1. A code excited linear prediction speech decoder, comprising: an adaptive codebook configured to generate an adaptive code vector; a random codebook configured to generate a random code vector; a synthesis filter that receives a signal based on said adaptive code vector and said random code vector, and that is configured to perform linear prediction coefficient synthesis on said signal; wherein said random codebook comprises: a pulse vector providing system configured to provide a pulse vector having a signed unit pulse; a comparing system configured to compare a value of adaptive codebook gain with a preset threshold value; a selecting system configured to select a dispersion pattern from a plurality of dispersion patterns stored in a memory in accordance with a result of said comparison; and a generating system configured to generate a dispersed vector by convoluting said pulse vector and said selected dispersion pattern.