Noises Off

Aug 1, 2008 12:00 PM, By Mike Levine

           
 

TIPS FOR GETTING THE BEST RESULTS FROM AUDIO-RESTORATION SOFTWARE

BONUS MATERIAL
Bonus Material: Denoise in the Name of the Law

Cleaning up clicks, pops, and crackle in recordings transferred from vinyl is perhaps the best-known application for audio-restoration (AR) software, but there are uses for it in virtually every situation where audio is recorded (for audio-restoration basics, see this month's “Square One” column). Whether you need to reduce background noise in film or video soundtracks, clean up audio for Podcasts, minimize the noise of computers and drives that leaks onto your music tracks, or repair clipped digital recordings, there's an audio-restoration product that can help you.

AR software comes in many varieties, from dedicated plug-in suites to standalone editors to noise-reduction features built into audio editors. (You may have such features included in software you own and not even realize it.) Although AR software titles offer the tools to tackle noise issues, these products will frequently yield the best results with subtle application. A heavy-handed approach to AR processing can often make a bad-sounding recording worse.

Full-featured AR programs or suites typically consist of several major tools. Common to most are a hum-and-rumble remover, a click-and-crackle remover, and a broadband-noise reducer. Manufacturers typically suggest that if you're going to use all three on a piece of audio, you'll get the best results by applying them in the order just mentioned. Some AR products also have noise gates, declipper modes for regenerating clipped sections, and other specialty tools.

In this article, I'll provide tips and techniques for using AR software in common audio-restoration scenarios. The information was gleaned from talking to professional users and software developers, and from my own experience with AR software. This is not a product roundup, but I will mention plenty of different programs and plug-ins along the way.

Noise Horizons

One of the most common reasons people turn to audio-restoration software is to reduce broadband noise. As the name implies, a broadband noise is one that covers a range of frequencies (as opposed to, say, 60 Hz hum). For instance, hiss, background noise, machine noise, and even people's voices would be considered broadband noise. Even with the prevalence of 24-bit recording and its improved dynamic range, hiss and background noise bedevil all types of recorded material, particularly dialog, interviews, and other spoken-word recordings (see the online bonus material “Denoise in the Name of the Law” at emusician.com).

Plug-ins like BIAS SoundSoap and SoundSoap Pro, Wave Arts MR Noise (from the Master Restoration Suite), and Waves Z-Noise and X-Noise (the latter is part of the Restoration Bundle); standalone applications like Enhancedaudio DC Seven and iZotope RX; and audio editors such as Adobe Audition, Apple Soundtrack Pro, BIAS Peak and Peak LE, Sony Sound Forge, and Steinberg WaveLab all offer denoising tools.

Denoising software requires that you sample a bit of program material, preferably a short section containing only noise, so that the software can calculate the frequencies and levels of the noise signal. This setting is sometimes called the “noise profile” or “noise print.” Then during playback, the software will use this profile setting to squash down any frequencies that fall below these levels at the specified frequencies. Essentially, it's a multiband gate.

FIG. 1: BIAS SoundSoap Pro’s denoiser module offers both a “learn noise” option and another that extracts a profile from a user-specified section of the audio that includes both source and noise.

“At every frequency,” says Bill Gardner of Wave Arts, “it knows what the level of noise is. So while the music's playing, if you're above that level, it decides it must be signal; it'll let it pass. But if it falls down to the level where it thinks it might be noise, it starts to attenuate it.” Although Gardner was speaking specifically about the MR Noise plug-in, what he said is descriptive of broadband-noise reducers in general.

Some denoising software can automatically detect the noise portion of the audio, even if you don't have a section of solo noise to sample. For example, Waves Z-Noise has a setting called Extract, and the standalone version of iZotope RX has one called Auto-Train. These functions search your audio for a section that contains only noise and sample it automatically. BIAS SoundSoap Pro (see Fig. 1) offers an option called Timed Learn Noise, which samples a user-specified segment of program material and then calculates an average noise profile from within it.

Tweak That Thing

Although a denoiser calculates its settings automatically from the noise profile, don't expect instant perfection. You'll typically need to do some additional tweaking of the threshold and reduction (sometimes called “attenuation”) controls — which are offered in virtually all full-featured noise-reduction software — to get the best results.

The threshold governs the level below which the reduction kicks in, at the frequencies identified in the noise profile. The reduction slider controls the amount of the process that's applied (similar to the ratio setting in a conventional noise gate). Experimenting with these settings helps you find the best compromise between reduction amount and sound quality. Too much reduction will cause glassy or watery-sounding artifacts, sometimes referred to as “musical noise” (see Web Clip 1; in his “Square One” column on audio restoration in this issue, Brian Smithers refers to these as “birdies”; I've also heard them called “space monkeys”). Other artifacts of the noise-reduction process include harsh-sounding audio and degraded transient response.