Not having ever looked at our FFT code, I won't address the issue of whether the documentation
is right or wrong (I suspect the documentation simply has a typo; base2 FFT's are the easiest
to implement, given that they're the only kind anyone ever seems to write about, so I can't
believe we have a base4 implementation, which would be less easily usable).
I'm +1 on providing window functions, given how commonly needed they are. I'm 0 on the spectrogram,
having personally spent time trying to use a naive spectrogram as a power spectrum estimator.
I only spent enough time on that task to come to the understanding that it wasn't as simple
as it seemed it should be. Maybe even that understanding is wrong. In any case, I would
prefer that the package provide a function that "does what most people think of when thinking
about a spectrogram" rather than the naive spectrogram. Unless my understanding, as I mentioned,
is wrong and the naive spectrogram is actually useful.
Al

Albert Davidson Chou
Get answers to Mac questions at http://www.MacMgrs.org/ .
 Original Message 
From: Carsten Krüger <C.Krueger@gmx.org>
To: dev@commons.apache.org
Sent: Monday, December 3, 2007 5:45:03 AM
Subject: [math] transform  fft documentation bug? proposal window function & sonogram
Hello,
the class documentation[1] of fft states
"We require the length of data set to be power of 2"
and the method documenation[1] this
"Perform the base4 CooleyTukey FFT algorithm (including inverse)."
One must be wrong:
base4 alogrithm means that the length of data has to be power of 4
fft without window function is pretty useless, it would be nice to
implement typical[2] window functions for example:
Hamming
Hann
Blackman
Trinangle
Rectangle
Welch
Gaussian
after having fft and windowfunction, sonogram/spectrogram[3] would be
nice and is easy to implement (10 lines of matlab code).
for example:
public double[][] sonogram (int fftsize, int overlap_factor, int
windowfunction)
greetings
Carsten
[1]
http://commons.apache.org/math/apidocs/org/apache/commons/math/transform/FastFourierTransformer.html
[2]
http://en.wikipedia.org/wiki/Window_function
[3]
http://en.wikipedia.org/wiki/Spectrogram

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