1. A method of producing an excitation signal to be used by a sound signal synthesis means to synthesize a sound signal, comprising the steps of: generating a codeword signal in response to an index signal associated to said codeword signal, said signal generating step using an algebraic code to generate said codeword signal; and prefiltering the generated codeword signal to produce said excitation signal, said prefiltering step comprising processing the codeword signal through an adaptive prefilter having a transfer function varying in time in relation to parameters representative of spectral characteristics of said sound signal to thereby shape frequency characteristics of the excitation signal so as to damp frequencies perceptually annoying a human ear.