| With the continual breakthrough and innovation of embedded system, network communication, multimedia technology, applications such as network security monitoring and multimedia communication are developed and used widely. Besides, traditional single-media communication is gradually replaced by the multimedia communication which is an integration of video, audio, text and so on. At the same time, people's requirement for the quality of service(QoS) also increases with it. One important problem is how to improve the real-time performance of transmission and maintain the synchronous relationship between audio and video in the premise of media quality. Therefore, this thesis researches on audio and video encoding and synchronous transmission based on embedded Linux platform. The main research contents and achievements are as follows:Firstly, the audio and video coding technology has been studied, and some popular encoding and decoding algorithms are analyzed and compared. After that, H.264 standard which has high compression rate, outstanding performance is selected as the video coding method, and G.729 A standard with low latency, low bit rate and high quality is selected as the way of speech coding. Then the traditional network transmission layer protocols are studied, such as TCP and UDP, their strengths and weaknesses are analyzed, furthermore RTP protocol is considered, which is more suitable for real-time audio and video transmission on the Internet. In addition, the synchronization technologies within and between audio and video streams are discussed, and then with reference to the principle of streaming media, an audio-video streaming synchronous transmission scheme based on time stamp is proposed.Secondly, the requirement of this audio-video acquisition and transmission system based on embedded Linux is analyzed. The hardware platform of the system has been designed and built, including embedded minimum system, audio-video acquiring and processing module, network communication module and so on. Besides, the software environment is deployed, consists of embedded Linux system transplantation, audio-video device drivers integration, audio-video coding libraries and RTP protocol stack's transplantation.Thirdly, the concrete procedure of audio-video's encoding and synchronous transmission on the embedded Linux platform is realized, including video's capturing and H.264 encoding, and audio's capturing and G.729 A encoding. Then the audio and video synchronization strategy is proposed on three periods known as capturing, encoding and RTP packaging. By means of multi-thread, shared memory and semaphore for Linux programming, the synchronous transmission of audio and video is finally achieved.At last, the system experiment of the audio-video coding and synchronous transmission on embedded Linux platform is implemented. Experimental result shows that the audio and video transmission is fluid and in real-time, the received voice and images both have high quality. In the end, with the help of Perceptual Quality of Service, the excellent audio and video synchronization performance is demonstrated. |
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