A family of low-complexity, high quality CELP speech coders are described which use two new techniques: Backward and Inverse Filtered Target (BIFT) for fixed codebook excitation search; and Tree-Structured Multitap adaptive codebook search. Incorporation of these new techniques resulted in very low complexity CELP coders at less than 16 Kb/s. The three coefficients for linear combination of the adaptive codebook are chosen from a tree-structured tap codebook. The best tap index in the primary codebook points to a secondary codebook where the search is further conducted. This procedure may be repeated many times, wherein each subsequent tap codebook points to yet another subsequent tap codebook, which points to yet another subsequent tap codebook, etc. A fixed ternary excitation codebook using a new technique called Backward and Inverse Filtered Target matching (BIFT), is used to encode the portion of the target signal that is left behind after the adaptive codebook contribution has been subtracted. BIFT combines the elements of the Backward Filtered Target response and Inverse Filtered Target response by element-by-element multiplication to define a new vector. A predetermined number of maximums of the new vector are chosen as the pulse locations and the signs assigned are the same as the signs of the corresponding elements.
Low-complexity speech coding with backward and inverse filtered target matching and a tree structured multitap adaptive codebook search