Immersive, interactive applications require on-line processing and mixing of multimedia data. In order to realize the lmmersipresence vision, we propose a generic, extensible, modular multimedia system software architecture. We describe here the Flow Scheduling Framework (FSF), that constitutes the core of its middleware layer.The FSF is an extensible set of classes that provide basic synchronization functionality and composition mechanisms to develop data-stream processing components. In this dataflow approach, applications are implemented by specifying data streams and their path through processing nodes, where they can undergo various manipulations. We describe the details of the FSF data and processing model that supports stream synchronization in a concurrent processing framework. We illustrate the FSF concepts with a tea/-t/me video stream processing application.