The vocoder is a well-established tool for time-scaling and pitch shifting speech and audio signals. Its theory is now well understood and improvements have been proposed to reduce artifacts commonly encountered when time expanding signals by large factors. In the literature the phase vocoder has been described primarily as a tool for time scaling rather than pitch shifting, the latter usually being achieved by a combination of time scaling and sampling-rate conversion. The authors focus mainly on pitch-scale modifications of speech and audio signals and discuss the drawbacks of the standard time-scaling and resampling technique. Two alternative techniques are presented which reduce the complexity and computational cost signficantly while offering dramatically extended capabilities. In particular, the new techniques, which operate solely in the frquency domain, enable chorusing, harmonizing, and nonstandard frequency mdodifications such as partial stretching (nonlinear frequency scaling), frequency inversions, and so on.
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