1. Requirements for image mosaic processorAs the pixel spacing of LED display screen becomes smaller and the viewing distance becomes closer, in order to achieve excellent display effect, not only the LED display screen itself is required to improve in image processing and assembly technology, but also higher requirements are put forward for the image splicing processor at the front end of LED display screen (hereinafter referred to as splicer):(1) Verify the output synchronization to avoid the asynchronous phenomenon of splicing pictures;(2) Optimize the image processing algorithm to maintain the high definition of the scaled image;(3) Customize the output resolution to deal with the irregular physical resolution of LED display screen.2. Splicing technology applied to small spacing LED display screen2.1 combination of splicer and small spacing LED displayA key application of splicer is that it can output multi-channel DVI signals to splice and display multiple display screens arranged in matrix, so as to make it a complete display area in logic.For the LED display screen, we can define the display area driven by an LED controller as an independent LED display screen. The current LED controller uses DVI / HDMI as the signal input interface and supports the maximum input resolution of 1920 1200 60Hz, the maximum bandwidth is 165mhz, and the maximum physical resolution of the driven LED display is 1920 1200With the increasing display area of LED small spacing products, projects of tens of square meters are common, and the physical resolution of LED display screen often exceeds 1920 1200, that is, each super large-scale LED display screen is composed of several independent display areas driven by several LED controllers. For the application of splicer, it only needs to provide several DVI output interfaces corresponding to the number of LED controllers and splice the whole LED screen.In the application of splicer in small spacing LED display screen, several key technologies deserve attention:(1) Output synchronization of signalThe multi-channel DVI signal output of splicer must have the problem of signal synchronization. When the asynchronous signal is output to the LED display screen, the picture will be torn at the splicing, especially when playing high-speed moving images. How to ensure the output synchronization of signal has become the key to measure the success or failure of a splicing system.(2) Graphics processing algorithmWe know that the point-to-point image display effect is the best. For the reduced image, if only ordinary graphics processing technology or general FPGA graphics processing algorithm is used, the edge of the image will appear sawtooth, even pixel loss, and the brightness of the image will decrease. The high-end image processing chip or FPGA system using complex graphics processing algorithm will maximize the display effect of the reduced image. Therefore, good graphics processing algorithm is a key technology of splicer applied to small spacing LED display screen.(3) Non standard resolution outputThe small spacing LED display screen is spliced by a matrix of display units of the same specification. The size and physical resolution of each display unit are fixed, but the whole large screen spliced is often not a standard physical resolution. For example, the resolution of the display unit is 128 96, can only spell 1920 1152, but can't spell 1920 1080 In the super large-scale splicing system, the LED display area driven by each LED controller may not be the standard resolution. At this time, the output of the splicer with non-standard resolution is critical. It can help us quickly find the appropriate splicing method, so as to reasonably allocate resources and effectively save the number of LED controllers and transmission equipment.2.2 splicer for small spacing LED displayAt present, splicers can be divided into four categories: embedded pure hardware architecture, PCI-E bus architecture, distributed network architecture and hybrid architecture.(1) Embedded hardware only architectureThe whole machine structure usually adopts the design of "backplane signal acquisition board main control board signal output board". The signal acquisition board performs signal processing such as video acquisition, scaling, superposition and format conversion. The processed signals are transmitted to the FPGA signal processing system of the main control board through the backplane bus, and the configuration of the main control FPGA is realized through the embedded ARM system Communication with upper PC, data exchange between systems and other functions, and output the signal to the display terminal through the signal output board.The structure of the splicer with pure hardware architecture is relatively simple and not prone to system failure; The acquisition board and output board are hot pluggable and easy to replace; It can realize the acquisition and processing of multi-channel and multi format signals; Backplane switching technology and output board unified clock technology ensure the synchronization of multi-channel signal output; The resolution of each DVI output signal can be customized, which conforms to the splicing characteristics of LED display screen.Many characteristics make the pure hardware architecture quickly become one of the mainstream products in the field of splicer. However, due to the use of FPGA as the core image processing unit, the quality of the algorithm determines the processing effect of a splicer, especially the image scaling algorithm. How to optimize to achieve a clearer display effect has become an important index to determine the product value of pure hardware splicer.(2) PCI-E bus architectureUsually, the splicer of bus architecture adopts PCI Express technology, and the available data bandwidth is up to hundreds of Gbps. The host is equipped with high-performance CPU and large memory. Different operating systems (such as 64 bit windows7) can be pre installed according to different application fields, and various application programs can be run directly. The splicer is equipped with multiple high-performance graphics output cards. Each output card has ultra-high internal bandwidth and video memory, and all output images are synchronized to eliminate image tearing between display units. At the same time, it is also equipped with multiple input cards, supports a variety of signal formats, and can process the input signals.PCI-E bus architecture splicer is a high-performance computer. All components use the most advanced and mature technologies of major hardware manufacturers. For example, Intel can be used as CPU and NVIDIA can be used as graphics card. All high and new technologies in the computer field can also be quickly integrated. This makes the PCI-E bus architecture splicer have incomparable advantages in operation speed, image processing, operation mode and so on.The threshold of PCI-E bus architecture splicer is very low. For simple applications, an industrial computer and a professional multi-channel output graphics card can be realized.On the other hand, how to solve the problem of system stability, how to design an intuitive and powerful control software, and how to solve various problems of data transmission under high bus bandwidth all need a strong R & D team and strong capital foundation, as well as the accumulation of experience. In other words, the high-end PCI-E bus architecture splicer not only needs to meet the most basic applications such as signal acquisition, processing and splicing, but also needs more investment in the design of system stability and software ease of use, so as to make the splicer meet various harsh application environments.However, it should be noted that most bus architecture splicers use Windows operating system. Once attacked by virus, the system may be paralyzed and stop displaying. Moreover, due to the use of customized graphics cards, the resolution of each output channel generally needs to comply with the VESA (Video Electronic Standards Association) standard. Non standard resolution output cannot be defined, and different resolution of each channel cannot be defined.(3) Distributed network architectureThe splicer of distributed network architecture usually adopts node hardware structure. Each input and output node is independent and connected to the central switch through twisted pair to transmit data interactively.Its core is a set of advanced video coding and decoding technology. Through various signal input nodes, the collected DVI, VGA, YPbPr, CVBS, 3g-sdi and other signals are processed and encoded. Through the special network communication protocol, the encoded video is transmitted through the central switch to the output node for decoding, converted into DVI digital signal and output to the display terminal.The synchronization of output nodes has become the key to the application of the system. One method is to send the synchronization code directly through the network to realize the synchronous output of multiple output nodes. However, due to the existence of network bit error rate, the output will still be out of synchronization after this mode runs for a period of time. Another method is to physically connect multiple output nodes through the sync interface, select one output node as the host, and actively send synchronization codes to other output nodes, so that all output nodes can receive synchronization signals at the same time, so as to realize real frame synchronous output, so as to ensure that the displayed image is complete and there is no tear at the screen splicing.At present, the application of distributed network architecture splicing system is more and more. Due to its distributed characteristics, it is convenient for the integrated wiring in the whole building and the centralized management of multiple display terminals in different areas. With the help of advanced visualization software, it can provide users with humanized, visual and comprehensive services.However, limited by bandwidth and codec technology, the distributed network architecture does not support the access of dual link DVI digital signal and HDMI signal. At the same time, because the coding, processing, decoding, signal synchronous output and other links all need frame cache, there is a gap in the real-time performance of data compared with other splicing technologies. In addition, the number of point-to-point points to be displayed exceeds 1920 For 1200 resolution images (more than two signal input nodes are required), the resynchronization output of multi-channel synchronization source input signals cannot be guaranteed.(4) Hybrid architectureHybrid architecture generally refers to the splicer or splicing system in which two or more of the above three splicing technologies are combined.For example, PCI hardware backplane bus architecture splicer, its system control and image processing are realized independently. PCI bus is responsible for system control and running the operating system in the background; The hardware backplane bus is responsible for video image processing. The system allows synchronous processing of a large number of high-resolution input signals. At the same time, it can still maintain real-time operation performance and best image quality at full frame rate, and ensure the synchronization of output signals. For important emergency places, it can ensure that the screen will never be black. Even if the operating system in charge of PCI bus fails or virus infection, it can ensure that foreign video images can be displayed at any time through the special backplane graphics processing bus.Through the hybrid architecture, it can be applied comprehensively, learn from each other, and greatly increase the stability of the system. This is also the development direction of splicing technology in the future, and has a broader application space.3. Application of small spacing LED displayAt present, small spacing LED display is widely used, including but not limited to:Military exercise command systemPublic safety display command systemPower dispatching systemTraffic network and aviation monitoring display systemEnergy industry production scheduling systemConference display system for government, enterprises and institutionsRadio and television media display systemPublic place information release systemAs a new generation of background wall display terminal, small spacing LED display is providing high-quality services for key systems in all walks of life.