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Title:Design and evaluation of real-time Voice-Over-IP (VOIP) systems with high perceptual conversational quality
Author(s):Sat, Batu
Director of Research:Wah, Benjamin W.
Doctoral Committee Chair(s):Wah, Benjamin W.
Doctoral Committee Member(s):Nahrstedt, Klara; Allen, Jont B.; Vaidya, Nitin H.
Department / Program:Electrical & Computer Eng
Discipline:Electrical & Computer Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Perceptual Quality
Conversational Quality
Subjective Evaluations
Multi-dimensional Quality
Comparative Evaluation
Human Factors
Model of Pair-wise Subjective Comparisons
Statistical Scheduling
Probabilistic Preference Model
Performance Evaluation
Abstract:This research addresses real-time multimedia communication systems that can achieve high perceptual quality for their users. It focuses on the fundamental understanding of multiple quality aspects perceived and their trade-offs in the design of run-time control schemes that adapt to changing network conditions and expectations of the users of a system. One of the main contributions of this thesis is the use of adaptively scheduled off-line subjective comparison tests to efficiently and accurately learn users' subjective preferences among the alternative trade-offs in order to guide the run-time control schemes to achieve high perceptual quality. In order to illustrate the application of the general framework and our methodology, throughout this thesis we study the design of real-time VoIP (voice-over-IP) systems that can achieve high perceptual conversational quality. The trade-offs in the design and operation of a VoIP system involve the design of speech codecs and strategies for network control, playout scheduling, and loss concealment. The perceptual quality of a conversation over a network connection depends on the quality of the one-way speech (listening-only speech quality or LOSQ) received and the delay incurred from the mouth of the speaker to the ear of the listener (mouth to ear delay or MED). In a conversation, each participant takes turns in speaking and listening, and both perceive a silence duration called mutual silence (MS) when the conversation switches from one party to another. When the connection has delays, the MSs perceived by a participant consist of alternating short and long silence periods between turns. As a result, listeners may experience lower perceptual quality when the MSs are highly asymmetric, some speakers appearing to be more distant than others, or some responding slower than others. The evaluation of conversational quality is a largely unexplored area. There are many objective metrics for assessing the quality of a VoIP conversation, but there is no single objective metric whose results match well with subjective results. Indiscriminate subjective testing is not feasible because it is prohibitively expensive to carry out many such tests under various conversational and network conditions. Moreover, there is no systematic method to generalize the subjective test results to unseen conditions. To this end, there are five key innovations in this research that will address the limitations of existing work and improve the perceptual quality of VoIP systems. Firstly, we have developed a methodology and test-bed to objectively and subjectively evaluate VoIP systems under repeatable and fair conditions using a black-box approach. We have applied this methodology and test-bed on four commonly used VoIP systems; Skype, Google-Talk, Windows Live and Yahoo Messenger. The results show that different systems operate differently in coping with network imperfections such as jitter and loss. We also observe that systems do not change their operation as a function of the one-way delay of the connection, or of the turn-taking frequency of the conversation. The results show thatWindows Live is preferred under a significant set of conditions but none of the systems is dominant under all conditions. We have also learned the mapping between easily obtainable objective measures and subjective preferences of users in order to allow any VoIP system to be comprehensively compared against others without expensive subjective comparisons. We later use the mapping learned to subjectively compare our newly designed system with the four systems already evaluated. Secondly, we have developed a general model of pair-wise subjective comparisons, based on individually identified properties, axioms and lemmas, that models comparisons on a continuous operating curve with a single control parameter. The model provides a basis for developing the method to schedule adaptive off-line subjective tests and for identifying the optimal point by fusing the information obtained from separate subjective evaluations on the same operating curve. The model can be used in formulating and solving any type of pair-wise comparison problem that exhibits the same properties identified. The model is flexible to allow the existence of multiple optimal points on an operating curve and includes a belief function framework that can guide the search for optimal points efficiently. Furthermore, the model is built on a statistical framework that allows for the confidence of individual evaluation results to be represented in the conclusiveness of the combined belief function. Thirdly, we have developed a method for tackling the control design problem of finding the optimal point in an N-dimensional space, which includes all the metrics that affect quality. The overall problem is transformed into two independent problems of finding the optimal point on a continuous but one-dimensional curve, and learning the mapping on a set of curves that adequately spans the K-dimensional (K < N) curve space, where K stands for the number of metrics characterizing the network and conversational conditions. Fourthly, we have applied the methodology to the design of play-out scheduling control for VoIP systems by conducting subjective tests and by learning from them under a comprehensive set of network and conversational conditions. We have also verified the accuracy and efficiency of our methodology using exhaustive subjective tests on a subset of the network and conversational conditions. The verification of our methodology on a real-life application also justified our model of pair-wise subjective comparisons and our optimal algorithm to adaptively choose upcoming comparisons using information learned from previously conducted tests. Lastly, we have generalized the learned results in the design of play-out scheduling control to conditions that are unseen at design time and observed only at run-time. Along with design choices in other components of the architecture, this results in a VoIP system that can achieve higher and more consistent perceptual quality than other four VoIP systems analyzed. We have also shown that our system performs very close to a non-causal ideal system where the POS decisions are made optimally using future information. Our model and methodologies are applicable to a wide variety of real-time multimedia communication system control design problems which operate under constrained resources, communicating over non-stationary IP networks, and for which the overall quality perceived by users of the system has multiple counteracting aspects which cannot be represented by a single objective metric.
Issue Date:2010-08-20
Rights Information:Copyright 2010 Batu Sat
Date Available in IDEALS:2010-08-20
Date Deposited:2010-08

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