DESIGN OF A PITCH SYNCHRONOUS INNOVATION CELP CODER FOR MOBILE COMMUNICATIONS

This paper describes the design of a speech coder called pitch synchro nous innovation CELP (PSI-CELP) for low bit-rate mobile communications . PSI-CELP is based on CELP, but has more adaptive excitation structur es. In voiced frames, instead of conventional random excitation vector s, PSI-CELP converts even the random excitation vectors to have pitch periodicity by repeating stored random vectors as well as by using an adaptive codebook. In silent, unvoiced, and transient frames, the code r stops using the adaptive codebook and switches to fixed random codeb ooks. The PSI-CELP coder also implements novel structures and techniqu es: an FIR-type perceptual weighting filter using unquantized LPC para meters, a random codebook with a conjugate structure trained to be rob ust against channel errors, codebook search with delayed decision, a g ain quantization with sloped amplitude, and a moving average predictio n coding of LSP parameters. Our speech coder is implemented by DSP chi ps. Its coded speech quality at 3.6 kb/s with 2.0 kb/s redundancy is c omparable to that of the Japanese full-rate VSELP coder at 6.7 kb/s wi th 4.5 kb/s redundancy. The basic structure of this PSI-CELP coder has been chosen as the Japanese half-rate speech codec for digital cellul ar telecommunications.