Field Test: iZotope RX Advanced Restoration Software

Jun 1, 2008 12:00 PM, By Jason Scott Alexander



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

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.

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