The application of high-resolution LED display is increasingly widespread. At present, the playback control system of domestic LED display is mainly realized by online mode, that is, PC + video transmission board, and the video is sent to LED in real time through Gigabit Ethernet or optical fiber. The display is completed on the video receiving board at the display. This method has a good man-machine interface and excellent video display quality, and is an important display means for online display of close-range LED displays. For ultra-long-distance, outdoor LED displays, online play obviously needs to be paid a high price to achieve, and in some cases even impossible. The offline mode shows its unique advantages in these occasions. The offline mode is characterized by unattended operation, strong specificity, low resource consumption, and economical portability. At the same time, the playback process and content can be intervened and modified in real time through certain control channels such as the Internet and GSM network. At present, domestic offline control systems are mostly implemented by embedded systems such as single-chip microcomputers, SOC, DSP, RISC processors. Most offline control cards can only display smaller resolutions, support pseudo-color or monochrome, and cannot play high-quality video. This design aims to realize a small dedicated LED offline playback system with high-resolution, high-quality video playback, streaming media playback, multi-region display and other functions. This paper will explain the hardware structure and principle of the system, the design and implementation of each module of the software, the performance test and the conclusion.
2 offline playback system hardware structure and working principle
At present, the domestic LED screen offline control system mostly adopts high-performance RISC processor as the core. The typical operating frequency of the SoC chip with ARM9 as the core is mostly between 200 and 600 MHz. However, the performance of the display is still relatively low. Most chips, such as the S3C24X0, implement display functions through an integrated LCD controller. They generally only support display resolutions up to 640X480 and 16-bit color depth. The LCD controller built into the processor chip sets a small portion of the system memory as a frame memory (
On the other hand, for general 2D graphics operations such as pixel copying, scaling, color space conversion, etc., software implementation is required, processor pair
In view of the above analysis, the use of ordinary embedded architecture for handheld devices can not meet the display requirements of full-color large LED screens. This paper selects the industrial-grade ARM chip S3C2440+SM501 display acceleration coprocessor solution, which fundamentally solves The above analysis of two key issues affecting display performance.
Figure 1 Block diagram of the offline playback system hardware
3 Establish an embedded Linux system platform
The Linux kernel has small size, high efficiency, mature and stable, open source code, rich resources, and the kernel directly provides rich network protocols, supporting multiple file systems and many other advantages. This topic transplants the Linux2.6.18 kernel, which has the advantages of good stability and good development tool support. This part of the work mainly includes:
1) Transplant u-boot for hardware platform.
2) Trim and cross-compile the Linux kernel. Porting USB storage disk, DM9000A network chip, SM501 graphics card driver to the kernel
3) Cross-compile busybox to generate Initrd file system image in ext2 format
4) Burn the kernel and Initrd image to Nand Flash via u-boot, set kernel boot parameters and u-boot boot command to start the kernel.
After the above work, a miniaturized embedded Linux platform was established. Due to space limitations, this article does not provide an in-depth introduction to the details of the embedded Linux platform.
4 LED display offline dedicated playback software design and implementation
The main design goal of the LED display offline playback software is to support video playback, picture display, multi-region display, streaming media playback, and remote control. Because the whole software project is relatively large, this article will only select a few key modules for detailed description, and focus on how to use the SM501 to achieve high-performance display and video playback. The streaming media playback and remote control sections are not introduced.
5 Conclusion
In this paper, high-speed MCU and SM501 embedded graphics are used as hardware platforms, which breaks through the bottleneck of embedded system performance and clear interface. The Linux 2.6 kernel was ported as a software platform on the software, which made a good balance between efficiency and portability. Using good software design ideas, the LED offline playback software with open architecture was developed. The system has been successfully applied to offline playback and control of full color LED displays.
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