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How to do real-time frequency inversion?

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I am trying to design a real-time analysis/resynthesis Sound to generate a literal frequency inversion of the audio input. This is mirror image inversion in the classical sense. Any thoughts on how to proceed? My experiments have generated freq shifts up or down - that's no problem - but the actual inversion math is the problem.

 - Brian B.
asked Jan 30, 2016 in Sound Design by bbelet (290 points)

2 Answers

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One approach is to first, negate the frequencies.  And then offset them so they are all greater than 0 again.  

Imagine you had a simple spectrum with frequencies (100 200 300 400).  When you negate the frequencies you get (-100 -200 -300 -400).  Now, if you add an offset of 500, you will get: (400 300 200 100).  Since you have changed the order of the frequencies but have not changed the amplitudes, the effect is an inverted spectrum.

For most live spectra, the effect will be to have very bright timbres, where the high partials are weighted more heavily than the low partials.

In the Kyma Sound Library, if you search by Sound name for Invert Spectrum of Live Input, you'll find an example.

Let us know how it goes!

answered Jan 30, 2016 by ssc (Savant) (115,850 points)
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Just realized that you may have meant interval inversion (rather than spectral inversion).  For that, you could start with a monotonizer to remove the pitch contour.  Then subtract a pitch tracker so the intervals go in the opposite direction and add an offset pitch so you can bring it back up to the mirror point.

If you search in the Sound library for Invert pitch shape, there is a Sound that you could try using as a starting point for your live melodic inversion.

answered Jan 30, 2016 by ssc (Savant) (115,850 points)
Yes - interval inversion of live instrument input. Thanks for these tips - I will investigate!
Hmmm.. I understand the basic signal flow you suggest, but am not sure which Sounds to use. E.G., what Sound uses the Monotonizer as input or parameter equation factor? I'm feeling a bit dense with this......  - BB
At the top of the Sound Browser, there's a section called Search.  Select Sound name as the search type and type "Invert pitch".  That should bring up an example from the Kyma Sound Library.
What is the input that you are trying to invert?  It's possible that the range of the frequency tracker needs to be adjusted to match the range of the inputs you expect to see.
Input is electric bass. Experimenting with the 'Invert Pitch' Sound, I have 40 hz for MinInputPitch and 800 hz for the MaxInputPitch.
  If I play in the highest octave on the bass (above 12thg fret G string) I get some good results, but anything lower is larger mush. Advice?  Thanks. - BB
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