We propose a method for fine grain QoS control of dataflow applications. We assume that the application software is described as the composition of actions (C-functions) with quality level parameters. The method allows to compute a QoS controller from this description, and average execution times, worst case execution times and deadlines for its actions. The controller computes dynamically feasible schedules and quality assignments for their actions. Furthermore, the control policy ensures optimal time budget utilization. A prototype tool implementing the method is shown as well as experimental results for a non trivial example. The results show the interest of fine grain QoS control for video encoders.