Temporal Integration for Audiovisual Call Quality

Abstract

Evaluating the perceived audiovisual quality of entire video calls requires to take into account the effects of time-varying degradations. Models of time integration have been developed which assess the fluctuating quality of transmitted speech and predict the call-final judgement based on a weighting technique of momentary quality ratings. In this chapter, the applicability of such models to the audiovisual case will not only be evaluated, but the models will be improved as well. To that end, a subjective test methodology is developed which allows the user perception evaluation of time-varying quality of 90 s long sequences organized in a simulated conversational structure. These sequences emulate the course of an entire video call and are composed of independent short samples (9 s). Audiovisual impairments are temporally distributed over the long sequences by applying different levels of impairment to the short samples, hence producing different predefined temporal quality profiles. A separate assessment of the perceived quality of the short samples on one hand and of the long sequences on the other hand allows to compare the perceptual effects between speech and audiovisual stimuli. It also allows the evaluation and optimization of existing call quality models that predict the quality at the end of a (simulated) speech conversation. These models proved to enhance the prediction accuracy in comparison to the plain average, and an optimization of the model parameters further refines the correlation of the estimates with the subjective data. The optimized models showed a higher correlation and a lower prediction error on independent test data.