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iZotope RX4

Restoration Software [Mac OS & Windows] By Eric James
Published April 2015

With their hefty low–frequency content, vocal plosives are obvious as the blobs at the bottom of the spectral display.With their hefty low–frequency content, vocal plosives are obvious as the blobs at the bottom of the spectral display.

The latest version of iZotope’s RX adds some interesting features — but do they have musical applications?

Restoration software of a kind which is relatively easy to use and which gives decent results is a comparatively new sector of the music software market. Someone will tell me that the DeNoise module in Sonic Solutions was probably in the vanguard here, but my own first encounter with this class of tool was CEDAR Retouch, fitted as an optional extra in the SADiE system which we used to record, edit and master classical recordings a decade ago. It was expensive, but invaluable: recording engineers’ nightmares such as piano pedal thumps, piano stool creaks, even lip smacks and the occasional cough no longer demanded a re–take, but could be eliminated, or substantially reduced, in post–production.

I originally bought iZotope RX2 to do some fairly heavy restoration work on a series of 1970s live operas that I was remastering for issue on DVD and accompanying CD. Sod’s Law dictated that the job came in just after I’d sold my SADiE system, and the built–in Spectral Cleaning facility in Magix’s Sequoia could not do all that was needed. Other offerings were beyond the budget, so RX2 fitted the bill perfectly, and I still think that the Advanced version is a whole lot of professional software solution for a relatively small outlay. Only after those projects were completed did I begin to realise how tightly woven into my mastering approach it would become.

In For Repairs

I have now been using iZotope RX2 since it was released in 2010. Apart from the DAW itself, it is the one piece of software that I have found to be indispensable, and I have used it on pretty much every mastering session. RX can play almost any file, and has frequently opened recalcitrant formats which had standard DAWs flummoxed. It has exceedingly good sample–rate conversion (with MBit+ dithering) and, of course, it can repair sonic damage, ameliorating those bad–luck moments in live recordings and unnoticed horrors in studio recordings which cannot be recalled and undone. Even with projects that did not call for large–scale restoration work, it was good to be able to identify such momentary irritations as vocal glitches, the base of a mic stand being kicked, the studio cat, mic capsule distortions, clunks, coughs and so on, and quickly brush them aside. And that was only the Spectral Repair feature, which provides a highly informative visual interface for spotting and addressing these problems.

In my mastering suite, with its revealing acoustics and speakers with extended bass response, I often encounter vocal plosives and very fast transient clicks that have been missed by the client and the engineer. The pictures show how these look in the RX4 main display: the click is shown in Linear mode, as digital clicks have content across the whole frequency spectrum, while the vocal pop is shown in Extended Log mode because plosives have a great deal of low–end content. Each of these issues took just a few seconds to eliminate entirely in RX4: lasso the problem area, press ‘R’ for repair, and the offending item is attenuated to inaudibility. More complex problem sounds have more complex repair modes, but with a little experience, they are barely any more difficult to implement.

A rogue click is clearly visible in RX4’s spectral display.A rogue click is clearly visible in RX4’s spectral display.

It’s worth noting that iZotope present RX as software that can find application across all kinds of audio work. However, although many of the features of RX that I find useful when working with musical content are also useful for non–music audio work, the converse doesn’t always apply. In other words, there are some tools in RX which are very useful in non–musical applications, yet have less immediate use in music production. iZotope themselves sort the modules according to function, into Restoration, Production and Utility groups.

Three: The Magic Number

Late in 2013, iZotope released version 3 of RX. Not only did RX3 sport an entirely new, and very much more ergonomically optimised user interface, but it also included interesting new modules in all of the functional groups, and several previously Advanced–only features were incorporated into the much more affordable Standard version. As these included the excellent sample–rate conversion algorithms, this made the standard version of RX very much more attractive to a wider base of music engineers. Hugh Robjohns reviewed RX3 in full in the February 2014 issue (–rx3.htm), so I don’t want to repeat too much of what was said then here, but it is worth re–emphasising the gist of the conclusion of that review, which was that RX3 is a worthwhile investment for anyone involved in professional music production, and that the upgrade from RX2 was also, as they say, a no–brainer.

In RX4 another advanced module, the Dialogue Denoiser, has made the same migration to Standard. iZotope are wise to have this divide, as not every engineer will need the Advanced–only functions, which are often quite specialised — and as I think the Advanced version is good value for money, this makes the Standard Version a bit of a bargain.

Pillar To Post–production

If RX3 represented quite a large leap forward from RX2, then RX4 is a smaller step in the same direction. New features include Clip Gain and a Clip Leveler, which do pretty much what their names suggest; less standard, and potentially more interesting, are features and modules such as RX Connect (see box), EQ Match and Ambience Match.

I mentioned earlier that some of RX’s existing features are targeted mainly at non–musical applications, and in fact, the two new modules in RX4 probably fall into this category. Ambience Match is not, despite the name, a convolution reverb, or indeed any kind of reverb. And while EQ matching — the idea of capturing the frequency responses of source and target tracks, and computing an EQ curve to make the former sound like the latter — arguably has a place in mastering and music production, the EQ Match feature in RX4 is so basic as to be of limited use. It is nothing like as sophisticated as Harbal (, nor even the Matching EQ feature in iZotope’s own Ozone mastering software. In the latter, the capture process produces visual curves for the overall EQ and difference EQ changes which can be overlaid to allow them to be compared and, if necessary, modified. EQ Match in RX is a greatly simplified form of this. The screenshot shows the UI dialogue, and the manual is quite succinct: after you have opened the EQ Match module, it tells you to “make a selection in a file; click Learn; make another selection; click Process.”

RX4’s EQ Match feature is more basic than its counterpart in iZotope’s own Ozone mastering software.RX4’s EQ Match feature is more basic than its counterpart in iZotope’s own Ozone mastering software.

For musical needs, I would say that this implementation falls short of being really useful. Most music engineers would want much more information and much more control over EQ changes applied to their tracks. But as I’ve indicated, this module is probably more likely intended for certain non–musical applications where such immediacy (and very good results given the simplicity) is an advantage in itself. I can illustrate this better by introducing the Ambience Match module at this point and showing how they operated together when I revisited an audio–book project I worked on last year.

Voices & Choices

The main recording for the audio book, which featured a number of different voices, was done in a London hotel. But then sometime later editing changes were made to the text itself, and so certain passages of the book had to be re–recorded. Some of this took place in my own small studio in Norfolk, using the mic with which we’d recorded the originals; but one particular reader could not travel that far, so recorded the changes at a more convenient local facility with a different microphone and sent me the results to be edited in. At the time, this gave me an awful lot of extra work to do. There were clearly mismatches of vocal tone and also background ‘room tone’ (low–level ambient sound), and though I could EQ the vocal sound to minimise the differences, the only way I could make sure that there was no clearly audible difference in the rooms was to cut a small piece of room tone from the very beginning of the original recording, edit it to a usable length and mix it in, ducking it with the vocals. It worked just about well enough, but it took an awful long time.

For the purposes of this review, I revisited this nightmare with RX4 and it took me 15 minutes. Though the voices were the same, the use of different spaces, placements, mics and preamps meant there were tonal differences between the original and later recordings. The original recording was warmer and smooth, the re–recording to edit in was less so in both regards — but the RX4 EQ Match module made a very passable attempt to live up to its name, and I think I would have been happy to use the result had I had the chance to do so.

The new Ambience Match feature is designed to ensure that room tone can be made consistent when editing together recordings from different sources.The new Ambience Match feature is designed to ensure that room tone can be made consistent when editing together recordings from different sources.

The room tone was very different between the two versions: although the original recordings were not exactly noisy, there was a very specific ‘hotel room’ sound to the silence, with a faint air–con motoring away somewhere in the basement. The re–recordings, made in recording studios, had much quieter, almost silent backgrounds, and this was easily perceptible when the original section segued into the edit. This is where Ambience Match came in. As you can see from the screenshot, it has the same minimalist interface as EQ Match. Fingerprinting the room tone from the original and adding it to new edits was very easy, and it took just a couple of experimental passes to get the level right. I had hoped that Ambience Match was going to be rather more — conceptually, it’s really just the Denoise module working in reverse, as the manual almost admits — but fitting horses to courses enabled it to show its proper strength. It also found a use in classical editing: many producers still insist that recordings do not fade to digital silence between movements and between separate pieces, so the editor has to edit in room tone recorded at the beginning of the session to give the illusion of a continuing live recital (ha!). Editing virtual silences together is a pastime for the seventh circle of Hell, so Ambience Match could be a real boon there.


As I have made clear, I find iZotope RX4 indispensable in almost every post–production project, including mastering and its manifold responsibilities. I also think the asking price is not a great deal for a professional facility to pay for a professional product, and that the ‘missing’ features of the Standard version that allow it to be offered pretty much at bargain price are less likely to be missed by music users. So RX4 is a great upgrade for users of RX2, and a great buy for those who are yet to feel the love at all. But is it a good upgrade for present users of RX3? I think it all depends on just how much use can be made by the purchaser of the half–dozen or so substantial new features. For those in film and speech post–production, this might be all of them; for some, like myself, who specialise in music but have a serious sideline in speech and restoration, that might be two or three; but for some music–only facilities, it might not be quite enough to justify the move just yet.

Only Connect

It is possible to use iZotope RX in two different ways: as an adjunct to a DAW, or in stand–alone mode. Even in RX2, there was already a facility which enabled Spectral Repair as a plug–in from within a DAW. In RX4 this has been replaced by a more fully featured ‘round trip’ capability called RX Connect. The idea is that, without leaving your DAW, you can either send a clip from your DAW for Analysis in RX (a one–way ticket) or you can send it for Repair and then return it to the DAW (a two–way ticket). One issue that can arise is that if your DAW does not ‘surrender’ its audio channels to RX when that is operating, then nothing routed through RX can be monitored. Enter an ingenious solution called RX Monitor, which gets around this problem by operating in the DAW as a virtual instrument through which the RX output can be played via the DAW’s non–surrendered channels.

I tried RX Connect in a mastering context, and it worked as advertised, but because there were a few menu items to negotiate and you need to pre-open the stand-alone version of RX, I found that it was just as easy for me to minimise the DAW, open RX in stand-alone mode, and work directly on the WAV file. Things changed dramatically, though, when I was working on a classical editing project where the fragments of music to be edited together were drawn from up to 100 different takes, hence 100 different WAV files. When doing this kind of work in the past, the technique I use now when mastering would not have been viable, as it would have meant opening all of those 100 different files, rather than the single one being worked on in mastering. Instead, I would deal with noises and suchlike by noting their time position in the overall piece, compiling the final edit and only then going to RX. It never occurred to me that there could be a better way — but this is where RX Connect starts to make much more sense. A process that is relatively unwieldy when compared to opening the stand-alone version for occasional work on a single WAV file becomes the clear winner when compared to opening and searching in 100 or more separate audio files! And also, as often happens, what seems a bit awkward when you are consciously concentrating on each step in the process becomes second nature and much easier as familiarity and muscle memory kicks in. Of course, there was a time when making up a DDPi file seemed like a lot of work...


  • Better integration for those who like to carry out their restoration tasks within a DAW.
  • EQ and Ambience Match can help when you need to edit together recordings made under different circumstances.
  • Still offers excellent value for money.


  • The new features are arguably more useful in post–production than in music work.


RX4 is a worthy update to perhaps the best–value restoration package on the market, though it perhaps won’t be an essential one for users who work only with music recordings.


RX4 £215; RX4 Advanced £729.02. Prices include VAT.

RX4 $349; RX4 Advanced $1199.