Spectrum stretcher Audiounit

Eva_platinum · Joined: Aug 19, 2005 Posts: 19 Location: Leeds

Hi, I'm new to the whole audiounits development or any software plug ins development for that matter...

I decided to start with creating a frequency stretcher using sonic birth (its interface is fairly easy for me to understand more or less) but I don't seem to find the right functions to do it...

Any one has any idea ???

By Frequency stretcher I don't mean pitch shifter... It's a tool Trevor Wishart used on some songs thanks to his Composer Desktop Project software... and I'm trying to recreate the same effect as a audiounit...

DrJustice · Posted: Wed Mar 28, 2007 4:02 pm Post subject:

What exactly do you mean by Frequency Stretcher? If it involves spectral manipulation, like stretching out the spectrum, then using the FFT modules is a possibility.

DJ
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Eva_platinum · Joined: Aug 19, 2005 Posts: 19 Location: Leeds

Yeah THat is what I meant... I'll have a look into the FFT

Thanks

Eva_platinum · Joined: Aug 19, 2005 Posts: 19 Location: Leeds

Some people are trying to do the same thing using Max/MSP unfortunately I don't understand anything to MAX/MSP nor their explanations on how to do it... Crying or Very sad

http://www.cycling74.com/forums/forums.html?t=msg&th=23411&rid=0&S=0a5a278c0c899d14cf8037724a7490d1

[Edit: made the link work, Blue Hell]

DrJustice · Posted: Wed Apr 04, 2007 6:09 am Post subject:

I couldn't get that URL to work, and I didn't find anything relevant straight away at their forum. If you fix that URL, I'll have a look.

DJ
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Eva_platinum · Joined: Aug 19, 2005 Posts: 19 Location: Leeds

Link working now apparently thank you!!

blue hell · Posted: Wed Apr 18, 2007 2:56 pm Post subject:

maybe I should have bumped the thread after fixing the link ...

DrJustice · Posted: Sat Apr 21, 2007 6:25 am Post subject:

I browsed through most of that thread. It seemed to revolve mainly around the practicalities of using the Max/MSP fft modules. You could always get an evaluation version of Max/MSP and try out the patches they've been posting there.

As long as you want to stretch the entire spectrum, the simplest solution is a remapping of the fft bins (harmonics). A refinement of this would be to allow the mapping, or stretching, to be variable, say using linear interpolation to pick amplitudes when filling in the output spectrum. The most precise method would be to resynthesize the output by properly transposing each single harmonic. This would involve adjusting the phases of each harmonic since it might not come from and/or land in a fft bin that represents the precise transposed frequency. The first method is simple to do in a graphical modular environment, the second and third methods may not be feasible as it would use an inordinate amount of modules.

If the stretching is wanted while retaining the fundamental frequency, then we get the added problem of pitch estimation, since we would only want to space out the harmonics that lies above the fundamental.

I'm sorry that I can't give more practical advice, neither a Max/MSP or Audiounits user. I just have a general interest in things like this. I'd recommend reading about the "phase vocoder" which is basically a way of using ffts (and also linear prediction and oscillator banks) to achieve various types of effects like pitch shifting, time stretching, vocoding, resynthesis etc..

DJ
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dewdrop_world · Posted: Sat Apr 28, 2007 1:36 pm Post subject:

I guess I should have posted earlier... this is pretty easy to do in SuperCollider.

This example uses a stock soundfile supplied with SC. You can use any input signal.