Abstract
In North America, the method of encoding information for videotex services has been standardized. The encoding scheme, the North American Presentation Level Protocol Syntax (NAPLPS), facilitates the transmission of graphics and text by representing display information in a highly compressed form. Two experiments examined viewers' responses to bit errors during simulated transmissions of NAPLPS codes. In the first experiment, graphic materials were transmitted; in the second, the materials were texts. In both experiments, a new measure of viewer satisfaction (requests for retransmission) and viewer evaluations of both image quality and usefulness indicated that bit-error probabilities in an operational teletext service should not exceed 1 x 10-4. The results of both experiments validated the new retransmission-request measure and provided some insight into the practice of teletext assessment.
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