VISIBLE SPEECH AS A FUNCTION OF IMAGE QUALITY - EFFECTS OF DISPLAY PARAMETERS ON LIPREADING ABILITY

Two experiments are reported that examine effects of certain visual di splay parameters on subjects' ability to lipread numbers. The quality of the image of the speaker's face is altered by varying temporal reso lution (frame rate), spatial resolution (number of pixels) and contras t resolution (number of grey scale levels). In Experiment 1, combinati ons 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 th ey are exposed. Effects of frame rate and grey scale ale again statist ically significant, and additive. Lipreading performance improves over the short duration of the experiment, but both image variables contin ue to affect performance. The effect size of the grey scale factor dec reases with practice, but not that of frame rate. The results are disc ussed with regard to the kinds of relevant visual cues that may be aff ected by image degradation. Implications for the planned development o f low cost digital communication are also considered. Trading off one image variable at the expense of another may be necessary to meet band width limitations, and data such as those presented will be necessary to decide on a combination of parameter values to satisfy human perfor mance and technical criteria.