Possible noise issue in EQ A2.0.3

[blatt]

There is a possible risk of the EQ creating noise. I used the EG to limit sound to 200 hz and up and empty (cleaned) areas got noisy visually and audibly.

[steve]

If you could describe the steps to reproduce the problem then we could look into it.

[blatt]

Hi. Make a steep curve in EQ and apply to some sound. that's it. The frequencies of the added new sound (parts that were earlier on silent) relate to the curve.

[blatt]

[steve]

I can't reproduce the issue.

Can you be more specific about steps to reproduce.
What operating system are you using?
Which version of Audacity? (look in "Help > About Audacity")
What settings do you have in Plot Spectrum and Equalization?
Are you applying Eq to absolute silence or to very low level audio?
What format is the track?
Are the Audacity Preferences at their default settings?
Anything else?

[Robert J. H.]

Maybe it is dithering noise, that's boossted?

[Gale Andrews]

I don't know how you got such an erratic green line in EQ (the green line is the curve that Audacity actually applies). Does it help if you use a longer filter, which should bring the green and blue lines closer together?

We cannot see enough of what you are doing. If the problem persists, please post a few seconds of audio in WAV format that you are applying the EQ to, and save the curve you are using as a preset then export and post it as an XML file (Save / Manage Curves).


Gale

[blatt]

start comp/OS, win7 home; pro
start audacity 1.3.13; 2.0.2; 2.0.3
create track mono; stereo
create 10 sec 0,1 strength tone ex 440 hz ...
apply random silences in the tone
select sound
open EQ,
create curve; ex preset walkie talkie
apply EQ
drag track line -> visual lines appear in some silent areas
view spectrum -> curve looks funny across the line

The pattern at my place seems to be that at each sound start and end position approx 0.088 sec of new low level sound is mid positioned with EQ anchor frequences. This would make a really fun effect elsewhere from the menu.

[steve]

drag track line -> visual lines appear in some silent areas
view spectrum -> curve looks funny across the line

Not seeing that here.
Is your Windows 7 32 bit or 64 bit?

[Robert J. H.]

How big is the amplitude of the modified silences (sorry, I can't see the graphics myself)?
If the disturbance is rather tiny, I can offer the following explanation:

The eq effect is built up with several Filters.
They are most likely so called IIRs (infinite impulse response).
As the name implies, an impulse that is fed into the filter will produce an impulse response that decays for a infinite time exponenttially. The limit is only the bit resolution, when small values can't be expressed by floating point values anymore.
Thus, when we have a signal, that is followed by absolute silence, any filtering will introduce non-zero values in the silent part. A little experiment to show this: create a mono track and execute the following script from the nyquist prompt.

Code: Select all

(abs-env (seq (snd-from-array 0 44100 #(1 1 1)) (s-rest 1)))
;(print (snd-samples s 500))

We-ve created an artificial impulse. You can open the nyquist prompt again, remove the semikolon and press the debug button to display the values (1 1 1 0 0 ...).
You can now apply the EQ effect "Walkie-Talkie" or whatever you like. The zero values have now changed. Again, open the Nyquist prompt and press debug to control this.
The value after 0.01 s is about -60 dB.
There are certain factors that have a less or greater impact on the amplitude of the "silence". Firstly the sound before this region. Secondly, the properties of the eq preset.
It is clear that active filters (such that can boost or cut) introduce greater changes than passive ones (e.g. classical lowpass and highpass).
If the noise that you've observed should really be of the kind explained here, we can truely say that it is not a bug but simply a given physical fact that causes this behaviour (in each and every sound program that uses filters).