第9章　多媒体操作系统 .ppt

所谓媒体片断（segment）就是一组音频和视频数据的索引，包括创建、修改和长度等时间信轨以外还会有“用户信轨”，如字幕轨。 A media segment is a set of references to audia and video data,including time information (creation,modification,duration),language,display or sound quality,media data type and data pointers.Future releases will have,apart from audio and video tracks,“custom tracks” such as a subtitle track. 所有信轨都可以并发地被看到或听到，一个movie的各信轨总是同步，movie不仅是可以播放的也是可以编辑的，可以进行删除、拷贝、粘贴等操作. All track can be viewed or heard concurrently.The racks of a movie are always synchronized.Since movies are documents they cannot only be played (including pausing,stepping through,etc.),but they can also be edited.Operations like cut,copy and paste are possible. movie文档也可以是其他文档中的一部分。QuickTime是可扩展的，可以利用加速器或压缩／解压卡等硬件。 Movie documents can be part of other documents. QuickTime is scalable.Hardware components like accelerator or compressor/decompressor cards can be employed. 为用户提供一组服务，允许将movie引入应用程序，这些应用程序可以直接操作movie中音频和视频数据的特征。 The Movie Toolbox offers a set of services to the user that allows him/her to incorporate movies into applications.These applications may directly manipulate characteristics of audio and video data of movies. 3.抖动（匮称延迟抖动）指的是数据到达目的地所需时间的最大偏差。 3.The jitter (or delay jitter) determines the maximum allowed variance in the arrival of data at the destination. （4）可靠性指的是多媒体任务传输和处理中所使用的错误检测和校正机制。错误可以被忽略、指出或校正。 （4）The reliability defines error detection and correction mechanisms used for the transmission and processing of multimedia tasks.Errors can be ignored,indicated and/or corrected. 值得注意的是，通过重新传输来进行错误校正对于限时任务一般是不合适的，因为重传输数据通常到达较迟。 It is important to notice that error correction through re-transmission is rarely appropriate for time-critical data beacause the retransmitted data will usually arrive late. 资源分配和管理的一个可能实现方案是基于客户和各资源管理器之间的交互。客户选择资源并通过一个QoS说明来指出其需求，从而请求一次资源分配。 One possible realization of resource allocation and management is based on the i