Download David Hands - University of Essex

David Hands
Title of thesis Mental Processes in the Evaluation of Digitally-Coded Television
Pictures
Position: Student/Research assistant
Degree awarded PhD
Duration of study 3 years (1994-1997)
Financial support Independent Television Commission (ITC) UK/ EU MOSAIC
Project
Supervisor Dr S.E. Avons (Psychology), Prof. M. Ghanbari (ESE), Prof D Pearson
(ESE)
Current affiliation BT, [email protected]
Abstract
Experiments investigating the mental processes involved in the subjective assessment
of digital television picture quality are reported. Initial experiments showed that a
recency effect was apparent in picture quality ratings for 30s video sequences. When
the poorest quality video appeared at the end of the video sequence subjects gave
worse quality ratings compared with ratings for sequences in which the poorest
quality video occurred at the beginning. Follow-up studies supported the existence of
a recency effect, but indicated that it was dependent on the cognitive demands of the
task. Recency effects in 30s video sequences were not evident when subjects' tracked
picture quality continuously using a slider mechanism. It is suggested that the absence
of recency under continuous assessment conditions is due to increased attention to
variations in picture quality.
A study was performed to determine if STM processes were responsible for the
recency effect in picture quality evaluations. Existing theories of STM suggest that
the addition of a new, improved quality, scene should reduce or eliminate recency. No
evidence for this position was found and evaluations of picture quality can best be
explained by judgement operations.
The effect of subject interest in the scene-content of television pictures on picture
quality evaluations was investigated. Two experiments failed to find any evidence for
a relationship between interest and picture quality ratings. An experiment in which the
duration of an impairment was varied found that impairment duration had no effect on
picture quality evaluations.
Evidence from these studies indicated that impairment intensity and the location of an
impairment can influence picture quality judgements, but impairment duration has
little effect. A multiple regression study manipulated these three factors to define a
model of picture quality evaluation. This analysis identified a peak intensity model
(PIM) for predicting retrospective quality ratings from continuous quality ratings.
Publications
Journal papers Patents
Aldridge, R., Davidoff, J., Ghanbari, M., Hands, D., Pearson, D. (1995).
"Measurement of scene-dependent quality variations in digitally-coded television
pictures", IEE Proceedings on Vision, Signal and Image Processing, Vol.142, pp.149154.
Conference papers
Aldridge, R., Davidoff, J., Ghanbari, M., Hands, D., Pearson, D. (1996). "Evaluating
digital television picture quality", ECMAST '96, Louvain-la-Neuve, Belgium, 28-30
May.
Aldridge, R., Davidoff, J., Ghanbari, M., Hands, D., Pearson, D. (1996). "Subjective
assessment of time-varying coding distortions", In Proceedings of the Picture Coding
Symposium, vol.1, pp.269-274, Melbourne, Australia, 13-16 March.
Aldridge, R., Davidoff, J., Ghanbari, M., Hands, D., Pearson, D. (1995). "Recency
effect in the subjective evaluation of digitally-coded television pictures", In
Proceedings of 5th IEE International Image Processing and its Applications
Conference, pp.336-339, Heriot-Watt University, Edinburgh, 4-6 July.