They're not a requirement for Japanese voicebanks since they don't occur phonemically in the language, but they can be used to make some utterances sound more natural or for vocal stylization, so a lot of voicebanks include them.
They're not a requirement for Japanese voicebanks since they don't occur phonemically in the language, but they can be used to make some utterances sound more natural or for vocal stylization, so a lot of voicebanks include them.