Low bit rate CELP using ternary excitation codebook

Code excited linear predictor coders hold promise to achieve high quality speech at low bit rates. We propose a ternary excitation based CELP coder with a new structure to achieve toll quality speech at 4 kbps. Speech quality is maintained by allocating more bits for the codebook index allowing for larger codebooks which provide better speech quality as quantization levels increase. To allocate more bits for the codebook index a backward adaptive 10-th order LPC predictor is used. Regular structure of the lattice codebook and convexity of error surface have been exploited to greatly improve the efficiency of the search algorithm. The storage requirement of the codebook is eliminated by transmitting the position of three weights used in generating the ternary codebook instead of the codebook index. Speech quality obtained using the new CELP structure is studied and results compared with the LBG and the Gaussian codebooks