On the (UN)importance of the contextual factors in HMM-based speech synthesis and coding

This paper presents an evaluation of the contextual factors of HMM-based speech synthesis and coding systems. Two experimental setups are proposed that are based on successive context addition from phonetic to full-context. The aim was to investigate the impact of the individual contextual factors on the speech quality. In that sense important and unimportant (i.e., not having significant impact on speech quality, also called weak) contextual factors were identified. The results imply that in speech coding the improvement in quality can be achieved just with reconstruction of syllable contexts. The sentence and utterance contexts are unimportant on the decoder side, and it is not necessary to deal with them. Although in speech coding the wider context was not necessary, in speech synthesis current syllable and utterance contexts are more important over others (previous and next word/phrase contexts).