Group Synchronization for Multimedia Systems

Introduction

An important feature of multimedia applications is the integration of multiple media streams that have to be presented in a synchronized fashion (Li et al., 1997). Synchronization is mainly the preservation of the temporal constraints within and among multimedia data streams at the time of playout. Temporal relations define the temporal dependencies between media objects (Blakowski & Steinmetz, 1996). An example of a temporal relation is the relation between a video and an audio object that are recorded during a concert. If these objects are presented, the temporal relation during the presentations of the two media objects must correspond to the temporal relation at the time of recording. Discrete media like text, graphics, and images are time-independent media objects, while the semantic of their respective content does not depend upon a presentation to the time domain. A discrete media object is frequently presented using one presentation unit. On the contrary, a time-dependent media object is presented as a media stream. In a continuous media stream (e.g. video), the presentation durations of all units of a time-dependent media object are equal (Li et al., 1997). For example, a video consists of a number of ordered frames. Each of these frames has fixed presentation duration.

Most of the components of a multimedia system support and address temporal synchronization. These components include the operating system, communication subsystem, databases, documents and even applications. In distributed multimedia systems, networks introduce random delays in the delivery of multimedia information, and there are four sources of asynchrony that can disrupt synchronization (Akyildiz & Yen, 1996):

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  Delay Jitter and Local Clock Drift.

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  Different Initial Collection Times. Let us consider two media sources, one providing voice and the other video. If these sources start to collect their media units (MUs) at different times, the playback of the MUs of voice and video from the two sources at the receiver loses semantic meaning.

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  Different Initial Playback Times. If the initial playback times are different for each user, then asynchrony will arise.

To ensure synchronized delivery of multimedia information, intelligent synchronization protocols/techniques are required. This article illustrates various issues on intra- and inter-media synchronization and presents the basic schemes for inter-destination media synchronization (IDMS). It presents IDMS standardization efforts and novel solutions for new multimedia applications. Finally, it outlines future research directions for IDMS.