This paper reports on a usability study from the perspective of deaf persons of a prototype videotelephony service implemented on a mobile network. The service offered combined video- and text communication. The objective of the experiment was to determine the minimal bandwidth requirements for deaf sign language users and to measure the effect of variable video picture quality on the use of the text telephone. The results of this study indicate that deaf persons who use sign language as their main communication mode are highly adaptable and prefer to use signs even if the video image quality is poor. At the same time, deaf persons expressed a preference for having a text messaging facility available in combination with videotelephony. It was found that the use of the text messages increased with deteriorating video picture quality. A bandwidth of 128 kbps was considered highly useable. Even at 64 kbps some users could communicate through sign language.
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