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Media processing in high-quality multimedia embedded systems (HQMES) has real-time constraints. Timely processing and rendering of video frames and audio samples is essential to meet user expectations. The nature of incoming media suffers unforeseen variations which have different resource requirements. Therefore, HQMES have to integrate policies for efficiently and smoothly adapting to these changes. Mode change protocols allow applications to switch their state (for instance, to transition from one quality level to another) by controlling the way in which the application tasks change from one state to another. This paper provides a solution for timely mode change protocols based on a contract model between applications and the execution platform. A new mode change algorithm, progressive mode change protocol, is introduced for applications with no tolerance to data loss during their transitions. The execution platform is based on a quality of service resource manager (QoSRM) that arbitrates the greedy execution of multimedia applications, and that is implemented on top of the services of a real-time operating system. A task model and a temporal characterization of multimedia application tasks is also presented as the basic platform for the QoSRM operation. Validation experiments show stable execution of applications with the proposed task characterization and progressive mode change protocol.