# 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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### Re: iTunes Visualizer Spectrum - What does it mean?

each is a different bucket of frequencies. which ones exactly are undefined.

### Re: iTunes Visualizer Spectrum - What does it mean?

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.

### Re: iTunes Visualizer Spectrum - What does it mean?

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.