In this contribution we report on a method to control the delay determination in CELP coders, which improves the subjective speech quality without increasing the bit rate. Possible values of the delay of the long-term predictor were scored with respect to previous delay values. This scoring is done by weighting the closed-loop performance of the long-term predictor by means of a weighting factor, which depends on the difference between the previous delay value and the actual delay value. The final decision on the delay value is made on the basis of the weighted performances. Optimization and evaluation of this method were studied in simulation experiments using conventional CELP with stochastic excitation as well as ACELP with adaptive excitation at low data rates [2,3]. CELP coders using the new delay determination achieve a significant improvement of subjective speech quality. With an ACELP coder a good speech quality can be obtained even at very low data rates.
Bibliographic reference. Balss, U. / Kipper, U. / Reininger, Herbert / Wolf, Dietrich (1993): "Improving the speech quality of CELP-coders by optimizing the long-term delay determination", In EUROSPEECH'93, 59-62.