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Field Test: iZotope RX Advanced Restoration Software


RX’s main interface displays the currently selected region as a curtain of FFT spectral analysis with nearby zoom-view and waveform opacity sliders.

Manufacturer iZotope is known for developing innovative, high-quality plug-ins, yet the company’s recent release of RX has generated more industry buzz in a shorter period of time than all of its previous products combined. The hoopla hasn’t been so much about what RX does — audio restoration, cleaning and repairs — but how it goes about doing it. iZotope’s research team spent the past four years developing entirely new algorithms and procedures from the ground up, which results in some novel methods for revitalizing noisy and severely damaged audio.

RX is a stand-alone application comprising a fully integrated suite of five modules: Declipper, Declicker, Hum Removal, Denoiser and Spectral Repair. Each module is also available as its own plug-in. In addition to a base version ($349), the $1,199 extended RX Advanced edition (reviewed here) allows even more precise control over the modules with enhanced algorithms and parameters. It also features iZotope’s MBIT+ dithering process and 64-bit sample-rate conversion for pro delivery. Both versions are available in the same download, meaning that you can start by purchasing the base package and unlock advanced features at any time for the difference in price. The application runs on Windows XP/x64/Vista machines and as Universal Binary on Mac OS 10.3.9 or later. Protection is through PACE iLok or challenge/response authorization; key not included.


My test system was a Mac PPC dual 1.8 GHz with 4GB RAM and OS 10.4.9 running through a Digidesign 96 I/O interface via Digidesign’s stand-alone CoreAudio driver Version 7.4.

RX opens to a rather large interface that’s reminiscent of a widescreen LCD (though it is resizable down to 900×600, minimum) and void of any color apart from the shades of gray that divide the GUI into its primary functions. Loading an audio file brings it to life, displaying a large blue waveform superimposed onto a glowing embers-orange curtain of FFT spectral analysis in the selection region panel. Nearby sliders let users blend waveform opacity for easier viewing against the spectrogram, as well as zoom-in on the time and amplitude/frequency scales. A full-length file waveform preview is always sprawled across the top of the display for shuttling.

Workflow is streamlined and highly efficient. Five buttons call up floating windows for each RX module, providing scalable access to their many parameters and commands while keeping screen clutter to a minimum. Processes can be automated on a single file or batch-run on multiple files. This is a huge time-saver when dealing with session folders or volumes full of similar source files needing the same treatment — just set it and forget it overnight. An Undo History panel, playback level meter, transport controls with looping option and a large time-position display (, samples, frames) round out the interface offerings.

Scrolling through the menu bar reveals a few handy features not available from the main window, including a gain control; 4-band parametric EQ curve with adjustable notch, high/lowpass filters; and a Help menu leading to a concise HTML manual. RX reads mono or stereo WAV, Broadcast WAV, AIFF, MP3 and WMA files, but can also directly import and convert audio from a video file (AVI, MPEG, WMV, DV, MOV and M4V) and save as WAV or AIFF with several resolution and dithering options.


One of my first sessions with RX happened to be a rescue job, where I needed to remix some tracks that a client had recorded into her DAW some years ago — apparently, before that client had discovered input limiting. Rather than simply “redrawing” the squared-off peaks of overcooked A/D conversion and analog tape distortion, the Declipper module uses advanced frequency band analysis to rebuild the audio.

Single-band mode is the best place to start as it’s very fast and often succeeds on the first try. I was knocked out by the results on severely clipped bass and Rhodes parts; transients were preserved perfectly while sustained notes came out sounding clean and smooth without any weirdness whatsoever. If the clipping is more complexly embedded (as with analog saturation), Multi-Band mode lets users select the number of frequency bands (8/16/32/64/128) used to perform interpolations. Using a higher number of bands doesn’t necessarily mean better-sounding results, though, and can actually add very unpleasant artifacts. I found that running single-band mode first to eliminate the digital peaks (appearing as extremely obvious “hair” in the spectrogram) and then auditioning various multiband passes to tweak out any remaining analog distortion was the best approach on areas of compounded clipping.

A third mode, Multi-Resolution, made only nominal difference on 90 percent of the material as compared to the other two modes. But it did something wonderful to moderately clipped parts containing high and harmonically rich material, such as the female lead vocal and violin, where it better retained the natural room ambience. All three modes feature a histogram of waveform levels that allow you to set the clipping threshold by visually finding the audio level where the waveform’s peaks are concentrated. This is quite handy.

The Denoiser lets you preview subtraction in real-time, shown here by the vertical red playback band.


Restoring an extremely rare but blemished Motown 1971 Sterling Ball Benefit souvenir LP provided ample opportunity to test the finesse of three other RX modules. The album was riddled with clicks, pops and scratches. I first set loose Declicker’s Automatic mode to search and destroy the offenders.

Setup was simply a matter of fine-tuning its maximum click width (0.1 to 10 ms) and sensitivity controls to suit the severity of the record’s condition. Ideally, these settings should remove most clicks without damaging transients in the program material. Looking at the spectrogram made this easy as I could visually identify clicks and pops of varying intensities to get an idea of their widths from the time ruler that runs along the bottom. I also chose to preview the process as “Clicks Only” to better hear what was being removed, tweaking settings to ensure that no drums were being mistaken for clicks, for instance, before hitting Apply. (This noise-solo’ed monitoring function is available in all RX modules.)

On Diana Ross’ “Reach Out and Touch (Somebody’s Hand),” a deep gouge traveling through several grooves created pops that at times lined up exactly on the kick during the intro. These proved too stubborn for automatic processing, so I switched over to manual mode, requiring that de-clicking be performed on each event individually. Using the zoom controls, or by moving the mouse wheel up and down, I was able to get a nice close-up view of an impulse and select exacting boundaries within which interpolation (adjustable in order, up to 400 samples) from surrounding data could occur. Like magic, each pop miraculously vanished and detected no residual effects, skewing or smearing of the kicks.

Once the larger pops were removed, it was much easier to hear smaller clicks and crackles that were still occurring in the background of the record. I found that making several passes with Declicker — each homing in on different sizes and intensities of impulses — gave far better results than trying to remove everything at once.

The Hum Removal module uses precision notch filtering to remove low-frequency buzzes and all sorts of related electrical garbage. The base hum can be set for 50- or 60 Hz, or “freely” assigned to any frequency you wish — handy for tuning into buzzes on a tape being played back at a slightly different speed from the recording speed. Up to seven harmonic filters can be enabled (including linear-phase variety in Free mode), each with independent or linked control over attenuation and separate slopes for odd and even harmonics. I even used these bands to tune into intermittent buzzes from computer power supplies and dirty data bus drains caught on DAW recordings. Low- and highpass filters (perfect for killing vinyl rumble) with adjustable frequency and Q can further shape the curve.

To clean up any remaining dust, I turned to the Denoiser module. The noise suppression used here is by far the most advanced I’ve seen. The problem with tools I have used in the past (particularly early ones) was that they relied almost entirely on gating and primitive frequency carving, cutting away the “bad” portions of the audio spectrum and compensating with others. On those systems, eliminating minor tape hiss was never a big deal, though it often came at the expense of losing some clarity and “air.” But removing substantial amounts of broadband noise would result in twittering, glugging and other comb-filtering artifacts. Later systems made use of more advanced audio fingerprinting or profiling techniques but still imparted a swirling, watery blur that I often found unacceptable and more distracting than the noise itself. Fortunately, RX isn’t sidelined by these issues.


RX Advanced provides independent control over the reduction of both the tonal and broadband noises that I spoke of earlier, each with sliders for threshold and noise reduction. This lets users aggressively suppress tonal noise, for example, while only applying a small amount of broadband noise reduction, or vice versa. I used it to zero in on grounding hum on electric guitar while cleaning up solid-state noise coming from the analog signal processors in the guitarist’s signal chain. The results were spectacular: transparent and completely natural sounding. In fact, about the only scenario that gave me trouble was having Denoiser differentiate between the spectrum of a snare drum and actual noise during a very quiet section containing an isolated rolling snare drum intro whose amplitude and spectrum must have closely matched both noise prints. After treating left and right channels with slightly different settings and much fudging around, I got an acceptable, though imperfect result.


A real bonus in RX is its amazing Spectral Repair module, which offers different interpolation modes geared toward specific types of repairs, including catastrophies where large amounts of information have been completely lost.

The Attenuate mode was great for pulling chair squeaks, page turns and coughs out of a live chamber music and spoken word recital I was editing. I chose this mode only for noises occurring during dialog, as it allowed me to adjust the attenuation strength while being careful to preserve background ambience. Using the Time-Plus-Frequency selector tool, I precisely drew a marquee around the offending occurrences in two dimensions, even solo-previewing each selection to make sure I was catching noise and only noise. Sometimes male coughs would fall smack-dab in the same frequency range as a male speaker so it became a judgment call as to how much attenuation was applied, but I always found a balance that didn’t compromise the intelligibility of the dialog or the natural feel of the ceremony. For noises largely outside the frequency range of program material, you’re better off cherry-picking in a similar manner, but then using the heavy axe of Replace mode instead.

The trickiest instance came when a large stage prop squealed into and during a crucial sustained note. This time, I chose Replace mode for its ability to remove brief noises that are mixed in with dense program material over a wide frequency spectrum. It does so by examining the surrounding audio for similar source material and intelligently re-synthesizing it to fill in the gaps (up to four seconds). My initial attempt wasn’t so great: The squeal occurred just prior to the note, causing the replacement to smear the transition. This happened because I had Replace set to its default so that it interpolates equally from the left and right of the noise.

There is a way around this: You can control the weighting given to material sourced before and/or after the replacement selection. In this case, I chose 100-percent weighting before the replacement, resulting in an unaffected transition. Brilliant! The advanced re-synthesis algorithms really do sound terrific, even capable of re-creating details like the vibrato on a singer’s voice or changes in pitch. Of course, it’s not entirely magic and replacement works best where surrounding material is similar and fairly constant.


I feel iZotope RX is the most comprehensive, customizable and best-sounding audio restoration and repair tool currently on the market. Available for less than $300 on the street, it puts RX easily within reach of any studio looking to generate extra revenue from audio-restoration sessions.

As the standard version offers so much, RX Advanced may appear relatively expensive. But to professionals working in no-compromise applications such as music recording, audio post/dialog cleaning, CD or DVD mastering, broadcasting, archiving or forensics, RX Advanced is an essential tool. I was blown away by the extreme transparency and unprecedented musical results. RX Advanced helps when all other audio tools and editing tricks fail.

Both RX and RX Advanced should be available in plug-in form by the time you read this. This is a key development as it will place the power of RX within any DAW. Anticipated are Pro Tools (RTAS, AudioSuite, HTDM), VST, Audio Units, MAS and DirectX versions. Eventually, all will ship bundled with the stand-alone version and iZotope says that current RX customers will be able to download the plug-ins free of charge.


Jason Scott Alexander is a producer/mixer/remixer in Ottawa, Canada.