Thanks for that, Gianni. It is an excellent and clear explanation. It
should be posted for all to see and learn from.
Bernie Krause
>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
>>
>>
>>
>>[Non-text portions of this message have been removed]
>>
>>
>>
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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
>
>
>[Non-text portions of this message have been removed]
>
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