We propose a method for fine grain QoS control of real-time applications. The method allows adapting the overall system behavior by adequately setting the quality level parameters of its actions. The objective of the control policy is to meet QoS requirements including three types of properties: 1) safety that is, no deadline is missed; 2) optimality that is, maximization of the available time budget; 3) smoothness of quality levels. The method takes as input a model of the application software, QoS requirements and platform-dependent timing information, and produces a controlled application software meeting the QoS requirements on the target platform. This paper provides a complete formalization of the quality control problem. It proposes a new control management policy ensuring safety, near-optimality and smoothness. It also describes a prototype tool implementing the quality control algorithm and experimental results about its application to a video encoder. Copyright 2005 ACM.