The MPEG-2 standard for video coding is predominant in consumer electronics for DVD players, digital satellite receivers, and TVs today. MPEG-2 processing puts high demands on audio/video quality, which is achieved by continuous and synchronized playout without interrupts. At the same time, there are restrictions on the storage media, e.g.., limited size of a DVD disc, communication media, e.g., limited bandwidth of the Internet, display devices, e.g., the processing power, memory and battery life of pocket PCs or video mobile phones, and finally the users, i.e., humans ability of perceiving motion. If the available resources are not sufficient to process a full-size MPEG-2 video, then video stream adaptation must take place. However, this should be done carefully, since in high quality devices, drops in perceived video quality are not tolerated by consumers.
We propose real-time methods for resource reservation of MPEG-2video stream processing and introduce flexible scheduling mechanisms for video decoding. Our method is a mixed offline and online approach for scheduling of periodic, aperiodic and sporadic tasks, based on slot shifting. We use the offline part of slot shifting to eliminate all types of complex task constraints before the runtime of the system. Then, we propose an online guarantee algorithm for dealing with dynamically arriving tasks. Aperiodic and sporadic tasks are incorporated into the offline schedule by making use of the unused resources and leeways in the schedule. Sporadic tasks are guaranteed offline for the worst-case arrival patterns and scheduled online, where an online algorithm keeps track of arrivals of instances of sporadic tasks to reduce pessimism about future sporadic arrivals and improve response times and acceptance of firm aperiodic tasks. At runtime, our mechanism ensures feasible execution of tasks with complex constraints in the presence of additional tasks or overloads.
We use the scheduling and resource reservation mechanism above to flexibly process MPEG-2 video streams. First, we present results from a study of realistic MPEG-2 video streams to analyze the validity of common assumptions for software decoding and identify a number of misconceptions. Then, we identify constraints imposed by frame buffer handling and discuss their implications on the decoding architecture and timing. Furthermore, we propose realistic timing constraints demanded by high quality MPEG-2 software video decoding. Based on these, we present a MPEG-2 video frame selection algorithm with focus on high video quality perceived by the users, which fully utilize limited resources. Given that not all frames in a stream can be processed, it selects those which will provide the best picture quality while matching the available resources, starting only such decoding, which is guaranteed to be completed. As a final result, we provide a real-time method for flexible scheduling of media processing in resource constrained system. Results from study based on realistic MPEG-2 video underline the effectiveness of our approach.
2004. , p. 238