FRAME DROPPING EFFECTS ON USER QUALITY PERCEPTION Ricardo R. Pastrana-Vidal*°, Jean-Charles Gicquel*, Catherine Colomes*, Hocine Cherifi° *France Telecom R&D, DIH/EQS/MAI, 4 rue de Clos Courtel, 35512 – Cesson Sévigné, France {ricardo.pastrana, jeancharles.gicquel, catherine.colomes}@francetelecom.fr °LIRSIA, Université de Bourgogne, Faculté de Science Mirande, 21011 Dijon cedex, France cherifi@crid-u.bourgogne.fr ABSTRACT Severe motion discontinuities are one of the most common degradations in video streaming. Frame dropping may be the source of this temporal discontinuity. When considering quality, it is essential to quantify user perception of the received sequence. We present a set of psychovisual experiments performed to characterize the effects of frame dropping. First, a previous study focusing on the impact of sporadically dropped pictures is briefly reviewed. Then, we present two experiments performed to quantify the effect of these temporal discontinuities for high temporal densities and distributions. We have found that quality impairment mainly depends on the duration and temporal density of discontinuities (burst of dropped frames). Finally, we propose a model that predicts the quality assessment from a group of assessors when a video is impaired by frame dropping. The model predictions show a high correlation with the observer's ratings. 1. INTRODUCTION There is 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, cell loss in the packet networks or restitution strategy could be the origin of this perceived jerkiness. At the source coding stage, temporal down-sampling is one of the most widely used techniques for bit rate adaptation; the sequence undergoes a frame dropping operation that affects the motion information. Packet loss or jitter could cause a sporadic or non-uniform frame discarding at the decoding process because of the buffering time limit [1]. The last picture that was received is then displayed until a new image has been reconstructed. The user then perceives a frozen image followed by an abrupt displacement of the objects. In the following text, we will use the term temporal discontinuity as a perceptual synonym of a dropped picture burst. Nowadays, psychovisual experiments are the only recognized way to characterize the perceived quality. Recently, a quality perception study conducted by the authors showed that subjects had a significant negative reaction to this temporal discontinuity [2]. Our previous study was mainly focused on the impact of sporadically dropped pictures; i.e., for a reduced number of discontinuities. However, several questions were left open in relation to the influence of these temporal discontinuities for high temporal densities and distributions. We have therefore been studying the effect of frame dropping at a constant rate either alone or combined with a burst of discarded pictures for different durations on perceived quality; the influence of different temporal distribution profiles was also studied. The corresponding psychovisual experiments and results are reported here. Furthermore, we propose a model to calculate the effect on perceived quality of several frame dropping conditions. 2. PREVIOUS STUDY We have organized the first study in two main parts: the visibility of frame dropping and its effect on the user quality. 2.1. Visibility findings We have found that the discontinuity caused by one discarded picture (80ms) is visible. We have also reported that the unequivocal detection (100%) of a burst of dropped frames is attained when the number of discarded pictures is equivalent to 200ms. Furthermore, this finding is valid for all selected contents.