In vocal tract prediction coefficient coding and decoding circuitry, a vocal tract prediction coefficient converter/quantizer transforms vocal tract prediction coefficients of consecutive subframes constituting a single frame to corresponding LSP (Line Spectrum Pair) coefficients, quantizes the LSP coefficients, and thereby outputs quantized LSP coefficient values together with indexes assigned thereto. A coding mode decision assumes, e.g., three different coding modes based on the above quantized LSP coefficient values, the quantized LSP coefficient value of the fourth subframe of the previous frame, and the above indexes. The decision determines which coding mode should be used to code the current frame, and outputs mode code information and quantization code information. The circuitry is capable of reproducing high quality faithful speeches without resorting to a high mean coding rate even when the vocal tract prediction coefficient noticeably varies within the frame.
Vocal tract prediction coefficient coding and decoding circuitry capable of adaptively selecting quantized values and interpolation values