An approximation to voice aperiodicity

Aperiodicity in voiced segments of speech may be ascribed to different causes. The magnitude of pitch perturbation is different in different spectral ranges of the signal. To see whether pitch perturbation can be effectively simulated by partially replacing voiced excitation by random noise, in appropriate frequency-time portions, experimental tests have been made on a computer-simulated channel vocoder. The buzz-hiss decision was made separately for three different frequency portions of the signal. The cepstrum technique was used for pitch detection, and separate buzz-hiss switching decisions were made at the synthesizer for each frequency portion. The switching thresholds were controlled, and deliberately "devoiced" versions were compared with regular vocoded speech. The fundamental frequency was determined by the lowband cepstrum. The result shows that partial devoicing of the high-frequency ranges definitely improves speech quality. Further, a comparatively large amount of devoicing is perceptually tolerable.