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iTunes Visualizer Spectrum  What does it mean?So the iTunes audio spectrum input is a structure consisting of 16 values. What exactly do these values represent? Do the values correspond to different frequencies, and if so, which frequencies do these values represent?
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each is a different bucket of frequencies. which ones exactly are undefined.
I ran an assortment of square waves at different frequencies through it; below around 128 Hz (I think) it showed up oddly (all the values were oscillating between 2 large values), and for higher frequencies one level would have a greater value than the others at each frequency.
I figured it out. iTunes creates a spectrum of 256 samples, ranging (for me) from 22050/256 to 22050 Hz in a linear fashion (~86 Hertz). The spectrum passed to Quartz Composer uses 16 different averages of these 256 samples. These are averaged by the following formula:
(Frequency = iTunes*22050/256 Hz)
QC  iTunes  # of Samples
0  [1,3]  3
1  [4,6]  3
2  [7,9]  3
3  [10,12]  3
4  [13,20]  8
5  [21,28]  8
6  [36,29]  8
7  [37,48]  12
8  [49,60]  12
9  [61,72]  12
10  [73,88]  16
11  [89,104]  16
12  [105,128]  24
13  [129,152]  24
14  [153,184]  32
15  [185,256]  72
I first figured this out when I created a sine wave at 11025 Hz (1/2 my maximum), and only index 12 in iTunes responded (with every other index equal to zero, which has never happened to me before). Then, after creating several (~100) more samples, I got a general idea of where certain frequencies were being stored. I narrowed them down to intervals of ~86 or 22050/256 Hertz.