【作者】 王广林；

【导师】 杨宗凯； 赵刚；

【作者基本信息】 华中师范大学 ， 计算机软件与理论， 2008， 硕士

【摘要】 网络技术与多媒体技术的发展,使得在网络上实现视频、音频、图像等多媒体数据成为可能。现代远程教育正是在这个背景下产生的,并得到了迅速的发展。同时,虚拟演播室成本的降低使其应用范围越来越大。它在教育领域的应用,一方面丰富了教学内容,另一方面也给教学方式带来了创新。本文提出把虚拟演播室和现代远程教育结合起来,充分发挥两者的优势,开发出虚拟课堂教学系统。在多媒体编码方面,为了满足虚拟课堂教学系统实时性的要求,采用硬件压缩编码。其中,视频编码采用MPEG-4编码器,音频编码采用MP3编码器。MPEG-4是一个多媒体通信标准,它的应用面非常广泛,既可以应用于高质量的数字电视,又可以应用于极低码率的移动多媒体通信系统。虚拟课堂教学系统采用ASP档次的编码硬件实现视频编码。在网络传输方面,根据虚拟课堂教学系统“一对多”教学模式的特点,采用组播技术实现多媒体数据的传输。相对于单播和广播,组播在实现一对多传输时能够大大地减轻骨干网负载,在传输数据量比较大的应用中这种优势更为明显。所以,组播技术特别适合于视频、音频、图像等多媒体数据的传输。在应用层,采用RTP/RTCP协议进行多媒体数据的传输和流量控制。本文对RTCP的发送时间间隔,丢包率的计算,时延抖动的计算进行了深入的研究。分布在世界各地的接收端,其网络接入状况可能不同,如,有的用户通过电话线接入Internet,有的用户通过综合业务数字网(ISDN)的方式接入Internet,有的用户通过Ethernet接入Internet等。这就对组播数据量提出了挑战,如果以接入速率大的用户带宽为标准提供组播数据传输,那么接入速率小的用户就无法实现视频数据的正常播放;如果以接入速率小的用户带宽为标准提供组播数据传输,那么接入速率大的用户就浪费了大量带宽,影响视频回放的效果。为了解决这一矛盾,本文用分层组播实现视频传输,满足了不同网络接入带宽用户的需求。最后,把学生端接收模块开发成一个ActiveX控件。通过把ActiveX控件嵌入网页的方法,给学生端接收多媒体数据提供了方便。学生端只要安装有浏览器软件即可。第一部分介绍了虚拟课堂教学系统所涉及到的关键技术。第二部分介绍了系统的设计和实现,包括虚拟课堂部分的设计和虚拟课堂教学系统直播部分的设计和实现。在实现虚拟课堂教学系统直播部分的同时,对其中涉及的关键问题进行了研究和分析。更多还原

【Abstract】 The network and multimedia technology made great development, it makes the transmission of video, audio and picture realistic. At the same time,modern distance education comes into being. With the cost of virtual studio decreasing,it is popular.The application of virtual studio in education enriches the courseware and innovates the method of teaching.The virtual class teaching system combines the advantage of modern distance education and virtual studio.The practice shows that it attracts the student’s attention and improves the efficiency of learning.Content-based encoding is the characteristic of MPEG-4.It takes the corresponding method according to the different content to achieve the high rate compressing.MPEG-4 is so popular that it can not only be used to high quality digital television, but also be used to extreme low rate application,i.e.mobile multimedia communication .Virutal class teaching system adopts the hardware encoder complying with MPEG-4 ASP profile.RTP/RTCP is a special protocol for multimedia communication.RTP is used to transmit the real-time data, but it does not control data flow. It needs RTCP’s assistance.The application uses the RTCP to control congestion and communicate other control information.So, it needs to make deep research on the interval of RTCP report’s transmission, packet lost rate and jitter.Corresponding to unicast and broadcast, multicast can definitely decrease the traffic load of bone network.Therefore, multicast is suitable for the transmission of multimedia. The control plane and data plane is implemented. The receiver module is embedded into a ActiveX control. Therefore, it made the installation of software convenient.Internet access maybe have different bandwidth.For example,some may be accessed through the telephone line,some may be accessed through ISDN ,some may be accessed through Ethernet.This is a big challenge.If the traffic rate is low,the client accessed through Ethernet wastes much of bandwidth,reducing the performance of playback;if the traffic rate is high,the client accessed through telephone line can’t receive the data regularly.To solve this problem ,the scalable multicast is realized.The different kinds of frame is transmited through different multicast address.At the multicast receiver,the data received should be sorted for playback. With the SR report ,the synchronization of video and audio is implemented using the RTP timestamp and NTP timestamp.更多还原