Introduction Since the beginning of this year, the market demand for full-color LED screens has shown a rapid upward trend. Large projects began bidding shortly after the Spring Festival. Although affected by SARS in April and May, implementation plans were only postponed, not cancelled. After SARS, many projects were accelerated. The contract value of major LED screen manufacturers increased by 20% compared to the same period last year, with some even nearly doubling. Many new entrants to the full-color LED screen field have also achieved excellent results. Compared to previous years, the market emerged earlier this year, with more large projects; over a dozen projects worth over ten million yuan have already been implemented. Entering the third quarter, many major projects entered the bidding stage. Another characteristic is the significant increase in projects invested by enterprises, including those for commercial advertising applications. It can be said that LED screens, as a commercial advertising medium, have once again gained favor in the advertising industry after nearly a decade of dormancy. Meanwhile, the export situation for full-color screens is also encouraging. In early September, the annual Optoelectronics Fair was held in Shenzhen, a major showcase of the LED industry over the past year. The dazzling array of LED screens, LED lighting, LED traffic lights, LED production equipment, and testing equipment indicates that a new boom has arrived for my country's LED industry. This exhibition is a landmark event, marking a turning point for the localization of my country's LED industry. Internationally renowned LED manufacturers and screen manufacturers are also beginning to relocate to mainland China on a large scale, making mainland China a manufacturing center for LEDs and related products. A vast market, a favorable investment environment, and low labor costs are gradually making China a manufacturing center for the world's electronics industry. Just like computers, mobile phones, and televisions, China is also a manufacturing center for LED screens. As a scientific and technological worker who has long been engaged in the research and development of LED screens, I have been paying close attention to the development of my country's LED screen industry and the latest achievements in the international large-screen display industry. Three years ago, in my article "A Preliminary Discussion on the Development of LED Screens," I discussed several application areas and characteristics of LED screens in my country, and proposed that full-color large screens in my country are mainly image projects. It should be said that this overall pattern has not changed, but the increasing demand for commercial advertising is also a very important development trend. It can be said that my country's full-color LED screen industry is ushering in a new boom. Carefully studying new developments in the market and technological advancements, adopting correct countermeasures, seizing opportunities, and rapidly improving and sustaining development are issues worthy of study by our peers. Here, I venture to raise some questions regarding technical indicators, hoping for guidance from colleagues and experts. 1. Image Projects: Aiming for International First-Class Standards Image projects refer to large-screen displays whose primary purpose is to enhance the city's image or a major event. They do not directly generate economic benefits, but they can increase the city's visibility, shape its image, improve the investment environment, create a commercial atmosphere, and contribute to economic development, thus indirectly creating economic benefits. Image projects are mainly government-funded, focusing on social benefits rather than, or primarily not, achieving economic benefits through commercial operation. Large LED screens in city center plazas or public leisure squares built in many parts of the country in recent years are typical example of image projects. Numerous LED displays in sports stadiums, essential supporting facilities for modern sports venues, also fall into this category. As image projects, they inevitably pursue the most perfect performance and effects possible, especially high-quality video images. In layman's terms, the goal is to create a giant color television set, something many domestic manufacturers have long strived for and made considerable progress in. However, we are still far from our ideal goal, and there is a significant gap compared to foreign full-color large screens. Earlier this year, a 114-square-meter full-color large screen was built on Hunan Road in Nanjing, a commercial street. Built by the British company UNITEK, it has been operating for over six months and, judging from its performance, is arguably the best large screen currently available in China. Having participated in the formulation of the screen's technical specifications and the final acceptance testing, I feel there are many aspects that domestic counterparts can learn from and emulate. Key technical indicators for a full-color large screen include: sufficiently high brightness, viewing distance, and viewing angle; good color reproduction; sufficiently high grayscale capability; sufficiently high scan refresh rate; sufficiently high video signal processing capability; and, in particular, good consistency. Significant effort must also be put into the structural design and manufacturing process. The technical specifications set during the bidding process for the Hunan Road large screen were: white balance brightness greater than 5000 cd/m²; white balance color temperature 6500K; video processing bit depth greater than 10 bits; grayscale level greater than 10 bits for each primary color, capable of displaying 1.07 billion colors; refresh rate greater than 400Hz; imperceptible color difference on the display, with a color difference of less than 4nm across all primary colors; imperceptible brightness difference on the display, with a brightness difference between adjacent pixels less than 5%, and no perceptible mosaic effect when displaying red, green, and blue monochrome grayscale images; MTBF greater than 5000 hours, MTTR less than 30 minutes. Objectively speaking, these specifications, not only at the time but also today, are not fully met by domestic manufacturers. In many cases, due to improvements in LED technology, brightness in normally designed large screens is no longer a problem and can meet requirements. However, brightness is only one important indicator, not the whole story. Currently, many people feel that domestic manufacturers' large screens have sufficient brightness but lack smoothness, and the overall effect is inferior to that of well-known foreign manufacturers. The reason for this is that many technical issues have not yet been resolved. From a technical perspective, it is necessary to focus on the following aspects: (1) Color matching and color mixing: The large screen is 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 judgment by the naked eye will not have a good effect. This involves the selection of light-emitting diodes and the design of the drive, which not only affects the photoelectric performance of the screen, but also the reliability. Especially in the design of indoor full-color screens, thermal design should be given sufficient attention, otherwise the effect and reliability will be problematic. (2) Color difference correction: Due to the difference in color coordinates, there is a difference between the color gamut of the TV video signal and the color gamut of the LED, which makes the colors of the broadcast TV program appear unrealistic. Therefore, color coordinate correction 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, they probably have not achieved color correction. (3) Consistency: The biggest difficulty of the full-color screen is consistency, or the most easily noticed problem by the audience is poor consistency. The screen appears dark and light in one place, commonly known as the mosaic phenomenon, which is the most annoying. There are many reasons for poor consistency, including the selection of light-emitting diodes, driving circuits, structural design and construction. However, unless the dispersion 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 screen is that they cannot observe the mosaic phenomenon (strictly speaking, it is very slight rather than non-existent, and there is some difference between the observation of ordinary audiences and professionals). This screen is very distinctive in terms of structural process, and does not use the module potting method commonly used in China, which is worth studying. Usually, light-emitting diode manufacturers divide light-emitting diodes into several grades according to brightness and chromaticity. Differences within the same grade can be solved through careful design and debugging. Poor structural design and construction is another major cause of mosaic phenomenon, and it is often only discovered after the screen is installed and put into operation, and it is too late to remedy it, so special efforts should be made. If there is a certain gap in the electrical indicators of domestic manufacturers' displays, then the gap in structure and process is much larger, which often seriously affects the display effect. (4) Gray level The experience of the Hunan Road screen and many foreign screens proves that the gray level must be high enough. Currently, high-end products generally display 1024 levels of grayscale, totaling 1.07 billion colors on a single screen. Domestic products mostly use 256 levels, a significant difference. Many manufacturers claim 1024/2048/4096 or even 16384 levels of encoding, achieving 16.7 million colors with 256 levels of non-linear display. Some argue that 256 levels of non-linear grayscale should be sufficient for large screens, as the HDTV standard also specifies 256 levels. However, according to my tests, this is not the case; the actual effect differs greatly. In fact, achieving grayscale levels higher than 1024 increases the technical difficulty considerably. A simple calculation clarifies this. At a refresh rate of 400Hz, with 1024 levels of non-linear grayscale and 32 brightness adjustment levels, if the correction factor is 2.8, the minimum width of the PCM pulse should be around 10ns, already reaching the bandwidth limit of the drive circuit. This is practically impossible to achieve; a combined approach of amplitude modulation and width modulation must be considered. There is also the issue of transmission bandwidth. When the number of rows and columns on the screen is large and the refresh rate requirement is high (e.g., above 400Hz), the difficulty of data transmission will also increase. Sometimes, fiber optic transmission must be used to ensure the quality of the image. Therefore, the grayscale level needs to be increased from 256 nonlinear levels to 1024 nonlinear levels. (5) Digital processing capability To obtain a high-quality image, a high-quality video signal source is required. For general large-screen users, the best signal they can get is broadcast television signal. DVDs and VCDs are only home-use. Therefore, digital processing of the input signal and improvement of image quality are essential. In fact, everyone has this experience: when playing selected programs, the display effect is very satisfactory, but if it is a live broadcast of television programs or a VCD/DVD, there will inevitably be various scenes and shots, such as dark scenes like night scenes or large areas of bright scenes with high brightness. At that time, the effect will be greatly reduced. How can we achieve satisfactory results under various scenes and shots? This requires digital signal processing, such as digital comb filtering, filtering and noise reduction, edge sharpening, motion prediction and compensation, color correction, nonlinear correction, etc. Of particular note is the scaling (SCALE) processing, a special problem for LED large screens, which requires significant research from the large screen industry. Since the number of pixels on a large screen is always less than that of the video information source, scaling problems are inevitable during playback. Losses in sampling and reproduction, as well as aliasing and defects, will cause a decline in image quality. How to reduce and minimize these losses requires digital signal processing technology. Digital image processing technology has made great progress and has achieved many results, such as the technological achievements in large-size televisions or projectors, which can be directly applied to large screen displays. The processing power of digital processing systems is concentrated in the number of bits and processing speed, and the use of DSPs or high-end FPGAs is an inevitable trend. (6) Reliability and Lifespan Reliability is the lifeblood of a display screen. Whether used for image projects or commercial operations, stability and reliability are of paramount importance. Reliability indicators are expressed by mean time between failures (MTBF) and mean time to repair (MTTR). An MTBF of 10,000 hours is a very high indicator, and it is not necessarily required in practice. Many manufacturers have put a lot of effort into improving the reliability of their systems, but they cannot provide convincing data on the level they have achieved. Conclusion At present, no reliability tests have been conducted on large screens in China. The foundation of reliability lies in rigorous reliability design and strict quality management control. A display screen is a large electronic device; with proper daily maintenance and regular upkeep, the required MTBF (Mean Time Between Failures) can be reduced, thus saving costs. The logic is simple: if the screen doesn't require continuous long-term operation but rather intermittent work, and during these intervals, maintenance eliminates potential problems, keeping the system in good condition, then malfunctions can be prevented during normal operation. In this way, even a 1000-hour MTBF can ensure stable and reliable system operation. The Hunan Road display screen has been operating normally for eight months since its opening, with no significant decline in key indicators. This is not only due to high system reliability but also to excellent maintenance. More importantly, the screen's structural design and spare parts are well-equipped, achieving an MTBF of 15 minutes—a model we should particularly emulate.