Support formant shifting (in addition to formant preservation)

[JackismyShephard]

I an wondering whether it would be possible (or perhaps already is) to perform formant shifting with this library. I have seen this library used in other projects for the purpose of formant shifting by doing something like

pitchshifter.shiftpitch(
                audio,
                factors=1,
                quefrency=formant_qfrency * 1e-3,
                distortion=formant_timbre,
            )

but that does not really have any effect.

I don't really know that much about formant shifting so apologies in advance if my question is not well phrased.

[jurihock]

stftPitchShiftDemo provides some demos for timbre shifting, without pitch altering.

Depending on your source signal you have to adjust the "quefrency" value, which is the reciprocal of the fundamental frequency of the source signal. See also readme and #52.

This specific algorithm may not work with all possible signal sources. If it doesn't work well for your source signal, please feel free to choose a different tool.