Speech coder and speech decoder

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, and
• a synthesis filter configured to receive a signal based on said adaptive code vector and said random code vector, and to perform linear prediction coefficient synthesis on said signal,
• said random codebook comprising:
• a pulse vector provider configured to provide a pulse vector having a signed unit pulse;
• a comparator configured to compare a value of adaptive codebook gain with a preset threshold value;
• a selector 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 generator configured to generate said dispersed vector by convoluting said pulse vector and said selected dispersion pattern.