FFTCrest Spectral crest measure

FFTCrest.kr(chain, freqlo, freqhi)

Given an FFT chain, this produces the spectral crest measure, which is an indicator of the "peakiness" of the spectral energy distribution. For example, white noise should produce a flat (non-peaky) spectrum, and therefore a low value for the spectral crest.

Optionally, freqlo and freqhi indicate the lower and upper frequency limits of the band to look at; by default, the whole FFT range (excluding DC and nyquist) is analysed.

In pseudo-equation form, the measure is calculated as follows:

Crest = S.maxItem / S.mean

where "S" is a list of the squared magnitudes in the spectral band. Note that this limits the value to being greater than or equal to 1. (Some research uses a logarithmic scale - you can apply the logarithm yourself if required.)

See also: FFTFlatness (which provides a similar type of feature)

Examples

s.boot;

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

(

{ // Example - vary mixture of white noise and pure tone with the mouse

var in, chain, crest;

in = XFade2.ar(WhiteNoise.ar, SinOsc.ar, MouseX.kr(-1,1));

chain = FFT(b.bufnum, in);

Out.ar(0, in.dup * 0.1);

crest = FFTCrest.kr(chain);

Out.kr(0, crest.poll(10, "crest"));

}.play;

)

(

{ // Same as above but subbands rather than whole spectrum. move mouse up&down too...

var in, chain, crest1, crest2;

in = XFade2.ar(WhiteNoise.ar, Blip.ar(numharm: MouseY.kr(200, 1, 1)), MouseX.kr(-1,1));

chain = FFT(b.bufnum, in);

Out.ar(0, in.dup * 0.1);

crest1 = FFTCrest.kr(chain, 100, 2000);

crest2 = FFTCrest.kr(chain, 2000, 10000);

Out.kr(0, [crest1.poll(10, "crest1"), crest2.poll(10, "crest2")]);

}.play;

)