[ecasound] Re: Noise reduction

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Subject: [ecasound] Re: Noise reduction
From: Kai Vehmanen (kaiv@wakkanet.fi)
Date: Tue Dec 28 1999 - 23:45:12 EET


Here's something I wrote a few months ago. If you happen to know
any (good, working) GPL'ed software (or algorithms) for
noise-removal, let me know!

One thing to try is to use a narrow bandreject filter and use a slow
oscillator controller for finding the optimal parameters... so
something like "-efr:20,50 -kos:1,20,20000,0.01,1.5". This applies a
50Hz wide bandreject filter. Sine oscillator slowly (1/100Hz) varies
the center-frequency between 20-20000Hz. Far from optimal, but might
work in some cases.

---------- Forwarded message ----------
Date: Wed, 3 Nov 1999 00:16:47 +0200 (EET)
From: Kai Vehmanen <kaiv@wakkanet.fi>
To: ecasound-list@wakkanet.fi
Subject: Re: [ecasound] a new kind of gate

On Tue, 2 Nov 1999, Jeremy Hall wrote:

> The gate that we currently have will rotate out the sound if it falls
> below a threshold, but I want one that does this, but also in the live
> sound can take out this cruft as well.

Btw; I'm yet not sure whether gates will act this way in the future.
I'll have to think about this some more.

> The application I am thinking of is digitizing a tape that has background
> noise associated with it. I wonder, would dynamics of the music be
> severely distorted?
[...]
> Would this be difficult to edit out?

This is a common problem. There're lots of ways to do it. One of the
better one's is CoolEdit's "noise-analyzing and removal". You select
a part of audio file that contains only noise. Based on this sample,
CoolEdit processed the entire file. Adding this feature ecasound is
not easy, as you can imagine. There are some specialized tools for
this. From Linux Audio Quality HOWTO:

"Brian ?? wrote to point out some tools useful for cleaning up noisy
soundfiles. Generally speaking, this will never produce results as
good as starting with a clean soundfile, but sometimes you have no
choice. denoi and dnr may be found at: http://www.sci.fi/~mjkoskin/.
Says Brian, "These routines ... take a sample of noise from the
beginning of the recording and filter it out from the rest of the
recording." I've found them both useful, though it takes some playing
with parameters. Also of interest is gramofile, available at
cardit.et.tudelft.nl/~card06/. Brian writes that "Gramofile is a
program to remove clicks and pops from LP recordings. It can handle
huge files and split them up into separate tracks. It doesn't do
denoising, but it does have some other filters."

I haven't had time to test these. If you've tried some of these and think
that they do the job, please, let me know about it. This goes for
other programs too. If you think some (open source) program has good
effects, tell me about it, and I'll try add those effects to ecasound.
Testing various designs, let alone designing them from scratch, takes
a lot of time.

--
Kai Vehmanen <kaiv@wakkanet.fi> -------- CS, University of Turku, Finland
 . http://www.wakkanet.fi/ecasound/ - linux audio processing
 . http://www.wakkanet.fi/sculpscape/ - ambient-idm-rock-... mp3/ra/wav
 . http://www.wakkanet.fi/kerttulin_listat/ - music&movies (in Finnish)


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