Re: [linux-audio-dev] JAAA Bandw parameter

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Subject: Re: [linux-audio-dev] JAAA Bandw parameter
From: Andrew Gaydenko (a_AT_etver.com)
Date: Sun Dec 12 2004 - 14:04:31 EET


Thanks! I'll try to say the same from user point of view :-)

So, if I understand well, gray drawing is some kind of noise filtering
which is possible when analysis takes place some period of time.

In othe words, a card SNR is blue, as our ears do not integrate a
sound during such long period of time. But for more o less constant
noise spectrum (which is true for any card), noise integration alow
us to expand measurement range (about 20db in my case) - gray spectrum.

Andrew

======= On Sunday 12 December 2004 13:20, Fons Adriaensen wrote: =======
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


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