Techniques for improving the quality of LD-CELP coders at 8 kb/s

The authors present an 8 kb/s low delay speech coder which is based on the code excited linear prediction (CELP) algorithm. They concentrate on analyzing the problem areas in backward predictive systems and solutions to alleviate these problems. The authors report on a forward linear predictive coding adaptive scheme to eliminate such distortions. Another approach is to use a windowing technique in the frequency domain on the error signal in analysis-by-synthesis procedures. The adaptation of the long term predictor (LTP) coefficients is also considered in a backward and forward mode for differential pitch coding of the LTP lags. The role of the excitation gain update rate is also examined in providing higher quality and better adaptation of the gains for an efficient vector quantization procedure. An 8-kb/s coder with a buffering delay of 4 ms has been informally subjectively tested, and the latest test results indicate a performance equal to that of the GSM at worst case