Symbolic quality control for multimedia applications

We present a fine grain quality control method for multimedia applications. The method takes as input an application software composed of actions. The execution times of actions are unknown increasing functions of quality level parameters. The method allows the construction of a Controller which computes adequate action schedules and corresponding quality levels, so as to meet QoS requirements for a given platform. These include requirements for safety (action deadlines are met) as well optimality (maximization and smoothness of quality levels). The Controller consists of a Quality Manager and a Scheduler. For each action, the Controller uses a quality management policy for choosing a schedule and quality levels meeting the QoS requirements. The schedule is selected amongst a set of optimal schedules computed by the Scheduler. We extend and improve results of previous papers providing a solid theoretical basis for designing and implementing the Controller. We propose a symbolic quality management method using speed diagrams, a representation of the controlled system's dynamics. Instead of numerically computing a quality level for each action, the Quality Manager changes action quality levels based on the knowledge of constraints characterizing control relaxation regions. These are sets of states in which quality management for a given number of computation steps can be relaxed without degrading quality. We study techniques for efficient computation of optimal schedules. We present experimental results including the implementation of the method and benchmarks for an MPEG4 video encoder. The benchmarks show drastic performance improvement for controlled quality with respect to constant quality. They also show that symbolic quality management allows significant reduction of the overhead with respect to numeric quality management. Finally, using optimal schedules can lead to considerable performance gains. © 2008 Springer Science+Business Media, LLC

Tavarua : a mobile telemedicine system using WWAN striping

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (p. 69-78).Tavarua is a platform designed to support mobile telemedicine systems over wireless wide area networks, WWANs. It utilizes network striping and other complementary techniques to send uni-directional near real time video and audio data streams from a imobile node to a stationary location. The key technical challenge is transmitting high-bandwidth, loss-sensitive data over multiple low-bandwidth, lossy channels. We overcome these challenges using dynamic adjustment of the encoding parameters and a novel video encoding technique (grid encoding) that minimizes the impact of packet losses. Using five WWAN interfaces, our system reliably and consistently transmits audio and diagnostic quality video, with median PSNR values that range from 33.716dB to 36.670dB, with near real-time latencies. We present a study of the characteristic behavior of WWANs, and a description of our system architecture based in part on the lessons gleaned from that study. Through a set of experiments where we transmit video and audio data from a moving vehicle we evaluate the system, focusing on consistency, reliability, and the quality of the audio and video streams. These experiments demonstrate that we can transmit high quality video and audio in varying conditions and even in the presence of hardware failures.by Jennifer Carlisle.S.M

A rate control algorithm for scalable video coding

This thesis proposes a rate control (RC) algorithm for H.264/scalable video coding (SVC) specially designed for real-time variable bit rate (VBR) applications with buffer constraints. The VBR controller assumes that consecutive pictures within the same scene often exhibit similar degrees of complexity, and aims to prevent unnecessary quantization parameter (QP) fluctuations by allowing for just an incremental variation of QP with respect to that of the previous picture. In order to adapt this idea to H.264/SVC, a rate controller is located at each dependency layer (spatial or coarse grain scalability) so that each rate controller is responsible for determining the proper QP increment. Actually, one of the main contributions of the thesis is a QP increment regression model that is based on Gaussian processes. This model has been derived from some observations drawn from a discrete set of representative encoding states. Two real-time application scenarios were simulated to assess the performance of the VBR controller with respect to two well-known RC methods. The experimental results show that our proposal achieves an excellent performance in terms of quality consistency, buffer control, adjustment to the target bit rate, and computational complexity. Moreover, unlike typical RC algorithms for SVC that only satisfy the hypothetical reference decoder (HRD) constraints for the highest temporal resolution sub-stream of each dependency layer, the proposed VBR controller also delivers HRD-compliant sub-streams with lower temporal resolutions.To this end, a novel approach that uses a set of buffers (one per temporal resolution sub-stream) within a dependency layer has been built on top of the RC algorithm.The proposed approach aims to simultaneously control the buffer levels for overflow and underflow prevention, while maximizing the reconstructed video quality of the corresponding sub-streams. This in-layer multibuffer framework for rate-controlled SVC does not require additional dependency layers to deliver different HRD-compliant temporal resolutions for a given video source, thus improving the coding e ciency when compared to typical SVC encoder con gurations since, for the same target bit rate, less layers are encoded