The nominal frequency resolution of a digital spectrogram is computed as
follows:
resolution =3D sample rate / FFT size
(it would be 43 Hz =3D 44100 Hz / 1024 in your set-up)
The bit-depth (8, 16 or 24 bit) does not affect the resolution.
However, the above resolution value corresponds to the =93height=94 of the
discrete pixels in the spectrogram image only. The actual physical frequenc=
y
resolution (the analysis bandwidth, as known from the ancient analog Kay
Elemetrics SonaGraph machines) is usually lower. The final bandwidth depend=
s
on the evaluation window (Hamming, Hanning, Rectangle, ... ) and whether
there is any zero padding mode activated. Assuming a Hamming window with no
additional zero-padding, the bandwidth would be about 56 Hz in your case.
Usually, a bandwidth of about 300 Hz (time constant =3D 3ms) is most
appropriate for bird sound analysis (which corresponds to the =93wide=94 se=
tting
in the old SonaGraph machines) because it provides a good compromise betwee=
n
time and frequency resolution. A narrower bandwidth would unfortunately als=
o
decrease the temporal resolution (rapid frequency modulations or short soun=
d
elements cannot be resolved).
The physical time resolution (the time constant) of a spectrogram is the
reciprocal value of the bandwidth (1/56 Hz =3D 18 ms). In digital
spectrograms, the amount of overlap between the consecutive FFT spectra may
further decrease the final temporal resolution. However, large overlaps wil=
l
never increase the final temporal resolution of a spectrogram. As in the ol=
d
analog sound spectrograph machines, the limit is still the reciprocal of th=
e
bandwidth.
Cool Edit does not provide a consistent way to adjust this overlap paramete=
r
(it selects the overlap according to the current zoom stage). So, it would
be difficult to specify the time resolution.
Therefore, I would recommend other programs for spectrogram-based
measurements (and also for exporting or printing spectrogram images). For
instance, the free Avisoft-SASLab Light software could be used instead. Tha=
t
software also indicates the actual time and frequency resolution of the
spectrograms. It is available together with the demonstration version of
Avisoft-SASLab Pro from
http://www.avisoft-saslab.com/downloa_htm (first item).
Regards,
Raimund
> Hi everyone,
>
> I was wondering if someone could help me with the following:
>
> I would like to determine the resolution of my measurements of frequency
> and
> time characterists in CoolEdit. At present, I use wave files at 44.1 kHz,
> 16
> bit and FFT of 1024.
>
> Given the sample rate, bit rate and FFT size, is there a simple formula t=
o
> determine the frequency resolution (Hz) and time resolution (ms)?
>
> Thanks and best wishes to all.
>
> George Sangster
> The Netherlands
>
>
>
>
> "Microphones are not ears,
> Loudspeakers are not birds,
> A listening room is not nature."
> Klas Strandberg
> Yahoo! Groups Links
>
>
>
>=20
>
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