Improving the performance of a mixed excitation LPC vocoder in acoustic noise

A number of improvements to the mixed excitation linear predictive coding (LPC) vocoder are presented. First, the authors have added more sophisticated frequency shaping of the pulse and noise in the mixture. They use a bandpass filter bank to attain a staircase approximation to any desired noise shape. Voicing strength in each frequency band is controlled by periodicity analysis of both the bandpass filtered speech and the bandpass speech envelope. Second, the authors have improved their pitch detection algorithm by using separate searches on the LPC residual and the input speech signal. Finally, they have added a fixed pulse shaping filter based on a spectrally flattened synthetic glottal pulse. The improved LPC vocoder performs much better in acoustic background noise, and it produces natural sounding speech in both quiet and noisy environments