A Low-Complexity Search Alagorithm for Speech Coders with Sparse Excitation

Abstract

A low-complexity joint position and amplitude search algorithm is proposed for deterministic sparse codebooks to be used in code-excited linear predictive coders. The conventional multipulse search is presented as an extension of a general two-stage sequential search, leading to a classification of multistage sequential searches according to the extent that orthogonality constraints are applied throughout the search stages. After its classification, an efficient implementation of the joint search is derived which incorporates backward filtering of the residual target vector and precornputation of autocorrelation elements, bringing about a reduction in complexity of approximately one third in comparison to the focused search within the G.723.1 codec. In particular, considering an extensive speech database, a floatingpoint version of the joint search method reduces down to one third the number of comparisons per subframe relative to the focused search. Moreover, the joint search performs about one thirtieth as many comparisons as the position-exhaustive search. Further, the complexity of a fixed-point implementation of the joint search is below one quarter that of the focused search and stands below those of the G.729A and lS-64l-A coders. Listening tests indicate an equivalence in perceived quality.