SYNCHRONIZATION OF MULTIMEDIA STREAMS AND COMMUNICATION SUPPORT FOR THE DISTRIBUTED HYPERMEDIA SYSTEMS

Abstract

Distributed Hypermedia Systems (OHS) is a networked multimedia information system currently being developed at the National University of Singapore. The system is intended to support interactive retrieval of distributed multimedia information by free association using hyperlink. This thesis describes the design of an intermedia synchronization scheme and the implementation of a communication support for DHS. In the proposed synchronization scheme, the synchronization requirements are assumed to have expressed using a script language. A parser at the receiver is used to interpret the script and converts the asynchronous temporal specification into a form of synchronous specification. As a result, each medium playout time is specified in relation to a common master clock that starts at time 0 and advances at the rate of real time. To ensure that each medium is played out at the scheduled time, sufficient time should be given to compensate for the various delays that would be incurred end-to-end Inter-stream synchronization is maintained by requesting each of the media streams required t seconds before its scheduled playout time, where t being the time needed to compensate for the end-to-end delay. In this scheme t , denoted as the control time, is a critical parameter. If t is underestimated, it may lead to data starvation at the sink and hence lost of playout synchronization. If t is overestimated the data streams will arrive before their playout scheduled time, and this may result in buffer overflow. As the delays at the network and the hosts involved are stochastic, t must be determined statistically. The method for determining I and the receiver buffer is presented in this thesis. As for intra-stream synchronization, a medium stream is divided into data units which are to be played out at their scheduled time. Remedy actions have to take place for data units that are not delivered in accordance to their playout schedules. The types of action are dependent on the medium types. For example in the case of audio and video, skipping of the data units that are lost or late is preferred to retransmissions, as the latter will incur additional delays that may increase the delay skew between the related streams. Dividing media streams into smaller data units of fixed duration also facilitates user interactions. Intensive experiments have been carried out to demonstrate the feasibility of the proposed scheme. Their results are presented and discussed in this thesis. The communication support linking the servers and the clients has to be established first , in order to establish a prototype test-bed of DHS for investigation into system requirements for supporting interactive multimedia information retrieval. In our design, the communication support comprises of a set of hyperbase front-end to back-end message passing procedures implemented on top of the BSD's sockets. To demonstrate the functionality of the communication support, a simple application consisting of the retrieval of some images, a voice information and some texts was implemented subsequently. As video capability has not been incorporated into the hyperbase frontend and backend of DHS, the intermedia synchronization scheme could not be tested rigorously.