Guest Editor's Introduction

Journal of Signal Processing Systems -     

Modeling the effect of reliability on performance

In many high-reliability systems, subsystem or component failures that do not cause a system failure can still degrade system performance. When modeling such systems, ignoring the effect of reliability on performance can lead to incomplete or inaccurate results. In the present work, performability modeling, the combined analysis of reliability and performance, is introduced; some examples of applications where performance and reliability need to be modeled together are given; a strategy for modeling the effect of reliability on performance is outlined and metrics that help quantify this effect are discussed. Some mathematical models for performability are introduced and an example is used to illustrate how such models can be applied     

Error-resilient transcoding for video over wireless channels

We describe a method to maintain quality for video transported over wireless channels. The method is built on three fundamental blocks. First, we use a transcoder that injects spatial and temporal resilience into an encoded bitstream. The amount of resilience is tailored to the content of the video and the prevailing error conditions, as characterized by bit error rate. Second, we derive analytical models that characterize how corruption propagates in a video that is compressed using motion-compensated encoding and subjected to bit errors. Third, we use rate distortion theory to compute the optimal allocation of bit rate among spatial resilience, temporal resilience, and source rate. Furthermore, we use the analytical models to generate the resilience rate distortion functions that are used to compute the optimal resilience. The transcoder then injects this optimal resilience into the bitstream. Simulation results show that using a transcoder to optimally adjust the resilience improves video quality in the presence of errors while maintaining the same input bit rate     

Forward error control for MPEG-2 video transport in a wireless ATM LAN

The possibility of providing multimedia services to mobile users has led to interest in designing broadband wireless networks that can guarantee quality of service for traffic flows. However, a fundamental problem in these networks is that severe losses may occur due to the random fading characteristics of the wireless channel. Error control algorithms which compensate for these losses are required in order to achieve reasonable loss rates. In this paper, the performance of error control based on forward error correction (FEC) for MPEG-2 video transmission in an indoor wireless ATM LAN is studied. A random bit error model and a multipath fading model are used to investigate the effect of errors on video transport. Combined source and channel coding techniques that employ single-layer and scalable MPEG-2 coding to combat channel errors are compared. Simulation results indicate that FEC-based error control in combination with 2-layer video coding techniques can lead to acceptable quality for indoor wireless ATM video.Work performed while the author was at AT&T Bell Laboratories on a D.O.E. fellowship program.     

Quality evaluation of motion-compensated edge artifacts in compressed video.

Little attention has been paid to an impairment common in motion-compensated video compression: the addition of high-frequency (HF) energy as motion compensation displaces blocking artifacts off block boundaries. In this paper, we employ an energy-based approach to measure this motion-compensated edge artifact, using both compressed bitstream information and decoded pixels. We evaluate the performance of our proposed metric, along with several blocking and blurring metrics, on compressed video in two ways. First, ordinal scales are evaluated through a series of expectations that a good quality metric should satisfy: the objective evaluation. Then, the best performing metrics are subjectively evaluated. The same subjective data set is finally used to obtain interval scales to gain more insight. Experimental results show that we accurately estimate the percentage of the added HF energy in compressed video.     

Design of quantizers for decentralized estimation systems

The authors consider parameter estimation in decentralized systems with distributed processors. They restrict the local processors to be quantizers and consider the optimal design of the systems to minimize the estimation error. They present necessary conditions for the optimal system based on the Bayes distortion functions and Fisher's information. The numerical results compare the resulting quantizers obtained by different distortion criteria     

Modeling one- and two-layer variable bit rate video

This paper presents a source model for VBR video traffic. A finite-state Markov chain is shown to accurately model one- and two-layer video of all activity levels on a per source basis. Our model captures the source dynamics, including the short-term correlations essential for studying network performance. The modeling technique is shown to be applicable for both H.261 and MPEG2 encoded video of a variety of activity levels. The traffic model is shown in a simulation study to be able to accurately characterize both the single-source buffer occupancy over a wide range of buffer sizes and the multiplexing behavior. The VBR video model is also used to model the enhancement layer of two-layer SNR scalable video. We show that two-layer encoding has significantly better statistical multiplexing gains than one-layer video, particularly when the network admits calls based on a leaky-bucket characterization