The relationship between intelligibility and naturalness in high quality synthetic speech

Abstract

The relationship between intelligibility and naturalness in synthetic speech was examined. There has been evidence that in extreme cases very unnatural speech may be intelligible [R. E. Remez et al., Science 212, 947–950 (1981)] and very natural sounding speech may be unintelligible [E. Abberton and A. J. Fourcin, Lang. Speech 21, 305–318 (1978)]. However, there is little evidence regarding the independence of naturalness and intelligibility in relatively high quality synthetic speech. This relationship was investigated in the present experiment using a paired‐rating design, in which listeners were required to rate the intelligibility and naturalness of a pair of words on each trial. The first item was always a digitized natural token of a word and the second item was always a synthetic version of the same word. The synthetic stimuli were designed to mimic the specific acoustic correlates of three male and three female talkers. Overall, the listeners rated the synthetic speech as highly intelligible and natural. In addition, a principal component analysis indicated that the naturalness and intelligibility of the different synthetic talkers were independent factors. These results argue that even in the case of high quality synthetic speech, these two attributes of the signal are separate. [Work supported by NIH (NINCDS).]