just over 128 or so values different combinations of those values are adjusted by a term i e the short term synthesis filter 63 to produce synthesized signal 69 the resulting synthesized signal 69 is compared to i e subtracted from the original speech signal 71 to produce an error signal this error term is adjusted through perceptual weighting filter 62 i e and fed back into the decision making process for choosing values from the fixed codebook 61 and the adaptive codebook 65 another way to state the closed loop error adjustment of fig2 a is shown in fig2 b different combinations of adaptive codebook 65 and fixed codebook 61 are adjusted by weighted synthesis filter 64 to produce weighted synthesis speech signal 68 the original speech signal is adjusted by perceptual weighted filter 62 to produce weighted speech signal 70 the weighted synthesis signal 68 is compared to weighted speech signal 70 to produce an error signal this error signal is fed back into the decision making process for choosing values from the fixed codebook 61 and adaptive codebook 65 in order to minimize the error each of the possible combinations of the fixed codebook 61 and adaptive codebook 65 values is considered where in the preferred embodiment the fixed codebook 61 holds values in the range 0 through 1024 and the adaptive codebook 65 values range from 20 to about 146 such error minimization is a very computationally complex problem thus applicants reduce the complexity and simplify the problem by sequentially optimizing the fixed codebook 61 and adaptive codebook 65 as illustrated in.