Proposal Noise Removal
|Proposal pages help us get from feature requests into actual plans. This proposal page is about improving the Audacity Noise Removal effect.|
Proposal pages are used on an ongoing basis by the Audacity development team and are open to edits from visitors to the wiki. They are a good way to get community feedback on a proposal.
- Note: Proposals for Google Summer of Code projects are significantly different in structure, are submitted via Google's web app and may or may not have a corresponding proposal page.
Effective noise removal is a difficult and highly subjective task, with trade-off between the degree of noise removal and the acceptable degree of quality damage to the remaining audio. There are a number of differing approaches to the task of noise reduction. The aim of this proposal is to bring together current work in Audacity noise reduction in one place with the aim of improving the Audacity Noise Removal effect to a level on par with, or superior to other noise removal applications.
- Marco Diego Aurélio Mesquita
Noise removal is one of the most common tasks among Audacity users. Transferring vinyl/tape to CD, recording a podcast, making a music recording, and many other common Audacity tasks can benefit from effective noise reduction.
Noise Reduction methods
Current Audacity method
Does the noise gating (downward expansion) take into account the full spectrum audio level as well as the level in the narrow band that is being processed?
The advantage of this method is that if both the noise and the "music" are broadband, then one would expect that low level noise would be largely masked when the music is loud and would not require removal. Considering that removing the noise will create some "damage" to the music, it would often be preferable to rely on this masking effect rather than attempting to remove the noise.
The obvious limitation of this method is that if the noise has a much broader frequency band than the "music", then it will not be masked and will remain obvious while the level of the "music" is above the noise level threshold.
Gating according to level of noise sample
Gating each frequency band according to the level of the noise sample at that frequency.
Bill 21Apr11: We have to be careful and precise in our wording. The above statement says to me "The signal level is compared to the level of the noise sample in each frequency band (call this the band threshold). If the signal is above the band threshold it is passed unaltered; if it is below the threshold, gating (or downward expansion) is applied." The question is, what do we mean by "signal level"? I could mean the overall signal level, or the signal level in the given frequency band. Treating the AudNR effect as a black box, I am led to the conclusion that it is the former. In the one other NR effect that I have experience with, it seems to be the latter.
In spectral subtraction, the average noise spectrum is subtracted from the average signal spectrum, performed independently in the frequency bands critical to human hearing. This reduces the level of each frequency band by an amount proportional to the level of that frequency in the noise sample. This could be done more softly (power subtraction) or more strongly (magnitude subtraction).
A noise reduction effect for Audacity developed by Jérôme M. Berger uses a technique that has been borrowed from image/video processing. The patch is available here.
Jerome describes the process as follows:
- The coring function is a soft thresholding function which gives very small gains for frequencies whose power is lower than the threshold and gives gains close to 1 for frequencies whose power is higher than the threshold.
- The idea being that frequencies whose power is higher than the threshold will be mostly signal which will mask the noise and must be kept, while the other frequencies will be mostly noise which must be removed.
- Mathematically, the coring function I used is: 1-exp(|FFT(f)|^2/s) where f is the frequency and s is a strength derived from the parameters.
The current version of this effect has a slope control for setting the "color" of the noise that is to be removed (dB per decade). Some examples of this filter in use can be found here: Examples. ("Bruts" are the originals, "Filtre" are filtered).
A significant difference between this effect and the Audacity Noise Removal is that Noise Coring reduces hiss from within other sounds as well as during quiet sections. This makes it particularly useful for reducing noise such as tape hiss, particularly from music where the hiss is evident through the music.
The current version of this patch has a flaw that it will sometimes create small glitches in the processed audio which is possibly due to the way that the envelope changes from one window to the next. Jérôme is currently looking at adding "time smoothing code" to cure this issue.
Summary of Current State
So far there has been significant improvement by the introduction of the Sensitivity Slider, and fixing the attack/decay times slider.
It has been observed that increasing the FFT size produces improved noise removal with less damage caused to the remaining audio.
A selection of test samples will go here.
Proposed Patches and Modifications
Patches and modifications will go here.
Test results should include the name of the source file, patches and modifications used, what settings were used, and full details of any additional processing. The source file should be listed in the Test Samples section above.
Test results will go here.