The Art of Noise Reduction

Tutorial: Page (1) of 1 - 05/05/04

The Art of Noise Reduction Noise Sucks (the quality from your projects) By Douglas Spotted Eagle Hum, buzz, hiss, drone, swish, whisper and more are the sounds of noise on tape. It plagues amateurs and professionals alike in any number of situations, usually beyond the control of the videographer. Refrigerators, air conditioners, noise from air movement in the room, camera motors, zoom lenses, noisy cables, and electrical hum are just a few of the noise criminals that stalk an otherwise great video recording.

Any video editor with a marginal modicum of desire to have good video uses noise reduction of one kind or another. Understanding and practicing proper audio techniques can go a long ways to getting a good signal on tape, yet sometimes or even many times, it is simply beyond rational ability to control.

Noise reduction tools come in two flavors only; good or inexpensive. There is no middle ground. Tools from WAVES, Bias, Cedar, and Sony are all highly effective noise reduction tools but they aren't inexpensive by any means. Equalizers are inexpensive, even free, yet are not terribly effective in most situations.

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Noise is a series of frequencies that are part of an undesirable sound. Using Fast Fourier Transformation (FFT) the most effective noise reduction tools allow selective removal of troublesome frequencies. Consider FFT as being an EQ with more than 1000 bands of control, and that will set you on the right track of understanding. Finding and isolating the noise-related frequencies from the desired frequencies contained in dialog or a performance is where the challenge comes in. Controlling them is even more challenging.. This is where the beauty of any number of noise reduction software packages come in. Choosing a package isn't that difficult. Knowing how to use it can be.

Figure 1 shows a video that contains a lot of room noise in the recording. The first step in the process of eliminating noise, is to find a section of the audio that contains only noise, no dialog, musical information, decay from a reverb in the room, or anything else except pure noise. In fact, it's a good idea on any video shoot to capture at least one minute of room ambience so that it might be used as a noise sample source in post-production. A "picture" or sample of this noise can then be taken, and used to compare with the noise in the production audio.

In Figure 1, I've identified such a section free of dialog, and have made a narrow selection of the noise. The selected area is actually fairly broad compared to the normal sample that should be taken. While the selection in Figure 1 is only about .200 of a second, noise samples of less than .050 are typically desirable. Less information in the sample is generally better, but this particular file has a very well recorded dialog, so the signal to noise level is quite high.

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Often, editors will attempt to remove all noise from a project with one process, and it is a serious mistake. Using very small time slices or selections, allows an editor to provide specific information to the noise reduction tool, with fewer and more specific frequencies contained in the noise sample. Using several noise samples, and multiple processes is one of the secrets to getting a good, clean file with significantly reduced noise.

Figure 2 is a zoomed in image displaying the selected area, it's relativity in volume or amplitude to the dialog contained in the audio file, and points out the noise we want to get rid of.

Figure 3 shows Sony's Noise Reduction 2.0's first dialog box. This has a check box to "Capture Noiseprint." A noiseprint is exactly as it's name implies, a picture, or "Print" of the noise contained in the selected area. By pressing the Preview button, an 'image' of the noise is created. The software will use this print as a means of comparing 'good' audio frequencies against the image, and selectively removing the 'bad' audio frequencies. The amount of removal is determined by the user with all of the software tools. A noiseprint profile may be saved by most of the noise reduction tools available, allowing the noiseprint to be applied to any files recorded in a particular environment. It's a good idea to store a noiseprint in the event that there is a lot of audio to be processed; some

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batch processing tools are capable of taking advantage of the noiseprint to remove noise in a large number of files in one processing path.

Figure 4 displays the audio file after the noise reduction software has been applied. Take note that the 'good' audio has not been visibly affected, and in fact, the ear can't hear any difference in the noise-cleaned file. All that is left is a solid, clean dialog. While it's in fact fairly rare to achieve complete removal of all noise without negatively affecting a dialog or music recording, if the signal to noise ratio is sufficient, and the dialog is clear and uncluttered with other noises, it's quite easy to get an extremely clean signal. Again, the secret is running multiple passes of whatever noise reduction tool you have access too, rather than attempting to do it all in one chunk. A few years back I was asked to do some forensic work on an audio file where a phone call had been recorded, but noise in the room caused the voice coming out of the ear piece of the telephone to be unintelligible. Since the noise source was fairly constant and the voice reasonably narrow in frequency response, I was able to remove the noise and provide a clean enough file for a stenographer to transcribe the audio to a text document.

To accomplish this however, it required somewhere in the

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neighborhood of 15 passes of the noise reduction tool, with extremely narrow selections of noise and careful tweaking of the amount of noise removed. Making matters worse was that I had to document each pass with exactly what was done to the file, and keep a comparison version of each pass for legal documentation. However, it worked very well, and in fact, I've now trained quite a few legal beagles in the use of noise reduction software.

But what if you don't have access to a good noise reduction software?

Remember that FFT is simply a monstrous algorithm that in the most simple terms is a large equalizer. Therefore, an EQ may be used to some effect, and if one is patient enough to stick with the numerous passes required to remove noise, then a marginal, and sometimes acceptable degree of noise reduction is possible. While not nearly as effective as a noise reduction application, it is indeed a poor man's method of achieving listenable results in many situations. Graphic EQ's rarely offer enough 'narrowing' of a frequency or band, and so a parametric EQ is really needed for this operation. Using a graphic equalizer without the ability to narrowly define a frequency range, will have an extremely negative impact on dialog or music that shares the same frequencies as the noise.

Sony's audio editing application, like many editing applications, offers a Spectrum analysis tool, that provides a 'window' into the selected area seen in Figure 2. Since we can see that the noise has a fairly high point relevant to the rest of the file at 7500 Hz, that gives us an idea of where the equalization process should start at. Noise is often harmonic. Therefore, if you can find a tone in the harmonic, often times merely dividing the noise frequency in half, or doubling it also provides a good clue as to where to start the work of reducing noise.

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Once the frequency range is identified, the equalizer is opened, and the offending bands identified. At this point, set the paragraphic EQ to it's tightest "Q" setting, defining as few frequencies or bands as possible, allowing for the greatest control. This is going to take a while to accomplish, so be prepared to run this process several times. It's not going to reduce the noise very much with only a single pass.

In this image, we can see the noise level prior to using the EQ to reduce the noise in the selected area. It's currently peaking at approximately -56 dB. The EQ is opened, but not enabled. Notice that the EQ is set for it's most narrow band width setting. Again, this is necessary to achieve noise reduction with an equalizer.

At this point, start working with the frequency identified by the spectrum analyzer, and preview the audio with the EQ enabled. Using the reduce/boost value found in the EQ, reduce the offending frequency all the way down. In this case, it's -40dB down. Slide the frequency around a bit, this might help you hear the effect the EQ is having on both the noise and the audio you wish to keep as natural and unprocessed.

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Divide the offending frequency in half, and use that as your next starting point. In the case of the 7500 Hz signal for instance, look at 3250 Hz as the next point of attack. This won't be exact, but the math provides a good starting point. Again, run the process as described above, and repeat as necessary. Typically it will take at least 10 passes or more to achieve any real result using this type of noise reduction processing.

Notice how much noise has been reduced in this image, compared to the previous image. Using an EQ, the noise is somewhat reduced in comparison to where it was originally. It's absolutely not perfect, but better by comparison. Patience, attention to detail, and exceptionally narrow bands of frequency are all part of the noise reduction recipe. If a number of files are going to need noise cleanup, consider the costs of time versus results, as it might be that the purchase of noise reduction software will prove less expensive than the time required, not to mention that the results of the manual EQ processing are less than stellar compared to the much quicker and higher quality results of using a noise reduction application.

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Wind, waterfalls, rivers, and traffic are all extremely difficult noises to remove, simply because the noise sources are constantly shifting the active frequencies, making the noise a constantly moving target. Trying to remove these elemental sounds from a recording is a significant challenge with even the best noise reduction application. Particular care should be taken to set up exceptionally small noiseprints. Expect the audio to be compromised no matter how great the effort might be. If these are constant elements found in video shoots, consider using hypercardiod microphones, shotgun mics inside of a good wind reduction system, lavaliers close to the sound source, or tightly held stick mics just out of camera range near the subject's mouth or sound source. Always face away from the wind rather than into the wind so that air is not blowing across the microphone's plates. This sort of noise is nearly impossible to remove or significantly reduce. Get talent to speak as loudly as possible in noisy environs. It will often add to the drama of a noisy area, and will provide viewers with the most important part of the information related to the video; the spoken word. Viewers will typically forgive less than stellar performances in challenging scenes if they can clearly hear what is being said. The bottom line is that it's far better to work on the front end of a production to prevent noise from entering the recording than it is to take the attitude that "you'll fix it in post."

Removing noise can be pleasurable or painful. In the end, it is absolutely better to do whatever it takes to minimize noise before the video shoot, but these pointers should help with the effort to remove noise that couldn't be controlled. Practicing with noise reduction tools will also help train the ear to listen for noise, and assist in the production efforts to reduce noises in the shooting environment. Having a reasonably noise-free working environment is also important. Computer fans, RAID fans, air conditioners without spillways, traffic sounds and other noise sources can all play a negative role in masking noise heard on your recording. While you should never mix with headphones, headphones are a good means of isolating and dealing with noise in your recordings. Check into noise suppression and room tuning products from companies like Auralex. For just a few dollars, you can make a significant impact on the ambient noise in your editing room while improving your overall listening environment.

Noise Reduction, regardless of which brand you choose, is something that every video editor should own and know how to use well. All professional audio engineers have used these tools for many years. There are many noise reduction tools out there to choose from at all price points. I'm partial to the WAVES and BIAS tools on the Apple platform and partial to the Sony tools on the Windows platform. I've had all three products save the day in otherwise bleak situations. Without this intending to be a review of the different products out there, I find that on the Apple platform, BIAS' new Sound Soap Pro is the absolute gem in the heap. It's just starting to ship. I presented this at NAB 2004 to wowed audiences, using it in Final Cut Pro.

On the PC side, it's a tossup, depending on budget. While the WAVES tools overall are superior plugins compared to most, Sony's Noise Reduction 2.0 has saved easily a thousand recordings over the past couple of years. It's slightly easier than WAVES, but also doesn't have the application integration with other noise reduction-oriented tools that WAVES has. When it comes to speed, Sony's tool is the fastest of any tool I've ever worked with, hardware or software based.

Noise Reduction Software manufacturers:

WAVES XNoise:
BIAS Inc Sound Soap/Sound Soap Pro
Sony Noise Reduction 2.0
Pinnacle Clean

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More tutorials and reviews by Douglas Spotted Eagle may be found here on the DMN Forums or on the VASST website, home of the shared veg files and other training media. "Spot" is the author of more than a dozen books on the subject of DV, audio for video, DVD authoring, and Sony Vegas. He is the co-author of "HDV-What You NEED to Know."

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