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Review: iZotope RX and RX Advanced Software Suite

The substantial corrective power of this audio restoration suite will reward and delight the dedicated user.

As easy as it may be to pitch-correct a vocal or correct bad timing, the most basic problems we may encounter can be more vexing, such as ground noise, distortion, and external sounds finding their way into the tracks with which we work.

Enter iZotope and its RX (and RX Advanced) software suite; it helps make such problems easier to manage, if you have the patience.


iZotope RX offers five different processor tools, each focused on a specific group of audio problems. Declicker, Declipper, Hum Removal, Denoiser, and Spectral Repair are all obviously named to indicate their purposes. These modules are available all at the same time via RX as a standalone application and are also available one at a time using RX as a plug-in within your DAW. The Advanced version includes additional parameters, extra algorithms, iZotope’s MBIT+ dithering and 64-bit sample rate conversion.

In Use

Pardon my brevity, but Hum Removal simply works. Whether 50 or 60 cycle, even in harmonic multiples, this module gets it done — and quickly, too — since you can preview the hum before removal. There are numerous advanced features, but you’ll find that Hum Removal is both intuitive and effective.

Declicker is like Hum Removal; it works easily, intuitively, and allows deep control when you need it. Mangled LP audio or seriously plagued digital tracks will require multiple passes of processing as you adjust “Click Width” to remove even surprisingly longer passages with multiple offenses. Automatic mode even offers control of residual output, interpolation order, and correlation length (in the Advanced mode only — controls the length of the window from which you pull corrective audio). Click previewing isn’t entirely accurate, but this module is otherwise a bulls-eye.

The single band mode of Declipper is the entry level of distortion correction and allows you to preview your settings before applying them. It works well, but please note: Repaired peaks may now exceed digital zero, and gain reduction should be employed. The multi-band mode is more effective at removing distortion, utilizing between eight and 128 bands, but previewing is not allowed here; plan on numerous test runs to finetune your parameters. (Hint: Use short passages for auditioning results.) Furthermore, the multi-resolution mode (Advanced only) also uses up to 128 bands and does the best job of all, albeit requiring patience on the user’s part.

Denoiser is somewhat effective in its Simple mode, but the Advanced mode is what most engineers will certainly use. Here parameters for both tonal (consistent at a particular frequency) and broadband noise are provided, and you’ll need these controls dependent on the type of noise you’re removing. Previewing my changes proved to be difficult and stuttering, so trial and error is the key. While offering powerful options, it will test your focus, as this module is very deep and far from intuitive.

Within my MOTU Digital Performer DAW environment, RX worked well as a plug-in, but sometimes caused slower operation and error messages. In particular, mastering sessions where I employed my Universal Audio UAD-1 plug-ins as well as RX provided the most performance “issues.” Problems like these can be alleviated by using RX as a standalone app (the manufacturer’s preferred method, actually); processing files within it and then exporting them into your DAWs session is the glitch-free way to go.

Using RX standalone with Spectral Repair, I tackled a bass guitar track that was loaded with a grab-bag of unwanted sonic distractions: ground noise, string thumps, finger slides, fret slaps, and occasional bursts of wicked noise (possibly a short in the instrument itself). The FFT analysis spectrogram allowed me to see both the waveform and the frequency content (with a nifty slider to adjust the blend and opacity), with events like a hand thump showing a “bubble” of low-frequency energy, finger slides as “stratus clouds” of swirls higher up the Y-axis (frequency), and those bursts of noise were ugly vertical spikes reaching all the way up. I selected the Attenuate mode and the Time- Frequency Selection Tool (for areas of processing across both domains), selected problem events by highlighting them in an opaque marquee, and then clicked Apply. The offending event was now gone with only one caveat: processing over too long an area (four seconds max.) or too wide in frequency response can cause unnatural gaps and shifts in the audio.

Spectral Repair will effectively remove most of the “classic” problems: chair squeaks, phone rings, and door slams, but spots of missing audio will require the Replace mode. I found “Multiresolution” to work best with (oddly?) a fewer number of bands (choose between 128 and 4,096) and very careful setting of “surrounding region length” (the audio from which replacement audio is pulled) and “before/after weighting” (i.e., whether the ideal replacement audio ahead, or behind, the missing area). The Pattern mode finds the most similar surrounding audio to fill the gap, ideal for drums or audio with repetitive patterns within it.

Much like Denoiser, Spectral Repair offers an advanced mode for heavy lifting, tasks that generally exceeded my needs (and patience). The Partials+Noise mode uses higher-quality interpolation, seeking harmonic content, and then synthesizes the filler audio. This is for difficult tasks like repairing audio with modulation and vibrato; naturally, it is much slower and tedious. Interestingly, I could test for harmonic sensitivity and “solo” the harmonics.


Overall, I am quite pleased with iZotope’s RX, but RX Advanced is what pros will really require. Don’t get me wrong, RX does a very good job and is a lot deeper than a review of reasonable length could ever cover. Its hum and click removal features are ideal and would have likely cost a lot of money (probably more than a contemporary DAW) back when I got my start in the late ‘80s. Declipper may be a handful in use, but such distortion correction is nearly miraculous and essential to most audio engineers. However, it’s the capabilities of Denoiser and especially Spectral Repair — in all their nuanced glory — that will delight restoration experts and devoted novices (the ones with patience and a desire to learn).

Rob Tavaglione has owned and operated Catalyst Recording in Charlotte NC since