1. A speech decoding method according to code-excited linear prediction (CELP) wherein the speech decoding method receives a coded speech and synthesizes a speech using at least an excitation codebook, the speech decoding method comprising: receiving a first time series vector with a first number of samples with zero amplitude-value from the excitation codebook and generating a second time series vector with a second number of samples with zero amplitude-value, wherein the second number of samples differs from the first number of samples; selecting one of the first time series vector and the second time series vector and outputting a selected time series vector; and synthesizing a speech using the selected time series vector.