1. An encoder for coding an input signal, comprising: adaptive codebook means for storing a variable set of excitation vectors; fixed codebook means for storing a fixed set of excitation vectors; codebook searching means for searching said adaptive codebook means to determine an optimal adaptive codebook excitation vector, and for searching said fixed codebook means to determine an optimal fixed codebook excitation vector; total excitation vector producing means for producing a total excitation vector from said optimal adaptive codebook excitation vector and said optimal fixed codebook excitation vector; voicing measure determining means for determining a voicing measure of said input signal, said voicing measure being voiced when said input signal includes voiced speech and said voicing measure being unvoiced when said input signal does not include voiced speech; and modifying means for modifying said total excitation vector by raising said total excitation vector to an exponent determined in accordance with said voicing measure of said input signal.