Evaluation of TTS Systems in Intelligibility and Comprehension Tasks: a Case Study of HTS-2008 and Multisyn Synthesizers

This paper explores the relationship between intelligibility and comprehensibility in speech synthesizers, and it designs an appropriate comprehension task for evaluating the speech synthesizers’ comprehensibility. Previous studies have predicted that a speech synthesizer with higher intelligibility will have higher performance in comprehension. Also, since the two most popular speech synthesis methods are HMM-based and unit selection, this study tries to compare whether the HTS-2008 (HMM-based) or Multisyn (unit selection) speech synthesizer has better performance in application. Natural speech is applied in the experiment as a control group to the speech synthesizers. The results in the intelligibility test show that natural speech is better than HTS-2008, which, in turn, is much better than the Multisyn system. In the comprehension task, however, all three of the speech systems display minimal differences in the speech comprehension process. This is because the two speech synthesizers have reached the threshold of having enough intelligibility to provide high speech comprehension quality. Therefore, although there is equal comprehensible speech quality between the HTS-2008 and Multisyn systems, the HTS-2008 speech synthesizer is recommended due to its higher intelligibility.