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
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