficulty; At the same time, metal compounds are filled in the packaging process, which has good color mixing effect and soft luminescence, and is more suitable for indoor application.Three in one surface sticker refers to that three light-emitting points of red, green and blue are encapsulated and synthesized in the same light-emitting tube. When you look closer, it is a point. When you zoom in, it is a line separated from the three light-emitting points. It has high requirements for materials and processes in the production process. It is mainly used for the production of high-end LED displays with small spacing and high density.Three in one surface sticker (separate surface sticker) refers to that the three light-emitting points of red, green and blue are packaged separately. After packaging, they are arranged into one pixel point, and then packaged with a mask for protection, which can prevent dust and scratch, and protect the light-emitting chip. Three in one is the separation of table stickers, three-point separate power supply, low power consumption, single lamp maintenance and low cost.Generally speaking, in order to improve the display effect of indoor screen, three in one surface paste, black light technology, gold wire packaging, etc. are selected.The LED control part is the core part to determine the display effect. The control circuit has a high-performance single-chip micro control chip. The controller sends the control signal and data to the LED driver chip through the internal control program. After receiving the signal, the LED driver chip generates the corresponding action, so as to realize the independent control of each red, green and blue LED light-emitting chip.1. Drive systemThe function of the LED driving part is to receive color data and drive the LED display screen to display according to the brightness value represented by the data. There are usually three modes: constant current, voltage stabilization and constant voltage constant current.The output current of the constant current drive circuit is constant, and the output DC voltage changes with the change of load resistance. The whole circuit is not afraid of load short circuit, but it is strictly prohibited to open the load completely. The voltage stabilizing circuit outputs a fixed voltage, and the output current changes with the increase or decrease of the load. The whole circuit is not afraid of open circuit of the load, but it is strictly prohibited to completely short circuit the load. The first constant voltage and then constant current mode is the most ideal driving circuit. It not only detects the LED current, but also controls the LED voltage, which is conducive to improving the LED life and reducing the power consumption. It is generally used in high-grade LED products.2. Control systemThe display effect of stage LED display screen depends on the size and time of current and voltage through it, so the control system mainly controls the power output of LED. At present, the LED power supply designed by PWM (pulse modulation) control mode has a conversion efficiency of 80% 90%, and the output voltage or current is very stable. It is a high reliability power supply.PWM can control the time ratio of LED on and off. By dividing the time ratio into several levels, the LED displays a corresponding number of gray levels (gray scales). The product of the gray level of the three primary colors is the number of colors that can be reproduced on the display screen in theory, which is generally 256 levels. When the number of colors reaches 16.7M, 24 bit true color information can be displayed.Generally, the frequency of PWM is greater than 100Hz, otherwise there will be flashing and scanning lines when watching. At present, the gray level of Wittem indoor rental screen is 14 bits and above, the refresh frequency is 1920hz and above, and the highest refresh rate in the LED industry is 6800hz. Under the normal viewfinder and rotation of the camera, the LED screen display is stable without flicker, and the gray scale is too smooth at the highest refresh rate without color mixing.3. Combination of drive system and control systemAs the load of the circuit, the connection between LED and the circuit of driving system and control system is related to the stability of the whole display screen. Generally, there are three modes: series, parallel and hybrid. Series connection is a connection method with low reliability, which is often used in low-end products. In the voltage stabilizing driving circuit, when a LED is short circuited, the voltage distributed to other LEDs will rise, which is easy to cause more damage.The parallel mode is applicable to products with low power supply voltage. In the constant current drive circuit, when one LED is disconnected, the current distributed to other LEDs will increase, which is easy to damage all LEDs in the circuit.At present, in order to improve the reliability of products, the mixed connection mode is generally adopted. The number of series and parallel LEDs is evenly distributed, grouped and connected in parallel, and then each group is connected in series. In this way, the voltage distributed on a series of LEDs is the same, the current through each LED is basically the same, and the LED brightness is the same. At the same time, it is best to add LED temperature negative feedback in the constant current output to prevent the LED temperature from being too high.The whole display screen is spliced by multiple module units. In order to ensure the reliability of power supply and data, hybrid connection must also be adopted to realize the loop backup function. When an abnormal fault occurs in one power supply, other power supplies will automatically conduct intelligent current sharing, so as not to affect the normal use of the system and more effectively improve the fault-free operation time of the system, Reduce failure maintenance time.4 Special requirements for screen body of stage cameraAlthough the high refresh rate can ensure that even under the shooting of high-speed camera, the large screen can still respond in time, and the picture conversion and transition are smoother and smoother; However, in the process of use, it will be found that when the camera lens is aimed at the LED display screen, inexplicable water wave like stripes and strange colors occasionally appear, and the water wave will change with the change of shooting angle and the adjustment of camera lens focal length, which will affect the display effect of live broadcast and recording, This is the moire phenomenon brought about by the digitization of TV camera.If the spatial frequency of the CCD (or CMOS) pixel of the photosensitive element is close to the spatial frequency of the (LED) stripe in the image, a molar stripe will be generated. Since the moire phenomenon is caused by the inherent structure of LED display screen, if one of the conditions for the generation of Moire - the grid structure of LED display screen or the grid structure of camera CCD (CMOS) disappears, the moire interference can be completely eliminated in theory.Domestic companies superimpose a layer of optical processing screen on the surface of LED display screen, which is composed of specific proportion of special light absorbing material and surface bead lens coating. Through the use of optical screen, the LED screen changes from grid light to continuous surface light. Through the amplification and display of discrete LED pixels, a continuous high-definition image is finally formed on the surface of optical processing screen, While improving the contrast, it retains high definition and eliminates the moire phenomenon.
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