Noise Removal Tests

[steve]

There's a free tool that may help in comparing the performance of different Noise Removal methods
http://libinst.com/Audio%20DiffMaker.htm

It's a Windows program, but works perfectly in Wine.

[billw58]

If you look at the flow diagram at the bottom of that page the software is merely inverting one recording then summing the two recording. It is apparently smart enough to deal with sample rate drift. But otherwise, for the purposes of our tests, why not just make a copy of the original noisy recording, invert it then mix that inverted copy with the noise-reduced samples. I just tried this and it is very revealing. Don't know why I didn't think of it before.

-- Bill

[billw58]

How does the DeNoise tone sound after processing? Is it much less artefacted? I agree we should be worrying about real life samples more than tone artefacts.
Gale

Gale:
The DeNoised tone is free of artefacts.

Tone tests are useful since they can exposes problems in the effect through both listening and spectrum plots. Spectrum plots are less useful with real-world samples, so we are left with listening tests. The "invert and sum" method can help expose artefacts in real-world samples. Spectrum plots of the result of "invert and sum" can be quite revealing.

For my first real world test I chose "Foxglove" by Bruce Cockburn - a fingerpicking solo guitar piece. I summed the stereo recording to mono, amplified to -2 dB then added noise at -40 dB. I processed it with Audacity NR with NR 20, Sens 0, Smoothing 100 and A/D 0.25. Then processed it with Denoise with NR 20, Noise Floor -40 (equivalent to Sens = 0) and noise profile = "white". Listening to the results, with the Audacity NR I can hear some slight degradation of the guitar sound, and I can hear the noise "pumping"; with DeNoise there is no apparent degradation of the guitar, and there is no pumping audible. Particularly revealing is the decay of the last guitar notes at the end. With the Audacity NR I can hear a "seashell over the ear" kind of effect; with DeNoise there is just constant hiss at -60 dB under the decaying notes.

-- Bill

[kozikowski]

<<<Audacity NR I can hear some slight degradation of the guitar sound>>>

Pretty much why I stopped using it for critical work -- or anything, really. It always does something to the sound. I wonder how that other package is doing it's work.

Koz

[waxcylinder]

And all of which confirms my personal view that Brian Davies has built some finel-honed tools to do some particularly narrow audio tasks extremely well - just as Chris has with his famous Compressor.

And Audacity still provides an excellent audio "workbench" to glue all these specificic external software tasks together.

WC

[billw58]

I wonder how that other package is doing it's work.

From the DeNoise manual:

The DeNoise application works in the frequency domain, dividing the incoming audio stream into
80 overlapping frames per second, and transforming each such frame into 1024 equally spaced fre-
quency bands (2048 if the sample rate exceeds 48 kHz).

For sound sampled at the CD rate – 44.1 kHz – this means that each band has a width of 22Hz. From
about 200Hz this enables fine-grained processing; however it is not satisfactory for the treatment of
rumble, hum, and the like. This is the reason for the development of a separate application (De-
NoiseLF) for the treatment of low-frequency noise.

Most readers will be familiar with the operation of graphic equalizers. Normally they are restricted
to no more than 30 bands, and they are adjusted infrequently. DeNoise operates as a type of “intel-
ligent” graphic equalizer. There are 1024 (or even 2048) frequency bands, and the incoming audio is
processed in overlapping frames at the rate of 80 frames per second. The gain in each of the bands
is adjusted continuously from frame to frame, using algorithms which attempt to judge what would
be perceived as noise, and what would be perceived as desirable audio. The objective is to suppress
the former without doing perceptible damage to the latter.

The emphasis is mine. Up to that point he's describing a multi-band noise gate. This corresponds to the description of the Audacity NR effect on the wiki http://wiki.audacityteam.org/wiki/Noise_Removal . Nowhere does Davies describe the algorithms in detail, but later in the manual refers to "psycho-acoustic algorithms". The implication is that he's doing something more sophisticated than simple noise gating.

Note: I know that the Audacity NR effect and DeNoise both use downward expansion rather than simple gating. It's just easier to write "multi-band noise gate" than "multi-band noise downward expander".

-- Bill

[steve]

The part that bothers me most about the Audacity NR effect is the low frequency "beats". I wonder if that's caused by "overlapping frames" that are at a lower frequency than 80 Hz and don't overlap very well. I've not come across this type of artefact in any other Noise Removal application.

[kozikowski]

Davies...DeNoise.

..using algorithms which attempt to judge what would
be perceived as noise, and what would be perceived as desirable audio...

Exactly. That would be McDonalds "Secret Sauce," the Coca Cola ingredient locked in a vault in Atlanta, etc.

And yes, without that, you have Tesco/7-Eleven microwave burgers and off-brand tap cola.

Ewwww.

Koz

[Gale Andrews]

There is also another bug in Noise Removal (1.3.13 alpha):
The Sensitivity and Attack/Decay sliders do not respond correctly to keyboard control. (tested on Linux only)

Confirmed on Windows so I added a bug:
http://bugzilla.audacityteam.org/show_bug.cgi?id=227

I have assumed the same problem on Mac too but if not, please change the OS information for the bug.

I guess we can agree then that at a given Noise Reduction setting, Bill's tone/noise sample has little/no hiss removed at Sens = 0 dB and very mild artifacts (based on mixing the processed audio with an inverted copy of the original). At high Sens settings much of the noise is removed (based on plots and running NR in Isolate mode), but tone artifacts are much worse, despite the plot of the low frequency spikes looking very similar to that for Sens = 0 dB.

I'd still argue that artifacting with high sensitivities and real world audio is nothing like as bad as with tones, based on testing some classical piano and low amplitude string samples, and that a lot of real world noise encountered will be narrow band (Audacity does fairly well removing hum from my USB turntable, I think).

I tend to use Goldwave for NR. If no-one else does, I might try some tests with it, though I think they will confirm my long-held subjective impression that Goldwave is to be preferred for NR over Audacity (this is partly because it deals better with intermittent vinyl artifact noise, not only that it produces fewer NR artifacts than Audacity).

I guess in the absence of getting Marco or a mainstream developer interested before 2.0, one possible kludge might be to start "Sensitivity" at + 5 or + 10 dB to make NR a bit more aggressive, then use some vague scale as in Click Removal. I still don't personally see further changes as critical for 2.0. Click Removal is also a relatively weak feature, but tends to get less discussion.



Gale

[billw58]

Davies...DeNoise.

..using algorithms which attempt to judge what would
be perceived as noise, and what would be perceived as desirable audio...

Exactly. That would be McDonalds "Secret Sauce," the Coca Cola ingredient locked in a vault in Atlanta, etc.
And yes, without that, you have Tesco/7-Eleven microwave burgers and off-brand tap cola.
Ewwww.

Davies is (IIRC) a retired mathematician. I don't think he's "invented" anything, but instead probably read the literature and implemented the best algorithms described there.

For example, if you look at the third plot in my original post you can see that full noise reduction (12 dB) does not occur until approximately one octave below and above the 440 Hz tone. A simple noise gate would not do that.

-- Bill