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How does formant shifting in synthesizers such as Reason's Maelstrom work?

I've been pointed to the PSOLA algorithm among others, but I cannot understand how exactly does it result to formant shifting, because its base operation seems to be "formant preserving pitch shifting".

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PSOLA can mean different things to different people.

the "Formant Preserving Pitch Shifting" that is done in the time domain requires a really good, tracking, pitch detector. knowing the pitch (which changes) at every arbitrary time means you know the period length.

so this Formant Preserving Pitch Shifting in the time domain, identifies the onsets of every period of the input waveform, when it is voiced. each period is isolated with a window and now becomes a "grain" or (my poorly-considered name for it) a "wavelet". then you launch these grains or wavelets out at the rate of the new fundamental frequency. but, to preserve the formants, you neither stretch (if down-shifting) nor scrunch (if up-shifting) the grains or wavelets. you overlap and add them.

the rate of firing out those grains determines the output pitch. the degree of stretching or scrunching the grains along the time axis determines the formant shift.

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  • $\begingroup$ This is fascinating. I don't understand how you can lower the pitch without stretching though. I get how the overlap would let you raise it, and I get that you could sustain the same pitch over time but really don't get how this technique or adding "wavelets" of the same pitch window would let you lower it. Little help? $\endgroup$ Commented Nov 7, 2022 at 13:13
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    $\begingroup$ @SacredGeometry Here is an example on how this can be accomplished in Puredata and the explanation: pd-tutorial.com/english/ch03s07.html $\endgroup$ Commented Apr 14, 2023 at 10:45
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Probaly you have seen this papers/ info allready:
VOICE CONVERSION USING PITCH SHIFTING ALGORITHM BY TIME STRETCHING WITH PSOLA AND RE–SAMPLING and Pitch shifting TC- Helicon

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    $\begingroup$ it's interesting how both of those references totally ignore more seminal work by Lent and me and Hamon et al. $\endgroup$ Commented Jan 29, 2016 at 3:06
  • $\begingroup$ this is the seminal reference to Hamon et. al.. $\endgroup$ Commented Jan 29, 2016 at 3:23
  • $\begingroup$ Sorry I don have access to all that type of information. So i miss probably the best sources ... $\endgroup$ Commented Jan 29, 2016 at 5:22
  • $\begingroup$ you can get a pdf of my paper from researchGate. $\endgroup$ Commented Jan 29, 2016 at 7:11

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