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? ------------------------------------------------------------------------------ WINDOWS 8 is here. Millions of people. Your app in 30 days. Visit The Windows 8 Center at Sourceforge for all your go to resources. http://windows8center.sourceforge.net/ join-generation-app-and-make-money-coding-fast/ _______________________________________________ SoX-devel mailing list SoX-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/sox-devel
