What a fantastic explanation, a true scholar! Thank you
Martyn Stewart
<http://naturesound.org> http://naturesound.org=20=20
425-898-8516
-----Original Message-----
From: Gianni Pavan
Sent: Tuesday, July 08, 2003 2:57 AM
To:
Subject: Re: [Nature Recordists] Sonograms
A weighting function must be applied to a signal segment before being
analyzed by FFT; the FFT sees the signal as a circular series of sample
and
the weighting avoids sharp transitions at the edges of the segment.
The most used ones are (there are others to be used in very specific
situations):
Hanning
Hamming
K-Bessel
Gaussian
The weighting has two main effects: one is to reduce the amplitude and
thus
the "importance" of the edges: this slightly increases time selectivity.
The other is a decrease of frequency selectivity: this produces a larger
peak in the frequency spectrum. These two effects are clearly related
with
the inverse relation among time and frequency selectivity. Longer the
signal, better the frequency selectivity.
The shape of the weighting function is very important: other than
affecting
time selectivity and thus the frequency peak width, it also affects the
shape of the frequency peak. Some produce sidelobes, that are secondary
peaks on both sides of the main peak. Weighting functions producing the
narrowest peaks also produce the highest sidelobes. Those with larger
peaks
has lower sidelobes...
Sidelobes become very evident when you set a window length shorter than
the
FFT size (in some programs this is called zero padding).
The presence of sidelobes is particularly evident when the signals are
very
clear, the signal to noise ratio is high, and when you plot the sonogram
with a very high dynamic range. I like to show 96 dB of dynamic range on
my
sonograms to see all components of the signal and of the background.
Thus I
prefer to use windows with no sidelobes at all. This means to have
larger
peaks I normally compensate by increasing the window length.
Unfortunately
few programs allows a complete control on all analysis parameters.
The Hanning function is the typical window used in most signal analysis
procedures; its shape is a cosinusoid.
Try to use the K-bessel and Gaussian ones to increase time selectivity
and
to avoid the sidelobes typical of the Hanning window. Linear (no
weighting
at all) and Hamming windows are not reccomended becouse of their
sidelobes.
Hamming has the best frequency selectivity but you have to pay this with
sidelobes spreading on the whole spectrogram.
To make some experiments I suggest to record or synthetize a constant
frequency and a frequency modulated tone and then to analyze them with
the
different windows you can set by changing size and shape.
Gianni
At 21.38 07/07/2003 -0700, you wrote:
>Here is one for the genius's, or maybe not so and that I'm just "thick"
>Subject: Sonograms.
>When deciding what Sonogram to use, can someone give me a general
>rundown on what names do what!
>EG:
>* Hann
>* Blackman
>* Hamming
>* Rectangular
>* Triangular
>
>What purpose does each name represent and how best to use it?
>Hope I don't open a "too bigger" can of worms here :-)
>
>Martyn
>
>
>
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--------------------------------------------------------------
Gianni Pavan
Email
Centro Interdisciplinare di Bioacustica e Ricerche Ambientali
Universita' degli Studi di Pavia
Via Taramelli 24, 27100 PAVIA, ITALIA
Tel/Fax +39-0382-525234
Web http://www.unipv.it/cibra
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