Talking about the application and technical indicators of LED full-color display screen

Since the beginning of this year, the market demand for full-color LED large screens has shown a rapid upward trend. Just after the Spring Festival, large projects began to bid. In April and May, although the SARS epidemic affected the market, it only delayed the implementation plan, not the cancellation of the project. After SARS, many projects were implemented at an accelerated pace. The contract amount of major LED large screen manufacturers increased by 20% compared with the same period last year, and some even almost doubled compared with last year. There are also many new companies entering the full-color large screen field, and their performance is also excellent. Compared with previous years, the market started early this year, with many large projects. There are more than a dozen projects with a total investment of more than 10 million yuan. Entering the third quarter, many major projects entered the bidding stage. Another feature is that the number of projects invested by enterprises has increased significantly, including applications in commercial advertising. It can be said that as a commercial advertising media, LED large screens have once again been favored by the advertising industry after being silent for nearly ten years. At the same time, the export situation of full-color screens is also encouraging. In early September, the annual Optical Expo was held in Shenzhen. This exhibition was a major display of the LED industry in the past year. LED large screens, LED lighting, LED traffic lights, LED production equipment, testing equipment, etc. are all on display, indicating that a new climax has arrived in my country's LED industry. The author believes that this exhibition is a sign that my country's LED industry has reached a turning point in localization. Internationally renowned LED light-emitting tube manufacturers and screen manufacturers have also begun to move to mainland China on a large scale. Mainland China will become a manufacturing center for LEDs and their products. With a vast market, a good investment environment, and low labor costs, China has gradually become a manufacturing center for the world's electronics industry. Just like computers, mobile phones, televisions and other electronic products, China is also a manufacturing center for the LED large screen industry.

As a scientific and technological worker who has been engaged in the research and development of LED large screens for a long time, the author has been paying attention to the development of my country's LED large screen industry and the new achievements of the international large screen display industry. Three years ago, in the article "A Brief Discussion on the Development of LED Large Screens", the author talked about several application fields and characteristics of my country's LED large screens, and proposed that full-color large screens are mainly image projects in my country. It should be said that this general pattern has not changed, but the increase in demand for commercial advertising is also a very important development trend. It can be said that my country's LED full-color large screen industry is ushering in a new climax. Carefully studying the new situation of market development and technological progress, taking correct countermeasures, seizing opportunities, and rapidly improving and continuously developing should be issues worthy of research by peers. Here I would like to raise questions about technical indicators and other issues, and hope that peers and experts will correct them.

1. Image Project: Aiming at World-Class Level

Image projects refer to the construction of large screens with the main purpose of adding style to a city or a major event. It does not directly create economic benefits, but it can enhance the city's reputation, shape the city's image, improve the investment environment, create a business atmosphere, and be conducive to economic development. It has good social benefits and thus indirectly creates economic benefits. Image projects are mainly government investments, focusing on social benefits, rather than or not mainly on achieving economic benefits through commercial operations. The LED large screens built in large squares in urban centers or citizen leisure squares in many places across the country in recent years are typical image projects. A large number of LED screens in sports stadiums are necessary supporting facilities for modern sports stadiums and also belong to this category.

Since it is an image project, it is necessary to pursue the most perfect performance and effect, especially high-quality video images. In layman's terms, it is to make a giant color TV, which is what many domestic manufacturers have been working on for a long time, and have made considerable progress, but it is still far from our ideal goal, and there is still a big gap compared with foreign full-color large screens.

At the beginning of this year, a large full-color screen with an area of ​​114 square meters was built at a commercial street on Hunan Road in Nanjing. The screen was built by the British company UNITEK and has been in operation for more than half a year. From the effect point of view, it can be called the best large screen in China. The author participated in the formulation of the technical indicators of the screen and the final acceptance test, and felt that there are many aspects worthy of learning and emulation by domestic peers.

As a full-color large screen, its key technical indicators are: sufficiently high brightness, viewing distance and viewing angle; good color reproduction; sufficiently high grayscale capability; sufficiently high scanning refresh rate. Sufficiently high video signal processing capabilities, especially good consistency, and great efforts must be made in structural processes. The technical indicators set by the Hunan Road large screen during the bidding are:

White balance brightness is greater than 5000cd/m2;

White balance color temperature 6500K;

The video processing bit is greater than 10 bits;

The grayscale of each primary color is greater than 10 bits, and 1.07 billion colors can be displayed;

Refresh rate is greater than 400Hz;

The display has no perceptible color difference, and the color difference of each primary color on the entire screen is less than 4nm;

The display screen has no perceptible brightness difference, and the brightness difference between adjacent pixels is less than 5%. When displaying red, green, and blue monochrome grayscale images, there is no perceptible mosaic phenomenon;

MTBF is greater than 5000 hours, MTTR is less than 30 minutes;

Objectively speaking, these indicators have not been fully achieved by domestic manufacturers' products, not only at that time, but also today. In many cases, due to the improvement of the level of light-emitting tubes, the brightness of large screens with normal designs is no longer a problem and can meet the requirements. However, brightness is only an important indicator, not everything. Now everyone generally feels that the large screens of domestic manufacturers are bright enough, not soft enough, and the overall effect is not as good as that of well-known foreign manufacturers. The reason is that there are more technical problems that have not been solved. The author believes that from a technical perspective, it is necessary to work hard in the following aspects.

(1) Color matching and mixing

Large screens are composed of a large number of light-emitting diodes. The photoelectric parameters of the light-emitting diodes used must be designed, calculated and tested to achieve a good white balance. Simple estimation and visual judgment are unlikely to produce good results. This involves the selection and drive design of light-emitting diodes, which not only affect the photoelectric performance of the screen, but also the reliability. Especially in the design of indoor full-color screens, thermal design should be given enough attention, otherwise both the effect and reliability will become a problem.

(2) Chromatic aberration correction

Due to the difference in color coordinates, the color gamut of TV video signals is different from the color gamut of LEDs, which makes the colors of TV programs appear unreal. Therefore, the correction of color coordinates is very important. The conversion calculation of correction is relatively complicated and difficult to complete with software. Judging from the products of various manufacturers, although everyone claims to have this color correction function, I am afraid that none of them can achieve color correction.

(3) Consistency

The biggest difficulty of full-color large screen is consistency, or the most easily perceived problem by the audience is poor consistency. The screen appears to be deep and shallow, commonly known as the mosaic phenomenon, which is the most offensive. There are many reasons for poor consistency, including the selected light-emitting diodes, drive circuits, structural design and construction. However, unless the discreteness of the light-emitting diodes used is too large, the consistency problem is mainly a design and construction problem. The first reaction of the audience to the Hunan Road large screen is that the mosaic phenomenon cannot be observed (strictly speaking, it is very light rather than absent, and there are some differences between the observations of ordinary audiences and professionals). This large screen is very distinctive in structure and technology. It does not adopt the commonly used module glue filling method in China, which is worth studying. Usually, light-emitting diode manufacturers divide light-emitting diodes into several grades according to brightness and color quality. The difference in the same grade can be solved by careful design and debugging. Poor structural design and construction is another major cause of the mosaic phenomenon, and it is often discovered only after the large screen is installed and put into operation, and it is no longer remedial, so special efforts should be made. If there is a certain gap in the electrical indicators of display screens produced by domestic manufacturers, then the gap in structure and process is much greater, which often seriously affects the display effect.

(4) Grayscale

The experience of the Hunan Road large screen and many large screens abroad proves that the grayscale must be high enough. At present, high-end products generally display 1024 levels and 1.07 billion colors on the same screen. Most domestic ones are 256 levels, which is a big gap. Many manufacturers propose 1024/2048/4096 or even 16384-level encoding, and 256-level nonlinear display of 16.7 trillion colors. There is an opinion that 256 levels of nonlinear grayscale should be enough for large screens, because the HDTV standard also stipulates 256 levels of grayscale. According to the author's test, this is not the case, and the actual effect is very different. In fact, to achieve grayscale above 1024 levels, the technical difficulty will increase a lot. A simple calculation will make it clear. When the refresh frequency is 400Hz, the nonlinear grayscale is 1024 levels, and the brightness adjustment level is 32 levels. At this time, if the Y correction coefficient is 2.8, the minimum width of the PCM pulse should be about 10ns, which has reached the bandwidth limit of the driving circuit. In fact, it is impossible to achieve this, and the method of combining amplitude modulation and width modulation should be considered. There is also the problem of transmission bandwidth. When the screen has many rows and columns and the refresh rate is high (for example, above 400Hz), the difficulty of data transmission will increase. Sometimes optical fiber transmission must be used to ensure the quality of the image. Therefore, the grayscale from non-linear 256 levels to non-linear 1024 levels requires a leap in technology.

(5) Digital processing capabilities

To get a high-quality image, you must have a high-quality video signal source. For general large-screen users, the best signal they can get is the broadcast TV signal, and DVD, VCD, etc. are only home-use. Therefore, it is essential to digitally process the input signal and improve the image quality. In fact, everyone has this experience: when playing selected programs, the display effect is very satisfactory, but if it is a live TV program or a VCD/DVD, it is inevitable that there will be various pictures and lenses, such as dark pictures such as night scenes or large areas of high-brightness bright pictures, then the effect will be greatly reduced. How can you have a satisfactory effect under various pictures and lenses? This depends on digital signal processing, such as digital comb filtering, filtering noise reduction, edge sharpening, telemotion prediction and compensation, chromaticity correction, nonlinear correction, etc. It is particularly worth mentioning that the scaling (SCALE) processing is a special problem of LED large screens, and the large screen industry needs to work hard to study it.