a speech coding and decoding apparatus for improving the quality of synthesized speech . a coding portion includes a filter for adding a short - term phase amplitude characteristic to an excitation signal , and a coding circuit for quantizing and coding a phase amplitude characteristic . a decoding portion includes a decoding circuit for decoding the coded phase amplitude characteristic , and a filter for adding the same phase amplitude characteristic as that in the coding portion . thus , it is possible to synthesize high - quality speech with good reproducibility of the phase amplitude characteristic of an excitation signal .