From Wikipedia:

Formants are the distinguishing or meaningful frequency components of human speech and of singing. By definition, the information that humans require to distinguish between vowels can be represented purely quantitatively by the frequency content of the vowel sounds. Formants are the characteristic partials that identify vowels to the listener. 

http://en.wikipedia.org/wiki/Formants

Traditional pitch shifting doesn't calculate formants so, as a basic rule, as soon as you shift a sound more that +/- 1/3rd from its fundamental frequency you start getting artifacts in terms of non_resemblance with he original sound. The more you shift the more artifacts you'll get….up to chipmunks and godzilla sounds.

Formant correct shifting is a very heavy process in which pitch AND formants are re_calculated to keep a higher level of fidelity with the original sound at any shift setting.

all the best

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