1. A speech synthesis method comprising: storing a number of formant parameters in a storage, the formant parameters representing formant frequencies and windowing functions; selecting predetermined formant parameters from the formant parameters according to a phoneme symbol string; generating a plurality of sine waves based on the formant frequency corresponding to the formant parameters selected; multiplying the sine waves by the windowing functions corresponding to the selected formant parameters, respectively, to generate a plurality of formant waveforms; adding the formant waveforms to generate a plurality of pitch waveforms; and superposing the pitch waveforms according to a pitch period to generate a speech signal.