Read Diatonic Shifter 2 User Manual. Such Audio Units could be used in host apps Apple’s like Logic (Pro), Garageband and AU Lab. This unit costs 29.99$. Interesting to learn that market for such type of software exists. Probably I could try to make something similar. E.g. this developer as a bunch of similar stuff in the Mac App Store including tuning app which I definitely could make.

Audio Unit manual by link above has quite a lot of very useful links. Some I would like to bookmark here. Pitch Shifting Using The Fourier Transform has detailed explanation of Short Time Fourier transform with implementation of which I am struggling at the moment including question of choosing of stride.

Site of company standing behind audiowish stuff mentioned above has not bad post about accelerate framework Vectorizing with vDSP and vecLib. Unfortunately it doesn’t mention question which bothers me a lot: can vDSP and veclib use as output vector one of its input vectors and how this use case influence performance.