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Re: FFT Format for iTunes Plug-ins.
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Re: FFT Format for iTunes Plug-ins.


  • Subject: Re: FFT Format for iTunes Plug-ins.
  • From: john <email@hidden>
  • Date: Wed, 11 Jun 2003 20:11:40 -0400

Hi Justin,

A quick and easy way would be to put together a simple audio file that has a single tone ranging in frequency from low to high, and then examine the FFT output and see what the format is! :)

Also, iTunes might use vDSP. Maybe you can find some info about its FFTs - here's the link:

http://developer.apple.com/techpubs/macosx/CoreTechnologies/vDSP/ vDSP.html

Hope this helps!

-- John


Hello,

I am working on creating an iTunes visualizer plug in using the sdk.
From the tech-note describing the sdk, it is mentioned that the provided array (visualPluginData->renderData.spectrumData) is a 512-point FFT of the sample data, but give very little additional information beyond that. I am fairly new to FFT and have not been able to confidently infer a few things. I would like to take the FFT data and display a frequency/power display. Plotting the array directly seems to be pretty close, but i want to make sure. (also it doesn't quite match up the the display in the track display part of iTunes)

So, some specific questions, hopefully someone can point me in the right direction.

Is the FFT data already processed into frequency/power?
Are the real and imaginary parts in the array? Perhaps something like RIRIRIRI... or RRRRiii
Since i tunes can play a variety of sample rates, is the fft performed on data that is always the same rate (say 44.1 khz)

Is there sample code for finding the strength of a given frequency (say 440hz) from the fft data provide by iTunes? I have seen similar code for different FFT implementation, but it looks like i need to know a little more about the format of what iTunes provides.

This might be slightly off topic for the list, if so please point me to the right venue.

Thank you, -justin
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References: 
 >FFT Format for iTunes Plug-ins. (From: Justin Bakse <email@hidden>)

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