A cascaded algebraic codebook structure to improve the performance of speech coder

This paper presents a cascade structure of an algebraic codebook to improve the performance of low bit-rate speech coder. A codeword of an algebraic codebook consists of a set of pulse amplitudes and positions. In general, the amplitude of each pulse is constrained to be either +1 or -1 due to the limitations of bit-rate and complexity. Thus, the performance of the codebook is varied depending on the characteristic of input target vectors. In this paper, we extend the algebraic codebook structure to two stages in order to provide flexible pulse combinations. While all pulses, M, are simultaneously selected in a classical one-stage algebraic codebook, the cascade structure searches the pulses with a two step procedure, i.e., L pulses at the first stage and (M-L) pulses at the second stage. Experiments confirm that our algorithm provides higher quality than the conventional scheme when the total number of pulses is same. In case of assigning 24 pulses per 8-ms subframe, a segmental SNR between target and synthesized signal increases 1.04 dB. In addition, at the same environment, the complexity of fixed codebook search is reduced by about 32%.