class:: SpecPcile
summary:: Find a percentile of FFT magnitude spectrum
categories:: UGens>FFT
related:: Classes/SpecCentroid, Classes/SpecFlatness

description::
Given an link::Classes/FFT:: chain this calculates the cumulative distribution of the frequency spectrum, and outputs the frequency value which corresponds to the desired percentile.

For example, to find the frequency at which 90% of the spectral energy lies below that frequency, you want the 90-percentile, which means the value of emphasis::fraction:: should be 0.9. The 90-percentile or 95-percentile is often used as a measure of strong::spectral roll-off::.

The optional third argument strong::interpolate:: specifies whether interpolation should be used to try and make the percentile frequency estimate more accurate, at the cost of a little higher CPU usage. Set it to 1 to enable this.

classmethods::
method:: kr

argument:: buffer
an link::Classes/FFT:: chain.
argument:: fraction
argument:: interpolate

examples::

code::
s.boot;
b = Buffer.alloc(s,2048,1);

// Simple demo with filtering white noise, and trying to infer the cutoff freq.
// Move the mouse.
(
{
var in, chain, realcutoff, estcutoff;
realcutoff = MouseX.kr(0.00001,22050);
in = LPF.ar(WhiteNoise.ar, realcutoff);
chain = FFT(b, in);
estcutoff = Lag.kr(SpecPcile.kr(chain, 0.9), 1);
realcutoff.poll(Impulse.kr(1), "real cutoff");
estcutoff.poll(Impulse.kr(1), "estimated cutoff");
Out.ar(0, in);
Out.kr(0, estcutoff * 22050.0.reciprocal);
}.scope;
)

// Audio input - try different vowel/long-consonant sounds and see what comes out.
// Specifically, change from "ssss" through to "aaaa" through to "wwww".
(
{
var in, chain, perc;
in = SoundIn.ar([0,1]).mean;
chain = FFT(b, in);
//Out.ar(0, in * 0.1);
perc = SpecPcile.kr(chain, 0.5);
Out.ar(1, LPF.ar(WhiteNoise.ar, perc)); //NB Outputting to right channel - handy on PowerBooks
Out.kr(0, perc * 22050.0.reciprocal);
}.scope;
)
::