Time-varying image quality : modelling the relation between instantaneous and overall quality

Experiments are described that evaluated a model linking instantaneously perceived quality judgments of long video sequences. Subjects evaluated a 3-min MPEG-2 coded video sequence by means of a continuous assessment procedure. Additionally, they rated overall quality of segments of 10, 30, 60, and 180 sec of the same video material. The model describing the relation between the instanteneous and overall quality ratings contains two main ingredients, viz. an exponentially decaying weighting function, simulating the experimentally established recall advantage for the most recently presented material (called the recency effect), and a non-linear averaging procedure stressing the relative importance of strong impairments. The fit of the mordel to the experimental data resulted in a decay time constant of 26 sec and a power of 3 for the nonlinear weighting. These findings suggest that subjects rely predominantly on the worst events of a sequence when determining their overall quality judgement.