A predicted residual signal is calculated from a current input speech signal and a past input speech signal, and a cross-correlation between the predicted residual signal and the past input speech signal having one speech sub-frame length stored in a first code book is calculated. In cases where the current input speech signal has no local peak, the cross-correlation becomes high, so that a synthesized speech signal is generated from the past input speech signal stored in the first code book or a predetermined sound source signal having one speech sub-frame length stored in the second code book. In contrast, in cases where the current input speech signal has a local peak, the cross-correlation becomes low, so that it is judged that a function of the first code book is depressed. In this case, a synthesized speech signal is generated from a group of short-length sound source signals having a total length equal to one speech sub-frame length stored in a short-length signal code book. Therefore, even though the current input speech signal suddenly has a local peak, because the synthesized speech signal is generated from the short-length sound source signals respectively having a speech length lower than one speech sub-frame length, the local peak can be expressed by the short-length sound source signals, an appropriate exciting sound source signal similar to the current input speech signal can be determined, and the synthesized speech signal can be adequately obtained.
Code excited linear prediction coder with a short-length codebook for modeling speech having local peak