Recency effect in the subjective assessment of digitally-coded television pictures

In MPEG-2 coding of television pictures, buffer overflow occurs during motion-intensive scenes; this introduces time-varying levels of coding distortion into the video. We report on the subjective effect of such distortion on viewers. It is known that standard ITU-R subjective testing methodology, which uses 10 s presentation times, is not easily able to deal with such variations in quality. Most of the applications of this methodology have been to situations where there is a single level of quality associated with a fixed set of engineering transmission parameters. However, if MPEG video is coded at a constant bit rate which is low enough to cause distortion, the quality of successive 10 s sections can be quite different from each other. At present, there is little reported experimental work concerning such quality variations in digitally-compressed television, although a study has been made of the time-varying distortions that occur in ATM video transmission due to sporadic network overload.