Incorporating Duration and Intonation Models in Filipino Speech Synthesis

In this paper we describe the development of an intonation model and a duration model to generate prosody for the Filipino language. Z-scores of normalized durations are used for the duration model and the Tilt parameters are used for the intonation model. The Filipino Speech Corpus (FSC) is the source of statistical data for modeling the duration and intonation. A Classification and Regression Tree (CART) generator is used to build the model for duration and intonation. The Harmonic plus Noise Model (HNM) is developed for the FSC. The diphones are concatenated to produce the synthetic speech and HNM is used to modify the prosody. The synthesized speech is evaluated using the Mean Opinion Score (MOS). Results show that the duration model and the intonation model needs improvement. HNM synthesis performs slightly better than TD-PSOLA (time-domain pitch synchronous overlap-add). S peech synthesis is the automatic generation of speech waveforms with the use of a machine. Speech is the primary means of communication between people; hence, speech synthesis is a natural way for man-machine interface. Synthetic speech may be used in several applications. An important application of speech synthesis is in reading and communication aid for the handicapped. With speech synthesis, digital talking books which will only require text inputs are now available. Synthesized speech gives the deaf and vocally handicapped an opportunity to communicate with people who do not understand sign language. Synthesized speech can also be used as an educational tool for special tasks such as spelling and teaching pronunciation for different languages. It can also be used with interactive educational applications. Synthetic speech can also be used in interactive games, automated call centers and hand-held devices for quick information. Initially, the research in speech synthesis is focused in producing intelligible speech. Current synthesizers are now able to produce intelligible speech, and the new goal is the improvement of naturalness of synthesized speech. A good model of prosody is necessary for synthesized speech to sound natural. Several Text-to-Speech synthesis systems of the Filipino language have been developed in the UP Digital Signal Processing (DSP) Laboratory. All implementations used the time domain pitch synchronous overlap-add (TD-PSOLA) method. Corpus and Liampo (1), in 2001, developed a continuous text-to-speech synthesizer of Tagalog words with 20 phonemes of the language used as basic acoustical units. In this project, only the intonation part of the prosody generation was put to use. They used punctuation marks at the end of sentences to determine the appropriate intonation pattern. The prosody evaluation test for the Tagalog sentences produced unsatisfactory results. In 2002, prosody development for Filipino Text-to-Speech systems (2) was done. In line with this, the Filipino Speech Corpus (FSC) was built. The FSC was used as the source of data for the project. Isolated words were used as concatenation units, and paragraphs and sentences in the corpus were used as source of prosody. Dynamic Time Warping (DTW) was used to align the pitch contour and produce the duration contour. Acceptability test shows that the produced synthesized speech is comparable to commercially available Text-to-Speech (TTS) systems. The system was used in in 2007 to generate the synthetic speech of the English text translated to Filipino (3). A concatenative synthesis of two-syllable Filipino words (4) was developed to address the problems associated with the Tagalog TTS Synthesizer. Filipino words were characterized to differentiate words of the same spelling but different meaning, depending on the pronunciation. The characterization took note of the pitch and duration of utterance of each word. The synthesizer accepts two-syllable text and outputs all the possible utterances for that word. Discontinuities at concatenation points degrade the quality of the synthesized word. Much of the prosody generation used in the previous Filipino TTS systems rely on a corpus of phrases which prohibits the synthesis of more phrases. In April 2005, an automatic means of modeling the duration of Filipino (5) was developed. Syllables were used as acoustic units. The duration of the generated synthetic speech was acceptable but the syllable concatenation using TD-PSOLA needs to be refined for commercial acceptability.