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iZotope RX 9 Advanced

Audio Repair Software [Mac / Windows] By Hugh Robjohns
Published December 2021

iZotope RX 9 Advanced

iZotope’s popular audio restoration tool has been treated to some significant improvements.

It’s that time of year when the clever people at iZotope refresh and update their impressive RX audio repair and restoration tool kit: it’s now reincarnated as RX 9, with both Standard and Advanced versions available, as usual. For this latest development, iZotope haven’t extended the platform by adding any brand‑new processing modules, but they have substantially upgraded three of the previous ones, as well as enhancing the underlying Editor platform. Two of the upgrades are restricted to the Advanced version and will benefit audio‑for‑video customers most, but more broadly based users will definitely appreciate the upgrades included in the Standard version, so there is something here for everyone.

As with the preceding versions, RX 9 supports mono, stereo, and multichannel file formats of up to 10 channels (so it can accommodate Dolby Atmos 7.1.2, for example) and while it can be operated as a standalone Editor, many of the individual processing modules can also be used as plug‑ins in most DAWs. As a bonus, the Spectral Editor in both Advanced and Standard can be used as an ARA2 plug‑in in Logic Pro, removing the need to toggle between RX and Logic to perform spectral edits. (Music Rebalance was RX 8’s only ARA‑compatible processor.) ARA isn’t supported for other DAWs — it doesn’t appear to be a very standard ‘standard’ — but hopefully that will follow in time.

The Advanced version of RX 9 benefits from major improvements to the Dialogue Isolate and Ambience Match modules — both being intended primarily for professional audio‑for‑video applications, of course. However, both the Standard and Advanced versions of RX 9 enjoy an upgraded De‑hum module, as well as substantial improvements to the History log and the way in which audio from previous processing steps can be ‘rolled‑back’.

While these revisions may sound relatively minor and inconsequential, the fact is that they deliver really major improvements in RX’s processing performance and workflow.


The upgrade to the De‑hum module is found in the addition of a new Dynamic Filter mode, accessed through a radio button at the top of the module. However, the previous way of working, using up to 16 static and strictly harmonically related notch‑filters, is still available through a Static Filter mode button, and in this mode all of the familiar controls remain and function just as before, including the linear phase and DC‑removal options.

The reworked De‑hum facility is a big improvement.The reworked De‑hum facility is a big improvement.Existing users of RX will be aware that this original Static technology wasn’t able to tackle unwanted ‘buzzy’ hum components extending above 1kHz, it didn’t work in situations where the hum components weren’t harmonically related, and it wasn’t ideal where hum harmonics extended over wanted dialogue, since the static nature of the filters could noticeably degrade the wanted audio. Nevertheless, iZotope have retained the option to use this technology, as it is a very fast and effective technique for removing simple low‑frequency hums with relatively few harmonic components.

Selecting the new Dynamic Filter mode changes the user interface and, at first glance, it appears a much simpler setup, with just three sliders, labelled Sensitivity, (number of filter) Bands, and Filter Q. This alternative new approach, though, is massively more powerful than the previous methodology, as it can employ up to 1024 dynamically variable notch‑filters, each operating with intelligently adaptive attenuation levels and an automatic gating function. The idea is that each individual notch‑filter is only engaged when the audio material needs it, and with only the required amount of attenuation to get the job done.

The benefit is that this approach better preserves the quality of the wanted audio, and the more sophisticated...

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