Lattice CELP for low bit rate speech coding

We propose a CELP coder based on lattice codebooks to achieve toll quality speech at low bit rates. The new CELP coder allocates 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 we use backward adaptation to update a 10th order LPC predictor. The LPC coefficients are computed using a modified Barnwell recursive window and the autocorrelation method of LPC analysis. A three tap pitch predictor is used for pitch synthesis. Spherical lattice codebooks have a regular structure which is exploited to greatly improve the efficiency of the search algorithm and because of the regular structure the excitation codebook need not be stored at the transmitter or the receiver. Speech frames of 20msec and 160 samples are encoded at a bit rate of 4kbps and the speech quality is studied and results compared with the LBG and the Gaussian codebooks.