Improvement of ADP-CELP speech coding at 4 kbits/s

The authors describe improvements in the ADP-CELP (adaptive-density-pulse code-excited-linear-prediction) coder at 4 kb/s. A fractional delay was introduced to the adaptive codebook. A fast exhaustive search procedure for a uniformly distributed fractional delay was studied for the case where the delay length is shorter than the codevector´s dimension. A novel method to determine the long-term predictor´s gain was also used. Four methods to find the density pattern of the ADP excitation were investigated. A modified adaptive postfilter was used to improve subjective speech quality