1. A method of training a codebook for use in quantizing or dequantizing harmonic amplitudes of a speech signal, comprising the steps of: selecting a first vector of said harmonic amplitudes, said first vector having a dimension corresponding to the number of harmonics associated with an first input pitch value; transforming said first vector to a zero-mean vector; interpolating the results of said transforming step, thereby obtaining an interpolated vector having a predetermined dimension; repeating the above steps for a number of vectors of said harmonic amplitudes, thereby obtaining a set of interpolated vectors all having said predetermined dimension; and training said codebook, using said interpolated vectors as input vectors for a codebook training process.