naturerecordists
[Top] [All Lists]

harmonics

Subject: harmonics
From: Gianni Pavan <>
Date: Sat, 22 Jun 2002 01:16:40 +0200
At 13.07 21/06/2002 -0400, you wrote:
>When using sonograms be sure and be aware that the math involved in
>producing sonograms produces artifacts in the display. In particular a
>lot of what looks like harmonics off the louder parts of the call is not 
>there.
>
>Walt
>

Walter,
         if harmonics appear on a spectrogram I think it is because 
something is in the signal, maybe due to distorsions (overloads) in the 
input stages of many small DATs (the "brickwalling" effect of the small 
SONY DATs is well known...) or in a poor sound card. I can't believe 
harmonics are generated by the math involved in computing digital 
spectrograms. Of course FFT computation should be done with the right 
algorithm and with large dynamic (I mean using float math instead of 
integer math and arranging scaling to avoid overflows). Although, it is 
true that the time-frequency uncertainty principle may cause confusion when 
analyzing pulse trains: they may appear as continuous tones with lots 
of  harmonics when the pulse interval is shorter than the analysis window. 
In such a case waveform inspection and a correct tuning of analysis 
parameters (fft size, window size, window shape, scanning step, dynamic 
range of the display) may help in revealing or at least in clarifying a bit 
more the real structure of a signal.
Just to complete my thought, I should say that the sampling rate often is 
not high enough to reveal the high frequency structure of a sound: a pulse 
train with 1/10000s pulse interval appears like a perfect sinusoidal tone 
at 10 kHz if sampled at 32kHz (with good anti-aliasing filters, of course); 
a 48 kHz sampling reveals a 2nd harmonic... only a very high sampling rate 
reveals its "pulsed" and not continuous nature.

Gianni

--------------------------------------------------------------
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 Laboratori
Tel/Fax   +39-0382-526208 Segreteria
Web       http://www.unipv.it/cibra



________________________________________________________________________
________________________________________________________________________

<Prev in Thread] Current Thread [Next in Thread>
Admin

The University of NSW School of Computer and Engineering takes no responsibility for the contents of this archive. It is purely a compilation of material sent by many people to the naturerecordists mailing list. It has not been checked for accuracy nor its content verified in any way. If you wish to get material removed from the archive or have other queries about the archive e-mail Andrew Taylor at this address: andrewt@cse.unsw.EDU.AU