Now that the hard part's over, we can go on to the digital restoration process. It consists of five steps:

Fixed-frequency noise is simply any unwanted signal that remains steady (or close to it) over time. Examples of it include DC offset from poor converters, hum and buzz from ground loops, and acoustical noises from air handlers or motors. The sonogram and frequency-amplitude graph in Fig. 2 reveal a 15.75 kHz subcarrier from a TV. You can easily remove the noise once you know it's there.

The solution is good old-fashioned filtering to reduce or eliminate the problem. Of the many available tools, some are easy, quick, and dirty; others are cumbersome but effective. All share the same approach: apply one or more filters to act on the unwanted signal.

We've always found it useful on mechanical recordings to carefully apply a highpass filter. That removes three common problems: DC offset (see the sidebar “Getting the Lowdown”), low-frequency “clunks” resulting from large stylus excursions, and rumble or other subsonic artifacts that can sneak into a recording.

The best way to apply a filter is to listen for what you want to remove, not for what you want to keep. Starting with a lowpass filter set to maximum cut, adjust the order (slope) and resonant frequency so you hear only the low-frequency garbage and no signal. Then note the settings and dial in a highpass filter with the same order and resonant frequency. Drop it in and out of the circuit as you listen, just to make sure it doesn't adversely impact the low end.

AC hum is the classic fixed-frequency contaminant, and it would seem easy to set a notch filter at 50 or 60 Hz and be done with it. However, the harmonics of the hum are often more of a problem than the fundamental. Doing a proper job in hum removal means setting up multiple notches in a harmonic series. Real-time and non-real-time hum-removal tools take care of setting up the series for you.

If you need a steep filter slope but have only first- or second-order filters (6 and 12 dB per octave slopes, respectively), just cascade what you have. Kellie Ware of Elemental Audio Systems (www.elementalaudio.com), whose Eqium filter package is highly recommended, suggests using several highpass or lowpass filters together to get a fast rolloff or a steeper slope.

A word about minimalism is in order here: these tools are very easy to abuse. Too radical a filter setting, usually too narrow a bandwidth or high Q value, will surely destroy any hope of a hi-fi result. Fight the urge to EQ out high-frequency noise in order to reduce hiss. That will interfere with your ability to reduce impulse noises later on. Throughout the restoration process, check your work against the original to make sure you're not cleaning too aggressively. A combination of analysis and critical listening will yield acceptable results for the next step in the process.

Impulse noises are those pops and clicks that betray the vinyl origins of a recording. Pops and clicks can also pop up (so to speak) in the digital world when someone doesn't pay attention to proper clocking procedures, when batteries become low in a portable recorder, or when DC offset causes a thunk at an edit point. Brief events, such as coughs and floor creaks, can sometimes be treated with impulse noise reduction tools as well.

Most declicking tools provide an automatic mode, and many also offer manual correction for individual clicks. Click detection algorithms look for very brief, rapid changes in frequency content. Of course, the trick is to distinguish clicks and pops from valid musical transients so that you don't, for example, inadvertently remove an important snare drum hit. We've found that some click detectors can be set so aggressively that they remove the entire signal.

Once the clicks have been detected and removed, they must be replaced with new samples. Declick algorithms range in sophistication from simple interpolators (that stretch and blend good samples over the offending region) to more clever versions. One very simple declicker that works with stereo playback of mono phonograph records simply picks the better of two channels in real time, assuming that pops and clicks don't usually occur in both channels simultaneously.

The most sophisticated declickers use spectral interpolation. They analyze the phase and frequency of the good audio sections on either side of the impulse (areas referred to as “wings” or “handles”). Then they generate a set of samples to seamlessly bridge the area of the click. Depending on the source signal, this type of interpolation can sometimes be used to fill quite large gaps, such as those caused by an intermittent cable.

Declicking is best done in stages. Remove the longest-duration clicks first, then shorter ones, then the smallest clicks perceptible as individual events. Andy Smith, sales and support specialist for DARTech (www.dartpro.com), says, “It's often a good idea to use multiple passes to get the most from your click removal tool. You'll discover that finding a good one-size-fits-all setting for click/pop removal will often not produce the same results as focusing on specific kinds of clicks. For that reason, we recommend that you use at least two passes with any click removal tool.”

Usually, automatic declicking does not identify every single click. In those instances, manual declicking tools are essential (see Fig. 3). The general approach is to zoom in tightly on the waveform of the offending impulse, framing it with the system's selection tools. Then choose Remove or an equivalent command. Although it's tedious when there are many clicks, you'll get a lot of satisfaction in seeing individual glitches disappear before your eyes and ears.

Many impulse noise reducers provide a Difference mode that lets you listen to the material being removed. Christoph M. Musialik, president of Algorithmix (www.algorithmix.com), suggests, “For the best results, use your own ears in combination with the Difference feature. Switch between the original input signal and the input/output difference (the part of the signal taken out by the descratching algorithms). This difference signal normally should not contain any parts of the original signal that you want to preserve.”

When reducing impulse noise, beware of the Pencil. Most edit systems have a tool to “redraw” a waveform and seemingly erase a pop or click. That usually isn't a good choice because the Pencil is operating in the amplitude domain, whereas repairs should be performed in the frequency domain.

Once the larger pops, clicks, ticks, and zipper noises are removed, you may be able to reduce continuous crackle and distortion. These are the toughest contaminants to remove. Crackle consists of impulsive changes so close together that they can't be identified individually or even distinguished from the signal in many cases. It's really a form of signal distortion.

The other common type of distortion is clipping (overloading), which is usually the result of one of two mechanisms: poor gain staging (as in a mic preamp) or groove-wall damage and surface noise on a phonograph record. Digital clipping occurs when a signal peak exceeds the binary range of the A/D converter or the internal signal processing.

Decrackling and clip removal tools are not as commonly found as declicking and broadband denoising, but they should be considered a critical part of a serious restoration tool set. Fig. 4 shows Syntrillium Cool Edit Pro's clip-restoration tool in action.

The final component in a complete restorer's arsenal is a broadband noise reduction tool. Think of a broadband denoiser as a large number of bandpass filters, each followed by a downward expander. Most denoisers use a learning mode that is key to their operation. You provide a small section of the noise uncontaminated by the signal, and the denoiser “learns” what is noise and what is not. It then determines where to set the thresholds for all the expanders.

Once the “noise print” has been taken, the audio source is passed through myriad bandpass filters (2,048 bands is common), each with its corresponding “gate.” If the audio signal in a given band falls below the threshold determined by the noise print, it's assumed that only noise is present, and that band is reduced by an amount determined by the operator. The sum of all the bands equals the original signal minus the noise.

The user interfaces for denoising tools come in many configurations. A traditional approach, pioneered by Sonic Solutions in the mid-'80s, is employed in Waves' X-Noise implementation (see Fig. 5), while a streamlined, 2-knob approach is used in BIAS SoundSoap (see Fig. 6).

After acquiring the noise print, you're free to mess with the controls until an acceptable result is obtained. Your ears are the most important factor. If the noise is not uniform, you may want to try two passes, the first to reduce high-frequency noise and the second to reduce noise in the midband.

Wideband noise is present to some extent in all recordings, and, as the Bard might say, therein lies the rub. Too much reduction will step on the high end, causing the material to sound dull or woolly. Subjectively, removing high-frequency noise often seems to dull down the program, even if there was no high-frequency content present in the original. Also, excessive or misadjusted denoising introduces a characteristic “watery” or synthetic sound. So, think about the destination for the material and your audience's expectations.

If you're new to this, try generating a pink- or white-spectrum noise file, and then with your available tools, become familiar with the look and sound of pure noise. I used the $25 Cacophony from Richard F. Bannister (www.bannister.org) to generate my test file, but Black Cat Systems' Audio Toolbox (www.blackcatsystems.com) and other utilities also provide that function. For a low-tech approach, record some interchannel noise from the FM radio band or check audio test CDs, which often have noise tracks that you can digitally transfer.

Once you've trounced all over the audio with these DSP denizens, you may want to apply some tasteful EQ or enhancement to what's left. In the final analysis, it really comes down to expectations. Some clients expect pristine results from severely damaged sources, and others know that GIGO controls our destiny: garbage in really does equal garbage out!

Now that we've looked at the overall process of restoration, let's take a look at the tools that are available.

A few years ago, if you wanted to do any kind of serious audio restoration, you had to shell out tens of thousands of dollars for dedicated hardware and software. But the landscape has changed. Whether you're doing delicate professional restoration for finicky clients, transferring your personal audio library to CD or MP3, or cleaning up problems with tracks to be used as production elements, there are now a great many tools that you can put to work.

With so many offerings, selecting the right tool can be difficult; you'll find a lot of overlap in functions and interface design. Price range certainly can give you a place to start. Products from the Big Three — Sonic Solutions (www.sonicstudio.com), Cedar Audio (www.cedar-audio.com), and Cube-Tec (www.cube-tec.com) — all have a long history in the professional restoration field and are still priced in the four- and five-figure ranges. Moreover, they require a lot of experience and training to get results that justify the price. Any of those products make sense only if you have a large contract in hand, are seriously hanging out the shingle as a high-end restoration shop, or are lucky enough to work in a facility that already has them. Likewise, if you're doing paid work for a client, it probably doesn't make sense to use low-priced tools designed for consumers.

On the other hand, if you're doing real work but don't have a mint to spend, you'll find a rich selection in the range from just under $100 up to $300. The good news is that these products are genuinely impressive in what they can deliver. The bad news is that you have to figure out which one to use.

For most of us, the computer platform is an important factor to consider, so you can eliminate some entries right away. There's no use looking at DirectX plug-ins if you're on a Mac. Beyond that, you should consider the trade-off of control versus efficiency and ease of use. Recent entries in the restoration tool market have emphasized simplification of user controls. Others retain more parameters to allow for more detailed adjustments. Getting the most out of these programs requires an investment of time as well, especially when it comes to restoring anything beyond the spectrum of easy LP and low-end analog-tape transfers.

At the far end, you can find a number of low-priced shareware and freeware products as well as some inexpensive shrink-wrapped products aimed at the consumer. If you'd like to try out some restoration work without a big commitment, this is the obvious way to start. Even if you're a professional, you should check out what's cheap or free. You never know when you'll come across a true gem or a tool that meets a specific need. The usual caveats for such products apply: be wary of viruses, and don't expect a lot of tech support.

Full-time restoration professionals still depend on expensive dedicated tools. The original big gun for audio restoration is NoNoise from Sonic Solutions (see Fig. 7), which grew out of original research at George Lucas's DroidWorks in the '80s. When DroidWorks was dissolved, some of the key staff went on to found Sonic Solutions. For several years, Sonic Solutions offered its then-unique processing only as a service for hire. The push to rerelease vinyl recordings on CD, along with the demand for new releases from deceased artists, created a land-office business.

The demand for Sonic Solutions' services soon outgrew the capacity of its single facility. At the same time, the digital audio workstation revolution was beginning to pick up steam, and the high-resolution processing employed in Sonic Solutions' proprietary system proved to be perfect for high-end mastering. The company decided that the time was right to introduce its process and underlying technology as an end-user product. The result was the Sonic System, with its premier processing option, the NoNoise suite.

NoNoise established the classic set of audio-restoration tools: Declicking, Decrackling, Broadband Denoising, and Complex Filtering (which often shows up as Hum Removal). Most of the newer restoration tool sets encapsulate this group in one way or another. While the NoNoise tool set is no longer unique, the Sonic Solutions implementation is still widely considered to be the benchmark against which other tools are compared.

While Sonic Solutions and Digidesign have competed fiercely for years, Sonic Solutions now offers NoNoise as a set of plug-ins for TDM and AudioSuite. Sonic Solutions itself has become more closely identified with DVD authoring than with audio and has spun out a separate company, Sonic Studio, which is carrying on the development and marketing of the Sonic System workstations and NoNoise processing tools.

Success breeds competition, and once the NoNoise processing became established, the next supplier of restoration tools to appear was Cedar Audio. In contrast to Sonic Solutions, Cedar Audio started with a strong focus on hardware boxes for real-time denoising and declicking. That focus continues today, and Cedar Audio enjoys a very strong presence in broadcast, thanks to its low-latency and highly reliable hardware boxes.

Cedar Audio has also spun its tool set into several plug-ins for Windows and TDM and offers dedicated versions for the high-end workstation from Studio Audio Digital Equipment (SADiE). The Cedar Audio tools for SADiE consist of eight plug-ins that run on any version of the SADiE hardware. Seven of the eight are more or less standard restoration processes, such as automatic declicking, denoising, and so forth, but the Retouch plug-in breaks new ground (see Fig. 8).

The Retouch tool provides a view of audio as a two-and-a-half-dimensional space, in which time and frequency are the horizontal and vertical axes, respectively, and color is used to indicate amplitude. Some other systems provide a similar sort of display, but Retouch goes further by providing real-time controls for scaling the contrast of the color/amplitude component. By adjusting the controls while observing the display, you can obtain a remarkably clear view that isolates individual sounds and their harmonics.

Once you have identified an offending artifact or noise (along with its harmonics), Retouch lets you draw rectangles on the screen that represent regions of time and frequency. You can enclose the duration and frequency of each component of the noise that you want to remove, and enclose adjacent “handle” areas before and after the noise to be used for interpolation. Retouch interpolates the waveform across the duration of each box just as in manual declicking, except that the processing is restricted to the frequency ranges defined by the vertical dimension of each box. You can select 0 to 100 percent interpolation and adjust the attenuation of the affected area.

In contrast to any other restoration tool, Retouch excels at the removal of individual anomalous noises, such as telephones ringing or cars driving by — situations that were previously used to demonstrate just what audio-restoration tools could not do! Retouch truly represents a new type of restoration technology, one that for the time being Cedar Audio and SADiE have to themselves.

The current reigning monarch of sheer audio-processing power is the AudioCube from German company Cube-Tec (distributed in North America by Sascom; www.sascom.com). The AudioCube features truly massive processing at sampling rates up to 384 kHz.

Tools that run on the AudioCube are called Virtual Precision Instruments (VPIs) to distinguish them from mere plug-ins. Files are stored in a 32-bit floating-point format for dramatically superior dynamic range. The AudioCube offers a total of 16 24-bit, 192 kHz VPIs, 20 if you include analytical tools.

According to the distributor, there are several restoration functions you can perform with VPIs on the AudioCube that cannot be done on any other system. For example, you can remove a noise floor that modulates over time, or remove motor noise (including that from the zoom motor) from camcorder files. The AudioCube has some major fans, including some of the most demanding restoration facilities around.

Between the rarefied world of the three big guns and the broad range of tools selling for less than $300, the pickings are rather slim. Waves Restoration Bundle offers a suite of four high-powered tools that follow the classic set of multifiltering for removal of hum and other constant noises, declicking, decrackling, and broadband noise reduction. They're offered as VST plug-ins for Mac or Windows ($1,200; www.waves.com) or in TDM format for Pro Tools ($2,400). The user interfaces provide a high level of detailed control, with graphical readouts to help guide the process (see Fig. 5).

If you are a Pro Tools user, NoNoise for Pro Tools (TDM or AudioSuite, $1,995) is also an option. It's relatively new on the market, and it's not yet clear how the Pro Tools version of this classic compares with its older (and costlier) predecessor. Digidesign also offers the single-purpose DINR broadband noise reducer ($995; www.digidesign.com) for use on all Pro Tools systems.

Most of the products that lie in the $99 to $299 range run in native mode on the Mac or PC. While native applications generally cost less than those requiring specialized hardware, the processes are necessarily limited by the horsepower of the computer that they run on. In the case of the complex processes used for restoration, that can translate into slower operation and/or lower resolution and audio quality. Different manufacturers, however, distribute the trade-offs in different ways, and today's ultrafast CPUs make true high-end processing at least a possibility on a personal computer.

Products in this price range fall into three broad groups that parallel the higher-priced systems. First, there are standalone programs that focus more or less exclusively on restoration tasks. Then there are digital audio workstations that include restoration processes among their standard tool set. Finally, there are plug-ins that can be used with any compatible host application. A few products straddle categories by running in either standalone or plug-in mode.

The standalone programs in this range are characterized by real-time operation — playing a file or accepting stereo input while processing and relaying the results directly to a monitor output and/or file capture function. There's a trade-off, however, in performing such processing without hardware acceleration: you have to accept the limitations of the host's processing speed. On the other hand, you can often achieve an acceptable result quickly by playing with the controls. It certainly provides an education in the effects of different parameters.

Most of the lower-cost standalone programs also use a simplified user interface. At first glance, Sound Laundry from Algorithmix is the exception (see Fig. 9). It offers an unusual structure consisting of a plug-in “shell” for Windows that hosts proprietary plug-ins (standard DirectX versions are in beta). Sound Laundry provides two kinds of signal analyzer, DC offset removal, a rumble filter, declicker, denoiser, and EQ. Its file playback and recording are also handled by pop-up modules, providing a distinctly different approach. Once you get used to it, though, the system proves to be powerful, and the results are quite good. The full Sound Laundry program sells for $299, with lighter versions available for $199 and $99.

This year's award for a fun user interface for denoising goes to BIAS SoundSoap (Mac OS X or Windows XP; www.bias-inc.com; see Fig. 6). SoundSoap performs broadband denoising and hum removal but not declicking. Although it offers fewer features than some other products, you have to love its innovative approach to control, its cheerful look, and its friendly $99 price. SoundSoap also runs in plug-in mode under VST or DirectX.

Arboretum Systems (www.arboretum.com) weighs in with several options, the most ambitious of which is Ray Gun Pro for OS X ($149). Ray Gun Pro offers click removal, hum and rumble filters, broadband denoising, and audio enhancement, all in real time. The user interface is pretty basic, with slider controls under multiple tabs. The only thing I don't care for about this arrangement is that I can get to only one set of controls at a time. Standard Ray Gun offers a much simpler set of controls, runs in standalone or plug-in mode (VST, AudioSuite, Premiere, RTAS, DirectX), and is available for OS 9, OS X, or Windows platforms at prices from $99 to $119, depending on the version. The company also offers Restoration-NR as a Windows DirectX plug-in for $199.

Programs that combine audio editing (and other kinds of processing) with audio restoration often provide a good level of versatility and flexibility. These products fall into two fairly clear groups: restoration/mastering programs that have evolved editing functions, and audio-editing programs that include restoration tools in their processing menus.

In the former category, three products stand out; all are for Windows. Alien Connections' Pristine Sounds 2000 ($249; www.alienconnections.com) is a tool for remastering vintage or new recordings. It includes broadband denoising and click removal along with an interesting frequency-space editor. For another look at restoration, check out the white paper “Introduction to Audio Restoration Using Computer Applications” on the company's Web site.

DART XP Pro from DARTech ($199) is also designed as an integral environment for restoration and mastering. It boasts a built-in CD-burning tool and audio-editing functions. In addition to denoising (with a real-time mode), frequency-selective click removal, and hum filtering, the program includes two kinds of real-time analyzer and vocal-canceling functions.

Diamond Cut's DC5 software (www.diamondcut.com) sells for $199. (A specialized version for forensic work that includes brickwall filters is available for $1,399.) DC5 offers the usual denoising and click removal but also provides a declipping tool (unusual at this price) and a fine-grained sampling-rate conversion feature for speed correction. Audio editing, EQ and dynamics processing, reverb, and CD burning round out a powerful and cost-effective package (see Fig. 10).

Full-blown audio-editing applications with built-in restoration features offer the greatest versatility in an integrated package. Along those lines, Syntrillium's Cool Edit Pro (Win; rebranded as Adobe Audition in July 2003; www.adobe.com) has to be one of the great values in music software at $249; that's why it won an EM Editors' Choice Award this year. Among its many excellent features are several restoration tools that are unusually powerful for a product in this class.

In addition to a clipping corrector, the program offers a flexible declick tool and separate denoise and dehiss tools. (Denoise requires a noise “footprint” to work; dehiss uses a generic tape-hiss curve.) Notably, the declick tool includes a manual command to fill an individual click. That's an important addition because automatic click detection never seems to find everything. (For a full review of Cool Edit Pro, see the December 2002 issue of EM.)

Although they're outside the limits of this price range, two other audio-editing programs are also noteworthy for their restoration capabilities. Steinberg's WaveLab 4.0 (Win, $599; www.steinberg.net/en) includes a very good declicker and denoiser along with a set of very capable analysis tools (see a full review in the November 2002 issue of EM). TC Works Spark XL (Mac, $749; www.tcworks.de) is another high-powered editing program that includes a respectable set of denoising and declicking functions, filters for hum and constant noise removal, and signal analyzers (see a full review in the May 2003 issue of EM).

Two collections of plug-ins round out the offerings in the $99 to $299 price range. Sonic Foundry's aptly named Noise Reduction ($279.97; www.sonicfoundry.com) and Virtos Noise Wizard ($99; www.virtos-audio.com) both run in Windows and offer potent DirectX suites of restoration-oriented processing, albeit with fairly generic user interfaces.

Not surprisingly, things get a little bit uneven once you drop below the $99 level. All of the products in this category run on the PC and generally lack the benefits of professional user-interface design.

Nevertheless, there are some gems to be found. We particularly like Algorithmix's Easy Tools ($59). It operates in real time in a similar manner to Sound Laundry but with a much simplified set of controls. Even so, it made my “bad vinyl” test samples sound good immediately, and it certainly fits the description of cheap and cheerful.

Coyote's Groove Mechanic ($39; www.coyotes.bc.ca) is about as basic as you can get in restoration, but it does a creditable, non-real-time job with material that isn't too scratchy. Wave Corrector from Ganymede ($45; www.wavecor.co.uk) is dedicated to declicking vinyl and also works well for the money, although it has the odd characteristic of processing your file the moment you open it. Jeffery Klein's ClickFix and ClickFix Lite ($45 and $20; www.jdklein.com/clickfix) are plug-ins that run only in Cool Edit Pro. I found them to be somewhat more effective than Cool Edit Pro's built-in declicker in single-pass operation.

Excla WAVclean and WAVhum ($30 and $35; www.excla.com) appear to be designed mainly for processing short WAV files intended as operating system sound effects in Windows. Milan Vidakovic's Glitch Eliminator ($20; http://solair.eunet.yu/~minya/Programs/ge/ge.html) is a similar product. All offer a limited number of options for more serious sound cleaning work.

Steinberg's Clean 4.0 ($39.99) is described as restoration and CD-burning software that removes hiss and crackle from recordings. It includes processing for surround and sonic enhancement (tube simulation), and also includes a lite version of WaveLab 2.5. Clean Plus ($99.99) provides a compact phono preamp/converter that connects directly to a USB port.

Also, a couple of freeware programs for the PC are available for download at www.sonicspot.com. Antipop by Vladimir Bashkirtsev is designed purely for click removal from WAV files in stereo at the CD-standard 16-bit, 44.1 kHz sampling rate. It runs in a command-line interface under DOS.

D/Noise from Fast Mathematical Algorithms and Hardware is intended for broadband noise removal and provides a graphical interface in Windows. The company, whose primary business is cleanup of visual images, cautions that the program is not intended for commercial restoration. (It says it's a work in progress.) The program uses a novel single-pass reduction procedure that is not explained in the user documentation. This is true tweakware. The parameters are rather inscrutable, but the price is right. Check out the company Web site (www.fmah.com) for a restoration of a truly hideous Edison cylinder of Johannes Brahms playing one of his own pieces.

Finally, Declick 2000 by Michael Paar (free; www.uuhome.de/mpaar/) consists of two plug-in filters that are specific to Cool Edit 2000 or Cool Edit Pro.

As you can see, powerful tools for audio restoration are available at every conceivable price point, and some once-difficult tasks can now be executed speedily and with reasonably good results. Still, true audio restoration remains a demanding craft. Rescuing the beautiful sound lying beneath the scratches, hiss, hum, camera noise, and other artifacts that can befall an audio signal — without making more problems than you solve — requires patience, skill, careful listening, and a real bent for solving difficult and nonobvious audio problems.