
Removing background noise in Audacity by differencing stereo channels - johns
http://weblogs.asp.net/jgalloway/archive/2013/07/17/removing-background-noise-in-audacity-by-differencing-stereo-channels.aspx
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ohazi
Nice trick, but don't forget that uncorrelated noise is always additive. I
know a few people who have built noise cancelling headphones (either for fun
or for a project class I once TAed) who discovered (after dealing with lots of
fun feedback issues) that their "quiet" output was noisier than the original
output.

There's usually a way to make this trick work, but it's important to have
accurate models of your various noise and interference sources.

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JTxt
Good tip.

I like to take a mono track, duplicate it, invert one of them, then make both
of them into a stereo track.

Now play it and play with the proximity of the left and right speakers. When
they're close, they cancel each other out. Similar principle of noise
canceling headphones.

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acjohnson55
Do you do this just for fun? Correct me if I'm wrong but this just seems like
a recipe for weird phasing issues.

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JTxt
Yes, it was just for fun, and to show my nieces and nephews that sound can be
canceled.

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dnautics
what if you take the windowed cosine transform, and then see where the
transform is within 10% of each other (or less than a threshold value). Then
zero out all such regions. With the technique you're using I'd worry about the
phase happening to be exactly 180 degrees if the frequency and position of the
noise source are 'just so' \- which might explain your observation of 'pops'.

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Joona
This is slightly off-topic, I apologize.

Is there a way to run Audacity's noise-removal real-time (or a few second
delay)? I've been trying to Google this, but without success.

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JonnieCache
There is plenty of real-time noise reduction software in VST plugin form.
Here's an (expensive, but very effective) example:
[http://www.sonnoxplugins.com/pub/plugins/products/restore.ht...](http://www.sonnoxplugins.com/pub/plugins/products/restore.htm)

Otherwise, you can get a long way with just traditional gating, EQing and
maybe a multiband compressor.

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Joona
Thanks, I should have mentioned that I don't know anything about audio, but
I'll look in to it!

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JonnieCache
A gate only lets sound through when it goes above a certain level, so if you
set the threshold just above the level of the noise, the parts of the audio
that are just noise will be silenced. This obviously works best if you're
treating something intermittent like speech or a single instrument, it
wouldn't work on a whole piece of music.

An EQ lets you attenuate certain frequencies, so if there are some parts of
the spectrum in your recording which are just noise, you could remove them,
eg. for speech you could remove all the information below 300hz and above
12-13000hz

Multiband compression is a bit too complex to explain here :)

The specialist plugins tend to use a combination of these techniques and then
their own special secret algorithms, which is why they're so expensive.

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Joona
Thanks for the explanation! I'm already using Noise Gate in OBS
(livestreaming), but the noise is still an annoyance. I'll definitely look in
to EQ. Thank you so much. :)

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JonnieCache
The lowest hanging fruit for you will probably be improving the source. Get a
better mic, make a pop shield out of an old pair of tights and a wire
coathanger, fill the room with soft furnishings, make yourself a ghetto
isolation booth by putting a duvet over your head and climbing into your
wardrobe, etc.

That last one might sound silly but many international-pop-career-launching
albums have been recorded like that.

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Joona
I've been thinking about it, but I don't really want to use 100€ on a new mic.
I'll try to "fix" this one first. :P Thanks for the advice!

