Clock hierarchies: an abstraction for grouping and controlling media streams

Synchronization plays an important role in multimedia systems at various levels of abstraction. We propose a set of powerful abstractions for controlling and synchronizing continuous media streams in distributed environments. The proposed abstractions are based on a very general computation model, that allows media streams to be processed (i.e. produced, consumed or transformed) by arbitrarily structured networks of linked components. Further, compound components can be composed of existing ones to provide higher levels of abstractions. The clock abstraction is provided to control individual media streams, i.e., streams can be started, paused or scaled by issuing the appropriate clock operations. Clock hierarchies are used hierarchically group related streams, where each clock in the hierarchy identities and controls a certain group, or subgroup of streams. Control and synchronization requirements can be expressed in a uniform manner by associating group members with control or synchronisation attributes. An important property of the concept of clock hierarchy is that it can be combined in a natural way with component nesting