Investigation of Effects of Different Synthesis Unit to the Quality of Malay Synthetic Speech

Synthesis unit of a speech synthesizer directly affects the computational load and output speech quality. Generally, phoneme is the best choice to synthesize high quality speech. But it requires the knowledge of language to precisely draw the segmentation of words into phonemes. And it is expensive to compose an accurate phoneme dictionary. In this study, another type of synthesis unit is introduced which is letter. In Malay language, the unit size of letter is smaller than phoneme. And using letter as the synthesis unit could ease a lot of efforts because the context label can be created in fully automatic manner without the knowledge of the language. Four systems have been created and an investigation was done to find out how synthesis unit could affect the quality of synthetic speech. Forty eight listeners were hired to rate the output speech individually and result showed that no obvious difference between the output speech synthesized using different synthesis units. Listening test showed satisfactory result in terms of similarity, naturalness and intelligibility. Synthetic speech with polyphonic label showed increment in intelligibility compared to synthetic speech without polyphonic label. Using letter as the synthesis unit is recommended because it excludes the dependency of linguist and expands the idea of language independent front end text processing.