Visible Speech as a Function of Image Quality: Effects of Display Parameters on Lipreading Ability

Abstract

Two experiments are reported that examine effects of certain visual display parameters on subjects' ability to lipread numbers. The quality of the image of the speaker's face is altered by varying temporal resolution (frame rate), spatial resolution (number of pixels) and contrast resolution (number of grey scale levels). In Experiment 1, combinations of these variables are examined in a repeated measures design and reliable effects of reducing both frame rate and number of grey levels are evident. In Experiment 2, the frame rate and grey scale settings are factorially combined on a between-subjects basis, and practice is given so that subjects can adapt to the display conditions to which they are exposed. Effects of frame rate and grey scale are again statistically significant, and additive. Lipreading performance improves over the short duration of the experiment, but both image variables continue to affect performance. The effect size of the grey scale factor decreases with practice, but not that of frame rate. The results are discussed with regard to the kinds of relevant visual cues that may be affected by image degradation. Implications for the planned development of low cost digital communication are also considered. Trading off one image variable at the expense of another may be necessary to meet bandwidth limitations, and data such as those presented will be necessary to decide on a combination of parameter values to satisfy human performance and technical criteria.