Before the implementation of the stage rental LED display project, it is necessary to make a design plan, especially when designing the screen body, it is necessary to consider important factors such as the display model, display content, installation site environment, cost, etc., and to ensure the production process, technical indicators, etc. It is suitable for indoor practical application requirements, and a reasonable design is carried out accordingly.

one. LED display dot pitch

Visual graininess mainly comes from the human eye has a certain resolution. When viewing two points at a certain distance, when the two points are close to a certain extent, the human eye will not be able to distinguish them. In the past two years, with the improvement of LED display manufacturing technology, the resolution of small-pitch LED display has been continuously improved, and the indoor display design has been upgraded from the initial selection of the smallest dot pitch specification to the selection of the appropriate dot pitch specification.

The pixel pitch is the distance between the lamp beads. For example, the pixel pitch of P3 is 3mm; the minimum viewing distance is the distance at which the human eye cannot distinguish the pixel particles, but this distance will damage the eyesight if viewed for a long time; the best viewing distance is the one where the screen is comfortable to watch Distance is also the clearest distance; pixel density is the screen definition index, the larger the value, the higher the resolution of the screen; full HD image video, screen 16:9 golden ratio, human vision is the best.

Through the analysis of the above table: P1.27, P1.54, P1.6, P1.9, P2.5 four kinds of rental LED display screens, which are small-pitch LED display screens (dot pitch below 2.5mm). The minimum viewing distance and the best viewing distance are very close to the viewing requirements of large flat-panel TVs. The resolution of the entire screen is as high as 4800×2400. Clearer images can be displayed on the same screen, but a high-definition signal can be displayed on a 12m wide screen. It needs to be magnified by 3 to 4 times, and it needs to be magnified by 2 times to display two high-definition images. At the same time, high-resolution display requires high-performance image processing equipment, and the overall cost of the system will be very high. The two specifications of P4 and P4.8 cannot display two high-definition images at the same time on a 12m wide screen, which cannot meet the display requirements of high-definition conference TV images. P3 can meet the requirement of displaying two high-definition images at the same time. According to the size of the display screen, the cost-effective P3 was finally selected as the display pitch of the main screen and auxiliary screen of the project, and P4 was selected as the display pitch of the monogram screen.

2. Packaging technology

One factor that determines the quality of LEDs is the main material, such as chips, brackets, glue (epoxy resin), gold wires, etc., and the other factor is the packaging process. Pixel packaging technology is related to the color saturation, viewing angle and other display effects of the entire display, production cost, quality stability, etc., and is one of the key indicators for selecting LED displays.

The mainstream packaging technologies for indoor LEDs mainly include in-line DIP and surface mount SMD. The in-line module is to package the light-emitting tube into a rectangular shape, and then put together three tubes of red, green and blue to make a pixel module, which is directly inserted on the PCB board and soldered. The process is relatively simple, the cost is low, and the dead pixel rate is high. , mainly used in outdoor display. Surface mount refers to pasting the packaged light-emitting tube on the circuit board, and then performing integrated circuit welding process to process it into a screen body. It has good heat dissipation and uniform color, and the entire display screen can be maintained from the front, which greatly reduces the maintenance cost and difficulty; at the same time, it is filled with metal compounds during the packaging process, with good color mixing effect and soft light emission, which is more suitable for indoor applications.

Three-in-one surface mount means that three luminous points of red, green and blue are encapsulated and synthesized in the same luminous tube. It is a point in close view, and it is a line separated by three luminous points when zoomed in. It has high requirements for materials and processes in the production process. It is mainly used for the production of small-pitch, high-density high-end LED displays.

Three-in-one surface mount (separate surface mount) means that the red, green, and blue light-emitting points are packaged separately. After packaging, they are arranged into 1 pixel point, and then packaged with 1 mask for protection, which can prevent dust, scratch, and protect Luminous chips. Three-in-one is separate surface mount, three points are powered separately, the power consumption is small, and single-lamp maintenance can be realized, and the cost is low.

Generally speaking, in order to improve the display effect of indoor screens, surface mount three-in-one, black light technology, gold wire packaging, etc. are used.

three. Design of LED Control Circuit

The LED control part is the core part that determines the display effect, and the control circuit has a built-in high-performance single-chip micro-control chip. The controller sends control signals and data to the LED driver chip through the internal control program, and the LED driver chip generates corresponding actions after receiving the signal, so as to realize individual control of each red, green and blue LED light-emitting chip.

1. Drive system

The function of the LED driving part is to receive color data and drive the LED display to display according to the brightness value represented by the data. There are usually three methods: 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 the load resistance. The whole circuit is not afraid of the load short circuit, but it is strictly forbidden 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 the load opening, but the load is strictly prohibited from being completely short-circuited. The first constant voltage and then constant current mode is the most ideal driving circuit. It not only needs to detect the LED current, but also controls the LED voltage, which is conducive to improving the life of the LED and reducing power consumption. It is generally used in high-end LED products.

2. Control system

The display effect of the stage LED display depends on the size and time of the current and voltage passing through it, so the control system mainly controls the power output of the LED. At present, the LED power supply designed by PWM (pulse modulation) control method has a conversion efficiency as high as 80% to 90%, and the output voltage or current is very stable, which 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 video wall panels can display a corresponding number of gray levels (gray scale). The product of the gray levels of the three primary colors is the number of colors that can be reproduced theoretically by the display screen, generally 256 levels, and 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 viewing. At present, Vitem indoor rental screens have a gray scale of 14 bits and above, a refresh rate of 1920 Hz and above, and the highest refresh rate in the LED industry is 6800 Hz. Under the normal framing and rotation of the camera, the LED screen display is stable without flickering, and the gray scale is too smooth at the highest refresh rate without cross-color.

3. Combination of drive system and control system

As the load of the circuit, the LED is connected to the circuit of the drive system and control system, which is related to the stability of the entire display screen. Generally, there are three ways: series connection, parallel connection, and hybrid connection. The series connection method is a connection method with low reliability. It is often used in low-end products. In the voltage regulation drive circuit, when a certain LED is short-circuited, the voltage distributed to other LEDs will increase, which is likely to cause more damage.

Parallel connection is suitable for 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 will easily damage all the LEDs on the circuit.

At present, in order to improve the reliability of the product, the mixed connection method is generally adopted, the number of LEDs connected in series and parallel is evenly distributed, grouped in parallel, and then each group is connected in series, so that the voltage distributed on a string of LEDs is the same, through each LED The current is basically the same, and the LED brightness is consistent. At the same time, it is best to increase the LED temperature negative feedback in the constant current output to prevent the LED temperature from being too high.

The entire display screen is spliced by multiple module units. In order to ensure the reliability of power supply and data, it is necessary to adopt a hybrid mode to realize the loop backup function. When a certain power supply fails abnormally, other power supplies will automatically perform intelligent Current sharing, so as not to affect the normal use of the system, more effectively improve the system’s trouble-free running time, and reduce fault maintenance time.

Four. Special requirements for screens in stage photography

Although the high refresh rate can ensure that even under the shooting of high-speed cameras, the large screen can still respond in time, and the picture transitions and transitions are smoother; The inexplicable water wave-like stripes and strange colors, and with the change of shooting angle and the adjustment of the focal length of the camera lens, the water ripples will also change, which will greatly affect the display effect of live broadcast and recording. This is the digitalization of TV cameras. Moiré phenomenon.

Moiré occurs when the spatial frequency of the CCD (or CMOS) pixels of the photosensitive element is close to the spatial frequency of the (LED) stripes in the image. Since the moiré phenomenon is produced by the inherent structure of the LED display, if one of the conditions for moiré generation—the grid structure of the LED display or the grid structure of the camera CCD (CMOS)—disappears, theoretically Moiré interference can be completely eliminated. Domestic companies superimpose a layer of optical treatment curtain on the surface of LED display, which is composed of a specific proportion of special light-absorbing material and surface microbead lens coating. Through the use of optical curtain, the LED screen changes from the original grid-like light to Continuous surface emission, through the enlarged display of discrete LED pixels, finally forms a continuous high-definition image on the surface of the optical processing screen. While improving the contrast, it retains high definition and eliminates the phenomenon of moiré.