Re: FFT Format for iTunes Plug-ins
Re: FFT Format for iTunes Plug-ins
- Subject: Re: FFT Format for iTunes Plug-ins
- From: Jim Wintermyre <email@hidden>
- Date: Wed, 11 Jun 2003 20:01:04 -0700
From what I recall when working on a iTunes plugin, the FFT data
provided by iTunes is pretty low resolution. It's just the magnitude
values, and each value is a UInt8. The SDK examples kind of
indicated that the spectrum values go from 0-128 but in my plugin it
looked to me like the values went from 0-255. You get an array of
512 FFT values times 2 (max) channels. The example plugins in the
SDK use this data and should make it fairly clear...
Depending on what you want to do, the FFT data you get from iTunes
might not be "good enough" for you. Unfortunately, you're kind of
stuck because you can't do an FFT on the actual audio, because... you
can't access the actual audio from an iTunes visual plugin. I would
guess this is for security reasons. You do have access to the
waveformData in the RenderVisualData struct, but those are only 8-bit
values as well.
Jim
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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