Working with Noise Reduction

Tutorial: Page (1) of 1 - 05/31/07

Working with Noise Reduction By Graeme Hague

The great thing about working in the digital environment is that things can come pretty close to perfect. There aren?t any of those nasty analog irritations like tape hiss or quality loss over multiple copies. For video there?s nothing like a high definition picture to feast your eyes on.

It?s a shame that the real world isn?t quite so perfect, particularly when it comes to audio. We don?t realize just how noisy a place can be until you?re holding up a microphone trying to make a clean and clear recording. There is always a barking dog, crowing rooster, screaming children, arguing neighbors, ringing phones, peak hour traffic on the nearest road... how big can this list get? Well, really big- okay, so maybe I?m the only one who needs to track down that pesky rooster.
Luckily, digital audio software can come to the rescue. But wait a second- there are a few tricks you can learn to reduce the amount of unwanted noise in a recording, before you turn to software like Sound Forge and Adobe Audition to get rid of the rest.

The Recording Process
When you?re recording audio you may have one choice in your equipment setup- particularly video cameras with built-in mikes- that will make a big difference. ?Auto Gain? on the input means the microphone?s level will automatically adjust itself for loud or soft signals. This is handy in most situations, because you don?t miss anything. It also means that when your subject isn?t making much noise the Auto Gain switches to a level of sensitivity that picks up every breath of wind and drop of a pin! These sudden swells in unwanted background noise as the microphone goes into high gain mode can spoil a good recording and sound amateurish. The answer is to learn how to set a recording level for your device (if you can), rather than play it safe with an automatic mode. You?ll find the recordings a lot more consistent and professional, while any softer moments can be enhanced in an audio editor later. The main danger is also by default turning off any protection that Auto Gain provides, letting your audio input peak, which in digital recording gives you a very horrible noise indeed. Stay well away from those red lines!

Using a separate microphone gives you one simple advantage in reducing noise, yet few users think of it. Most microphones, with the exception of seriously omni-directional lapel mikes and field microphones, are designed to pick up sound within a predesigned area called their polarity pattern. Beyond this pattern and especially behind the microphone, sound is ignored or rejected. So if you find yourself recording in a noisy place and go looking for a quiet corner, instead try to identify where most of the unwanted sound is coming from and turn your back- and the rear of the microphone -toward it. Let the polarity pattern shut the noise out. Some of the best microphones are recognized not for what they can pick up, but for how much they reject.

Turning to software
Noise Reduction functions (NR) in audio editing software have come a long way and some truly excellent results can be had, but the problem is still in defining for the software just what it- and isn?t -the noise you want to remove. For instance, when you?re using vinyl restoration programs, the software is always struggling to pick the difference between a stylus crackling with dust on the record and some percussion instruments. They exist in the same audio spectrum.

That?s the trouble. A lot of extra noise in any recording is usually in the same audio spectrum as the material you want to keep. Which is why you need to narrow the focus of your Noise Reduction facility right down. The secret is to provide the NR with a sample of only the noise you want to remove.

Figure 1

Figure 2. The thick band of noise in Figure 1, (top) is the air conditioner rumble that doesn?t get above 80Hz- a low hum. After selecting a portion away from the dialogue and analyzing it, next the entire file is selected and the Noise Reduction Profile applied. Figure 2 shows the result with the band of hum almost completely gone

In Figure 1, using Audition we have a mono recording of an interview where unfortunately a rowdy air conditioner has added a low rumble in the background. By zooming in to a pause in the dialog at the end and selecting just a sample of that rumble you?re telling the NR ?this is the noise I want to remove.? Figure 2 shows you the result.

After you?ve selected a noisy portion of the recording, you create a Profile of that sample to analyze. Snapshots in Profile is like a resolution setting of the data you?re examining. Normally you?d expect a higher figure to give you a more accurate process, but you have to remember that you?re going to apply the NR to the recorded dialogue, to the entire file, and you don?t want the NR affecting that too much. So a ?coarse? setting, if you like, will sometimes identify what is clearly that air conditioner rumble and ignore anything it isn?t sure about. This may mean leaving behind some of the noise, but that?s better than removing audio you?d like to keep.

With NR you always have to experiment- always. Presets rarely work from one file to another. Rather than look at all the parameters in detail- they?re well covered in your software?s Help -let?s try an overview of the NR function so that you can understand why each file needs to be treated individually and how adjusting any of the settings might make a vital difference.

Again, the problem is for the NR to be told what is unwanted noise and what isn?t, as the software isn?t figuring this out for itself. By subtly altering the parameters and testing the results, you are shifting the filters until the best result is achieved. You are deciding what to remove, not the software. Because each noise, like that air conditioner rumble, is unique in frequency range and amplitude, the available filter settings can provide an infinite amount of different NR levels. It?s also important to understand that NR is about balance- about how much noise to remove or leave behind. Extreme settings don?t create complete silence. Beyond a certain point you?ll begin to introduce distortion. This is because NR works not by removing noise, but by converting it to a form of mild distortion that?s much quieter and less obtrusive than the original, unwanted sound. Therefore, applying too much NR emphasizes this replacement distortion and you hear a burbly, phase-like artifact.

Ultimately, Noise Reduction can work brilliantly on small problems like tape hiss or low end rumble. The louder and more varied the noise you want to remove, the harder you?re making it for the NR to get good results. So it comes back to getting a clean recording in the first place. Think about your microphone settings and placement, and avoiding unwanted noise in the wave file as you record it. Next, in your audio editor you pin-point the troublesome sound for a Profile. Then Noise Reduction can do its best work, as long as you don?t ask too much of it.

Page: 1

Related Sites: Digital Producer , Audio Video Producer , Digital Post Production , Digital Pro Sound , Oceania , BN - Sony
Related Newsletter: DMN Newsletter , Timeline Newsletter , Loud Newsletter , Digital Media Net , DMNForums , Tutorial Finder

To Comment on This Article, Click HERE

Most Recent Reader Comments:

Working with Noise Reduction by DMN Editorial at Jun. 01, 2007 7:05 am gmt (Rec'd 3)

Click Here To Read All Posts
Must be Registered to Respond (Free Registration!!!, CLICK HERE)