SPEECH ENCODER CAPABLE OF SUBSTANTIALLY INCREASING A CODEBOOK SIZE WITHOUT INCREASING THE NUMBER OF TRANSMITTED BITS

In a speech encoder, a gain codebook switching circuit is supplied with short-term prediction gains from a short-term prediction gain calculator circuit and with mode information through an input terminal and compares the short-term prediction gains with a predetermined threshold value when the mode information indicates a predetermined mode. As a result of comparison, the gain codebook switching circuit produces gain codebook switching information which is delivered to a gain quantizer circuit. The gain codebook quantizer circuit is supplied with adaptive code vectors, excitation code vectors, impulse response information, and the gain codebook switching information, and gain code vectors from a particular gain codebook connected to one of a plurality of input terminals that is selected by the gain codebook switching information. For the excitation code vectors being selected, the gain quantizer circuit selects combinations of the excitation code vectors and the gain code vectors in the gain codebook selected by the gain codebook switching information.