6. Conclusion 5. Related Work

based media is now found in audio/video servers (e.g., [1][18]). These systems are likely precursors of more general audio/video databases. We have presented a data model that addresses the representation of time-based media. The model includes the notions of media objects, media elements and timed streams. Three general structuring mechanisms are used: interpretation, derivation and composition. Many of these ideas have evolved from existing systems. Our goal has been to unify these ideas, provide a more formal foundation, and express them in a way suitable for data modeling. Most of the examples in this paper deal with audio and video, we have given less attention to other media types such as graphics, music, and animation. However , the data model does encompass these types. The key is derivation: animation and music deal with symbolic representations from which audio or video sequences are derived. For example, music representations allow the specification of note sequences. A synthesizer then takes these sequences and derives audio sequences. Similarly video sequences are derived (via rendering) from representations of animation. Implementation of database systems capable of supporting time-based media presents many new problems in such areas as scheduling, synchronization, resource allocation, and storage management. Authorization and electronic copyright need to be addressed. Support for time-based media requires basic changes in database architecture and the form of the database/application interface. The notion of timed streams introduced in this paper leads to a perspective where database operations are viewed as extended activities that produce, consume and transform flows of data. A database architecture based on activities and their interconnection is explored in [5]. 19 Figure 5). It is the higher-level abstractions–media objects and elements, multime-dia objects–that are seen by applications. The areas that have influenced the data model include digital video and digital audio processing systems (e.g., editors, composition tools) and developments in multime-dia platforms. QuickTime in particular has served as a concrete example of the mechanics of media interpretation and timing. QuickTime defines three structures, called media, tracks and movies–these correspond to what we have called non-derived media objects, derived media objects and multimedia objects. The main difference is that derivation is more general than QuickTime's media-to-track mapping. In QuickTime a track is created by rearranging the timing of media elements. Derivations, however, encompass changes in content and type in addition to changes in timing. The notion of timed streams bears some resemblance to work in temporal …