Speech encoder capable of substantially increasing a codebook size without increasing the number of transmitted bits

Abstract From the Canadian patent

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, …