2. The method of claim 1 , wherein said codebook stores, for the plural number of predetermined speech parameter vectors, respective codes representing the respective predetermined speech parameter vectors, and said step (a) further includes a step of quantizing each set of speech parameters obtained from respective one of the plurality of the frames in the portion of the input speech by using said codebook to obtain the code.
2. The method of claim 1 , wherein said codebook stores, for the plural number of predetermined speech parameter vectors, respective codes representing the respective predetermined speech parameter vectors, and said step (a) further includes a step of quantizing each set of speech parameters obtained from respective one of the plurality of the frames in the portion of the input speech by using said codebook to obtain the code. 10. The method of claim 2 , wherein said step (b) includes a step of calculating a conditional probability of normal state by linear interpolation of said independent normal-state appearance probability and said conditional normal-state appearance probabilities.