12 Dec
2004
12 Dec
'04
6:17 a.m.
On Sun, Dec 12, 2004 at 06:54:46AM +0300, Andrew Gaydenko wrote:
1. Can anybody explain me (I think, Fons Adriaensen
can :-) the
JAAA "Bandw" parameter meaning?
The bandwidth determines how much detail you see on the frequency scale.
If you have a small bandwidth, you will be able to separate two signals that are
very close together in frequency, while for a larger bandwidth they will merge into
a single peak. So why not use a very small bandwidth all the time ? The reason is
that small bandwidth requires a longer FFT, and you will see less detail in time.
So there is a tradeoff to be made between resolution in time and in frequency.
2. When Bandw parameter is rather small, I see two
spectrums - main (blue)
and, below the first, - the second one (gray). What is the last spectre spectrum
meaning?
From the README :
'Bandw' sets the FFT length, and hence the bandwidth of the analyser. Depending
on this value, the size of the display and the frequency range, you may sometimes
see two traces. This happens when the resolution of the analyser is better than the
display, so that one pixel contains more than one analyser value. In that case, the
blue trace is the peak value over the frequency range represented by each pixel, and
the gray one is the average value. The first one is correct for discrete frequencies,
and the latter should be used to read noise densities.
To see this, switch on both the sine and noise generators, and loop back into JAAA.
Set a bandwidth and frequency range that gives you the two separate traces. Now
put a peak marker on the tone, and a noise marker somewhere on the noise. You will
see that the noise marker is on the gray trace.
--
FA