On spectral flattening techniques in residual-excited linear prediction vocoding

Residual excited linear prediction (RELP) vocoder is considered to be a promising approach to medium rate (4.8 to 9.6 kbits/s) vocoding that does not require pitch extraction. One critical part of the RELP vocoder that affects greatly the synthetic speech quality is the spectral flattener in which high frequency components are regenerated from the baseband residual signal. In this paper several spectral flattening techniques so far proposed are first reviewed, and their advantages and disadvantages are discussed. Then, two improved methods are proposed. Those are the hybrid method and the split-band coding method. These new methods yield excellent synthetic speech quality when the transmission rate is 8 kbits/s.