noisered vs Audacity's noise removal filter

[Daniel Jensen <jensend@iname.com>]

I find that Audacity's "Noise Removal" spectral noise gate filter gives 
me much better results than SoX's noisered effect. However, since I have 
over a thousand files requiring noise removal which could all use the 
same noise profile, and since for that kind of a job Audacity's 
rudimentary batch processing is tremendously inferior to using the 
command line, I really wish I could use SoX.

The primary problem seems to be that SoX gives "tinkling" artifacts like 
those in badly encoded mp3s. My preliminary guess was that this meant 
that SoX is making the determination of whether frequencies are above or 
below the profiled noise gate level independently for each FFT bin and 
window, with the result that individual bins rapidly pop back and forth 
across the threshold.

Glancing briefly at the noisered source (not enough to really tell what 
it's doing) I see that ever since the initial checkin some barebones 
attempt has been made to smooth out that decision, with the humorously 
applicable comment "Audacity says this code will eliminate tinkle bells. 
I have no idea what that means." I think this must mean that the code 
was adapted from the method used by the old noise removal filter in 
Audacity 1.0/1.2, without a lot of comprehension of what was going on.

At some point in their 1.3 beta cycle Audacity improved their noise 
filter substantially by adding frequency smoothing and attack/decay time 
and associated parameters. (Again, I haven't yet looked closely enough 
to say how these are implemented.) In their wiki they say "if you set 
both of the "smoothness" sliders to zero, you'll get something very 
similar to the old noise removal algorithm in legacy Audacity 1.2.x 
which is prone to artifacts and distortion..." Sure enough, setting both 
the frequency smoothing range and the attack/decay time to zero gives 
results basically identical to SoX's.

It would also be nice to have a more comprehensible scale. Audacity's 
effect asks you to specify "noise reduction" in dB; this is much more 
intuitive than noisered's 0-1 scale and does a decent job of allowing 
you to match noise levels between different clips. (A sharp transition 
where the background noise level suddenly changes can in many 
circumstances be more of a problem than the noise itself).

Obviously since Audacity's filter is GPL'd  while SoX's effects are 
LGPL'd, code can't be shared. (Even if SoX decided that this could go 
under the GPL, since Audacity's filter is in WxWidgets-heavy C++ this 
wouldn't help much.) I know very little about how clean-room one has to 
be in reimplementing their ideas to avoid the result becoming a 
derivative work.

Thoughts?

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