This paper proposes a real-time estimation method of perceptual QoS (i.e., user-level QoS) of audio-video transmission over IP networks. The estimated quality reflects cross-modal influences of audio and video in the framework of ITU-T Recommendation J. 148. As objective measures of audio and video for the use of the perceptual QoS estimation, we select application-level QoS parameters for media synchronization quality. The perceptual quality is expressed in terms of the user-level QoS parameter of the interval scale, which is referred to as the psychological scale. As the multimedia quality integration function, we propose QoS parameter mapping between the application-level and the user-level. We made subjective quality assessment of simulated audio-video transmission over an IP network by the rating-scale method. We then calculate the psychological scale, utilizing the law of categorical judgment. Furthermore, we obtain the QoS mapping relations by the principal component analysis and multiple regression analysis. The regression formula thus derived can estimate the psychological scale with high accuracy unless the video frame rate is low. Finally we present an extension of the proposed method to continuous time estimation of user-level QoS.
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