Like other display devices, the LED display has been accompanied by technological advancement and upgrading since its birth. Compared with other display devices, it is large in size, ultra-thin and high-resolution, and the LED display screen is close to the grain. The technical aspects of long-term viewing comfort have also been greatly improved; and with the rapid breakthrough of technologies such as small-pitch high-density and low-brightness display screens, LED display screens have been entered indoors, with DLP splicing, LCD splicing, PDP splicing, Engineering projectors and the like have become the mainstream of indoor large-scale display, and are more and more widely applied to indoor multifunctional auditoriums, conference centers and dispatching command centers.
This paper comprehensively analyzes the design, installation, testing and maintenance of LED lighting in an auditorium in combination with engineering construction.
Basic situation of the project
1.1 Project construction needs
The display system of the multifunctional auditorium is divided into three parts: the main display (hereinafter referred to as "main screen"), the auxiliary display (hereinafter referred to as "auxiliary screen") and the monogram screen. The main screen construction demand is to build a large-format, high-definition large screen, display multiple computer digital signals, video recording of the venue, remote monitoring or video signals of different venues, etc., to achieve the display of video, explain materials, video conferences, etc. At the same time, we must also take into account the stage performance background; the auxiliary screen construction needs to display a standard definition signal, to achieve the display of conference-assisted content, and can also be used as a poster, subtitle, etc. during the performance; the monogram screen needs to display multiple colors of text.
The stage part of the auditorium is 20m wide, 17m deep and 18m high. The mouth is 15m wide and 7.8m high. The stage is equipped with stage machinery such as lights and curtains. The ear wall on both sides of the platform is 3m away from the ground, 8m wide and 6m high. The design main screen display area is at least 72m2 (12m×6m). The front of the screen is the place for the podium meeting and performance. After the screen is the actor passing and maintaining the space. The first row of the audience is at least 17m away from the screen; the auxiliary screen is displayed on the ear wall. The area is at least 12m2 (4m × 3m).
1.2 Determination of display technology
For DLP splicing display wall, LCD splicing display wall and LED electronic screen, no matter what method is adopted, it must meet the display requirements of auditorium video conference and live performance, and analyze and compare the three display modes in combination with the scene.
1) DLP splicing display wall
The advantages of DLP splicing system are high contrast, fine color, high resolution and seam less than 0.25mm. The disadvantage is that the display effect is greatly affected by the stage lighting. The brightness and color will be difficult to be uniform due to the different attenuation of the projection equipment. Short, high maintenance supplies costs.
2) LCD splicing display wall
LCD splicing unit size is generally 1397mm, 1524mm, unit brightness is less than 1000cd/m2, patchwork minimum is 2.4mm, life is 60,000 hours, the advantages are thin body, low noise, low heat dissipation, easy maintenance, suitable for close-range viewing; Because the unit size is small, the seams are more and more obvious after splicing the large screen.
3) LED display
LED display indoor screen brightness is greater than 1500cd/m2, can be spliced ​​into a seamless display wall with any area, good color consistency, life expectancy of 100,000 hours, low requirements for the application environment; the disadvantage is obvious viewing of particles at close range, if the display area Not big enough, the image needs to be compressed, affecting the clarity of the picture display.
Compared with the three display technologies, for the DLP mode, in order to clearly view the screen display content at a distance of 17m, it is required to use a high-brightness, high-contrast laser light source DLP projection for multi-screen splicing, which has high engineering cost and possible application performance. Dust and props are touched, so DLP splicing display walls are not ideal;
For the LCD mode, the video conference image display may have a poor display effect of the person's face segmentation, and at the same time, the brightness of the screen is not high enough for the auditorium environment at a distance of 17 m;
For the LED display mode, since the viewing distance is at least 17m and the display area is about 72 m2, the shortcomings of the LED splicing wall are completely negligible, and as the cost of the electronic product decreases, the overall cost is not high, and the system has a long life and low Application environment, easy to maintain, suitable for conference TV and stage performances.
Key technologies that affect the quality of the display body
When designing the screen, the LED display should consider three important factors: the content of the display content, the space conditions of the space, the size of the display unit module or the pixel size. At the same time, it must ensure that the production process and technical indicators are suitable for the practical application of the room. Combine the cost of the entire project with reasonable design.
2.1 Display the choice of dot pitch
Visual graininess mainly comes from the human eye has a certain resolution, watching two points at a certain distance, when the two points are close to a certain degree, the human eye will not be able to distinguish. In the past two years, with the improvement of LED display manufacturing technology, the resolution of small-pitch LED screens has been continuously improved. The indoor display design has been upgraded from the initial selection of the smallest point-distance specification scheme to the selection of the appropriate point-distance specification scheme. Generally, the display specifications adopted by the system are determined by comprehensive comparison from the minimum line of sight, the best line of sight, the system cost, and the screen display resolution.
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Table 1 compares the typical display specifications. The specifications of P5, P4, P3, P2.5, P2, and P1.6 are common indoor specifications. “P4†indicates that the pixel pitch is 4mm, and the minimum viewing distance is human eye resolution. There is no distance from the pixel particles, but this distance will damage the vision for a long time. The best viewing distance is the comfortable distance to watch the screen, and the clearest distance.
It can be seen from the comparison of Table 1 that the P2.5, P2, and P1.6 specifications, the minimum line of sight and the best line of sight are very close to the viewing requirements of large flat-panel TVs. The resolution of the whole screen is as high as 4800×2400, in the same A clearer picture can be displayed on the screen, but a high-definition signal needs to be enlarged by 3 to 4 times on a 12-m wide screen, and two high-definition pictures need to be enlarged by a factor of 2; while high-resolution display requires high-performance image processing equipment. The overall cost of the system will be high.
P5 and P4 can't display two high-definition pictures simultaneously on a 12m wide screen, which can't meet the requirements of HD conference TV screen display. Both P3.1 and P3 specifications are required to simultaneously display two high-definition pictures. According to the size of the display screen, the P3.1 with higher cost performance is selected as the display point distance of the main screen and the auxiliary screen of the project, and P4 is selected as the display point distance of the monogram screen; the LED wick selects the domestic first-line brand, such as Shilan, Jing Yuan, dry photos, Sanan, etc.
2.2 pixel packaging technology
One factor that determines LED quality is the main materials, such as chips, brackets, glue (epoxy), gold wire, etc. Another factor is the packaging process. Pixel packaging technology is related to the color saturation, viewing angle and other display effects of the entire display screen, production cost, quality stability, etc., is one of the key indicators for selecting LED display.
LED indoor mainstream packaging technology mainly has a surface mount and in-line mode.
Surface labeling refers to attaching a packaged light-emitting tube to a circuit board, and then performing an integrated circuit soldering process to form a screen body. It has good heat dissipation and uniform color, and the entire display can be maintained on the front side, which greatly reduces the maintenance cost and difficulty. At the same time, the metal compound is filled in the packaging process, the color mixing effect is good, the light is soft, and it is more suitable for indoor applications.
The in-line module encapsulates the LED tube into a square shape, and then puts together the three tubes of red, green and blue to form a pixel point module, which is directly inserted into the PCB board for soldering. The process is relatively simple, the cost is low, and the dead point rate is high. Mainly used in outdoor displays.
The pixel structure of the indoor screen has two ways: surface mount three-in-one and surface mount three-in-one (separate surface mount).
The three-in-one surface sticker refers to the red, green and blue light-emitting points packaged and integrated in the same light-emitting tube. The near-point is a point. The enlarged view is a line separated by three light-emitting points, which requires high materials and processes for the production process. Mainly used in the production of small pitch, high density high-end LED display.
The three-in-one surface label (separated surface label) means that the three red, green and blue light-emitting points are packaged separately, arranged in a pixel after packaging, and then encapsulated by a mask to protect against dust, scratches and protection. Light emitting chip. The three-in-one is to separate the surface mounts, the three points are separately powered, the power consumption is small, and the single lamp maintenance can be realized, and the cost is low.
In general, in order to improve the display effect of the indoor screen, the surface mount three-in-one, black light technology, gold wire package is used; the relatively large package brands are Taiwan Yiguang, Dazu Laser, Foshan Guoxing, Shenzhen Lusheng, Zhejiang Ying Specially, Chongqing Silian, etc.
2.3 LED control circuit design
The LED control part is the core part that determines the display effect. The control circuit has 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. After receiving the signal, the LED driver chip generates corresponding actions, thereby implementing separate control for each of the red, green and blue LED light-emitting chips.
2.3.1 Drive System
The function of the LED driving part is to receive the color data and drive the LED display to display the brightness value indicated by the data. There are usually three ways of constant current, voltage regulation, constant voltage and constant current.
The current output by the constant current drive circuit is constant, and the output DC voltage varies with the load resistance. The entire circuit is not afraid of load short circuit, but the load is completely open. The voltage regulator 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 being open, but the load is completely short-circuited. The first constant voltage and constant current mode is the most ideal driving circuit. It is necessary to detect the LED current and control the LED voltage, which is beneficial to improve the life of the LED and reduce the power consumption. It is generally used in high-end LED products.
2.3.2 Control system
The display effect of the LED depends on the magnitude 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 PWM power supply designed by PWM (pulse modulation) control method has a conversion efficiency of 80% to 90%, and the output voltage or current is very stable, and is a high reliability power supply.
The PWM can control the ratio of the time between the LEDs on and off. By dividing the time scale into several levels, the LEDs display the corresponding number of gray levels (grayscale). The product of the gray level of the three primary colors is the number of colors that can be reproduced theoretically on the display screen, generally 256 levels, and the number of colors reaches 16.7M, and the information of 24-bit true color can be displayed.
Generally, the frequency of the PWM is greater than 100 Hz, otherwise there will be flashing and scanning lines displayed during viewing. Now the large screen with a gray level of 10 bits, the refresh rate is mostly 800 ~ 1000 Hz, the highest refresh rate of the LED industry is 6800Hz. When the camera is normally framing and rotating, the LED screen display is stable and flicker-free, and the gray scale is excessively smooth at the highest refresh rate, and there is no cross color.
2.3.3 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 the control system to the stability of the entire display. Generally, there are three ways of series, parallel and hybrid.
The series connection method is a low-reliability connection method, which is often used in low-end products. In a voltage-stabilized driving circuit, when a certain LED is short-circuited, the voltage distributed to other LEDs will rise, which may cause more damage.
Parallel mode is suitable for products with low power supply voltage. In a constant current drive circuit, when one LED is disconnected, the current distributed to other LEDs will increase, which will easily damage all LEDs on the circuit.
At present, in order to improve the reliability of the product, the hybrid mode is generally adopted. The number of LEDs in series and parallel is evenly distributed, the groups are connected in parallel, and each group is connected in series, so that the voltages distributed on one string of LEDs are the same, through each LED. The current is also basically the same, the LED brightness is consistent, and it is better 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 composed of multiple module units. In order to ensure the reliability of power supply and data, it is also necessary to adopt a hybrid mode to realize the loop backup function. When an abnormal power failure occurs in one power supply, other power supplies will automatically perform the intelligent function. 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 the fault maintenance time.
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