| Most of traditional video monitoring system is based on the wiring to complete the video information transmission, the need for field wiring, installation of camera equipment, and then through the network cable, serial port connected to the receiving place and then monitoring. In this way, there are many troubles such as the construction of the complex, line modification and installation difficulties, making use of the high costs. Therefore, the technology of wireless video monitoring is developing fast in recent years as an effective complement and extension. The devices of embedded wireless video monitoring do not require complex construction, they are convenient and cheap. The key technology mainly involves several areas, such as embedded platform technology, image encoding/decoding and processing technology, wireless internet technology. In particular, we need to build the embedded OS for different platforms running the program.The program runs on mini2440 development board, the system of the platform includes three parts, bootloader, Linux kernel, root filesystem. They are all open-source programs. Two of them need to be modified according to our own programs. The overall design of the system's application comes after the establishment of the platform's system, according to the existing monitoring products and the features of wireless video monitoring designed for the applications'overall functions and GUI. First, photo browser application, it can browse the photos which come from the screenshot function in the wireless video monitoring application module. Second, video player application, it can play the video files which also come from the videotape function in the wireless video monitoring application. Last, adding interfaces of MP3 player and game applications in order to improve the overall design.Designing GUI according to each application's functions, the implementation of each GUI is based on open-source library SDL. Applications use touch screen to control the GUI and the response of function. Programming the testing and correction application of the touch screen, and then coming into touch screen response thread. Amending some of the thread's code according to each application, it can be easily added into application.Wireless video monitoring application must also inseparable from communication technology, this application uses wireless card to communicate and the card type is ZD1211b, is based on WI-FI protocol. The card's transmission distance is about 200 meters with the effective bandwidth of 54M. The distance and bandwidth fully meet the system application. The driver of wireless card and touch screen is already in the Linux kernel, selecting the card and touch screen driver when configuring the kernel. The wireless card uses RTP to control the transmission. Using RTP to create a session and then using RTCP to control the session. This application uses open-source library JRTPLIB with a specific version of JRTPLIB-3.7.1. This version is a second development of the library. JRTPLIB is a real-time transport protocol which is based on UDP. When establishing a RTP session, there will be a RTCP to control it automatically. RTPLIB-3.7.1 is written in C + + language, it can run in a variety of operating system. Of course, adding your own RTCP implementation is also allowed. RTCP has its own interface to program.After receiving the video, the video information needs to be decoded and display on the LCD screen. The open source libraries FFmpeg and SDL can do that. FFmpeg is a very good open source solution of audio/video encoding and decoding, while both format conversion, camera drivers, audio, player, streaming media and other functions. It is very powerful and can be used in many kinds of operating system such as Linux and Windows operating system. It already supports for very large audio/video formats, if some do not have the encoding format, expansion pack of format also can be easily added to FFmpeg's solution.SDL is a cross-platform and support library for games and multimedia, can be a very good run on multiple platforms such as FrameBuffer console of Linux, XWindows, as well as DirectX widely used Windows operating system and so on. On its official web site there are a lot of secondary development SDL-based library files, the system of applications using the SDL-1.2.13 release.These two libraries need to be cross-compiled and built into shared library, in the decoding process using two formats. One is MEPG2 which is a video format and the other is JPEG which is image format. In fact the format is not important as long as the specified decoder ID when decoding. The key is what format of video or pictures being sent to. After trying a variety of video and pictures, the two formats above have been proved suitable for the application. X264 encoding format is very high efficient, can largely reduce network bandwidth consumption. But the X264 encoding algorithm complexity is too high that the Samsung S3C2440A processor can not handle it. The performance of JPEG format is much better when the screen jitter. So at last JPEG image is a better choice besides that the camera obtains the compressed JPEG format images by hardware which can reduce the algorithm complexity of sender as well. In the LCD video display is achieved through the SDL library functions. |
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