Four factors of stage rental LED display engineering project design（二）

 3、 Design of LED display control circuit

The LED control part is the core part that determines 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. Outdoor LED floor drive system

The function of 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, constant voltage and 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 is necessary to detect the LED current and control the LED voltage, which is conducive to improving the LED life and reducing the power consumption. It is generally used for high-grade led interactive floor panels.

2. Outdoor LED floor control system

The 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, it is a PWM power supply with high voltage conversion efficiency of 80% ~ 90%, and the reliability is very high.

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 flickering and scanning lines when watching. At present, the gray level of Wittem indoor rental screen is 14} bits and above, the refresh frequency is 1920 Hz 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 outdoor LED floor drive system and control system

As 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 on the circuit.

At present, in order to improve the reliability of products, the mixed connection mode is generally adopted. The number of LEDs in series and parallel 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 brightness of high brightness outdoor LED floor is the same. At the same time, it is best to add negative feedback of LED temperature in constant current output to prevent 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, the mixed connection mode 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, more effectively improve the fault-free operation time of the system and reduce the fault maintenance time.

4、 Special requirements of stage camera for LED display screen​

Although 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 with the change of shooting angle and the adjustment of camera lens focal length, the water wave will also change, which will greatly affect the display effect of live broadcast and recording. This is the phenomenon of Moire caused by the digitization of TV camera.

If the spatial frequency of the pixel of the photosensitive element # CCD (or # CMOS) is close to the spatial frequency of the (LED) stripe in the image, the moire will be generated. Since the moire phenomenon is caused by the inherent structure of the LED display screen, if one of the conditions for the generation of Moire - the grid structure of the LED display screen or the grid structure of the 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. The screen is composed of special light absorbing materials with a specific proportion and surface micro bead lens coating. Through the use of optical screen, the # LED large screen changes from grid light to continuous surface light. Through the amplification and display of scattered # 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 phenomenon of moire.