Over the past few years there has been an increasing interest in real time video services over packet networks. When
considering quality, it is essential to quantify user perception of the received sequence. Severe motion discontinuities are
one of the most common degradations in video streaming. The end-user perceives a jerky motion when the
discontinuities are uniformly distributed over time and an instantaneous fluidity break is perceived when the motion loss
is isolated or irregularly distributed. Bit rate adaptation techniques, transmission errors in the packet networks or
restitution strategy could be the origin of this perceived jerkiness. In this paper we present a psychovisual experiment
performed to quantify the effect of sporadically dropped pictures on the overall perceived quality. First, the perceptual
detection thresholds of generated temporal discontinuities were measured. Then, the quality function was estimated in
relation to a single frame dropping for different durations. Finally, a set of tests was performed to quantify the effect of
several impairments distributed over time. We have found that the detection thresholds are content, duration and motion
dependent. The assessment results show how quality is impaired by a single burst of dropped frames in a 10 sec
sequence. The effect of several bursts of discarded frames, irregularly distributed over the time is also discussed.
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