Video Transmission of American Sign Language and Finger Spelling: Present and Projected Bandwidth Requirements

The bandwidth for communicating ASL sentences, isolated lists of ASL nouns, and finger-spelled names by a television type of raster scan was measured for a heterogeneous deaf population. Video sequences of a signer were shown to subjects at 60 frames/s (without interlace) with a raster composed of from 9 to 79 lines/frame corresponding to a bandwidth of from 1.1 to 86 kHz. Most subjects could interpret ASL sentences with little loss at a bandwidth of 21 kHz; the most expert ASL users received sentences at 40-50 percent correct at a bandwidth of 4.4 kHz. Even with the present nonoptimal raster, expert signers require a bandwidth for ASL communication that is just four times greater than the bandwidth required by experienced speakers for equivalent performance with auditory speech. With a more judicious choice of raster parameters, with better display conditions, and with better camera position, it seems quite probable that the present raster-bandwidth requirement could be lowered. Transmission at ordinary (3 kHz) telephone bandwidths will require more sophisticated picture coding, perhaps with different codes for ASL, finger spelling, and speechreading.