iZotope RX

iZotope are the latest developers to release a suite of tools for tackling problem audio. As well as the usual noise–reduction algorithms, RX also includes clip reduction and an advanced 'spectral repair' module.

Following on from the likes of Waves, BIAS, Sonic Solutions and Wave Arts, iZotope have released a comprehensive suite of restoration software. RX contains five modules: Declipper, Declicker, Hum Removal, Denoiser and Spectral Repair. At launch it was a stand–alone product only, but iZotope have since released an update that brings these modules into digital audio workstations as plug–ins. Supported formats include RTAS/Audiosuite, VST, MAS, Audio Units and Direct X, so there won't be many host applications unable to open their doors to this restoration suite.

In Pro Tools, which I used for this review, you can set up the plug–ins in RTAS mode, copy the settings across to the Audiosuite version and render the files, so taking the load off the system and making the Session playable on other systems without the plug–in. For testing purposes, I used the same sample files that I used in my restoration shootouts from August 2005 (www.soundonsound.com/sos/aug05/articles/ptrestoration.htm) and November 2007 (www.soundonsound.com/sos/aug05/articles/ptrestoration.htm). All tests are for one pass using one plug–in, taken to the point where artifacts just become audible.

The suite comes in two versions: RX and RX Advanced. The Advanced version offers advanced manual declicking and 'multi–resolution' declipping, 'multi–resolution' spectral repair modes, additional denoiser parameters and, in the stand–alone version only, a dithering control panel and a resampling control panel.

Denoiser

The broadband Denoiser module offers three quality settings, but the highest can't be used in real time.The Denoiser module follows the traditional learn–and–process model used by most denoising plug–ins. RX and RX Advanced offer three different algorithms. One works in real time, one can work in real time on some systems, and the third (C) is available only for off–line processing, but offers the highest quality of processing.

A wealth of additional parameters is available in the Advanced version of Denoiser.The basic RX version offers two controls: Noise Reduction adjusts the threshold and so determines the amount of noise reduction, and Smoothing helps reduce the artifacts created by denoising. The Advanced version has a raft of additional controls across two pages. There are now two Threshold controls, one for tonal sounds and the other for broadband sounds. Both these parameters have Reduction controls to go with them, and this just the start of the Advanced section. I would recommend some serious time is spent with the manual to work your way through the rest of the Advanced section, if you choose to go for this version.

I first tried the Denoiser as an RTAS plug–in on my 'hiss' test file. I selected a section of the file with just noise, hit the 'Train' button, and Denoiser analysed the sample. In Simple mode it was then a matter of adjusting the Reduction and Smoothing controls for the best sound. I found that if the Smoothing control was set too high, the hiss would be modulated up and down with the audio. Lower settings reduced that, but at the risk of introducing artifacts.

Next I tried the Audiosuite version to see what improvements I could achieve with the off–line algorithms. Both of these take longer to work, with the 'C' algorithm taking around 15 minutes to process a one–minute file. With this test file, the difference between the three algorithms wasn't enormous, although re–training the plug–in each time I changed the algorithm did seem to produce a better result.

I then tried the Advanced mode, but I have to say I wasn't overly impressed with the amount of improvement I seemed to be able to get using the RTAS version. When I tried Advanced mode using the 'C' off–line algorithm it was virtually impossible to preview the results, as the buffering took so long: I only got snatches of audio that made it too difficult to evaluate what was happening, let alone make changes. The standard version worked well and is quick and easy to set up in both the RTAS and Audiosuite versions, but the Advanced version doesn't really work well in plug–in form. Sonically, RX Denoiser performed as well as Waves X–Noise, Z–Noise and the Sony Oxford denoiser, but not as well as Wave Arts MR Noise or Sonic Solutions' No Noise.

Hum Removal

Up to eight harmonics can be notched out using the hum filter in RX Hum Removal.This module also follows the format that's familiar from other hum–removal plug–ins. As well as the fundamental frequency of the hum, it can tackle up to eight harmonics. These can all be linked or unlinked, or odd and even harmonics can be linked, and there's also high–pass and low–pass filtering.

On my 'No Earth' test file (see box below), RX's Hum Removal worked about as well as Waves' X–Hum or BIAS' Sound Soap Pro, but unlike its Wave Arts and Sony Oxford counterparts, the RX plug–in left some high–frequency buzz behind, even with all eight harmonics in circuit. However, on the 'Tape Motor' test file, RX Denoiser was actually the best of the bunch, with a slightly cleaner sound than any of the others.

Declipper

Unusually, the RX suite includes a module dedicated to restoring audio that has been clipped.This is the first of the restoration suites we have looked at that has a dedicated module for treating digital overloads, although in my second restoration shootout, in November 2007's Sound On Sound, I tried using declick and decrackle modules from the various plug–ins to tackle clipping, with reasonable success. RX Declipper has few controls. A Quality drop–down menu offers Low, Medium and High options and, depending on your computer, you may only be able to use the higher quality options off–line; I was unable to use High Quality in real time on my G4 1.42GHz 'mirror door' Mac without getting Pro Tools –9128 errors, even with the H/W Buffer Size right up at 2048 samples.

The Clipping Threshold control should be adjusted so that the red line is just below the clip point, which usually shows as a blue line in the histogram display. However, the Threshold control's minimum setting is –8dBFS. This means that if the clipping took place earlier in the process and the peak level of the file you are trying to process is below –8dBFS, RX Declipper won't touch it. This is a pain for broadcast work, as the maximum signal level is normally expected to be no more than –10dBFS, and that means lifting the level of the file, either by normalising it or using the Audiosuite Gain plug–in, to get it within range.

The final control is the Makeup Gain control. This has a default setting of –6dB, which at first seems strange. The reason is that once you restore the peaks to a clipped file, its maximum level is higher, so you need to compensate for fear of ending up with a clipped file again! In experimenting with the Declipper module, I soon realised that it produces much better results if you can play a reasonably long file into it — say 30 seconds or so — rather than a short clip.Overall, it works better than the declicker plug–ins I tried in November 2007 — not surprisingly, as it's designed for the job.

Declicker

For removing clicks from vinyl records, the Declicker module is simple and very effective.The Declicker plug–in is one of the simplest plug–ins to use, and indeed, when I played the 'Vinyl Record' sample for the first time I thought something was wrong, because the clicks had all but gone. After a further couple of passes, adjusting the Sensitivity and Maximum Click Width, all the clicks had gone, which puts this module at the top of the list alongside the Sony Oxford and Sonic No Noise plug–ins. I couldn't set the Quality drop–down menu to High without problems in RTAS mode, but Medium was good enough to deal with all the clicks in the test file.

Spectral Repair

The Spectral Repair module displays your selected audio segment in a spectrograph–type display, allowing you to highlight an area to be treated. Spectral Repair has been designed to remove or attenuate unwanted sounds, such as squeaky chairs, coughs, dropped objects, mobile phones ringing, and so on, from recordings. It will also close up gaps in recordings by using advanced resynthesis techniques. Spectral Repair is an off–line tool, and so is only available stand–alone or as an Audiosuite plug–in.

Performing a 'spectral repair' is a three–stage process. First, you capture an audio clip by highlighting a section in the Pro Tools Edit window and hitting the Capture button. Once the plug–in has finished analysing the audio, it will open the Spectral Repair editor window, from where you can view and edit the segment graphically, preview the changes, undo, redo and so on until you are happy, before committing the changes in the normal way, by hitting the Process button on the main plug–in window.

The captured audio is shown in a zoomable graphic display, with time running along the 'X' axis and frequency along the 'Y' axis, allowing you to visualise different elements of the sound, and you use the mouse to draw round the area you want to treat. There are several tabs, which represent the different treatment modes available. The Attenuate mode is suitable for recordings with background noise, and when unwanted sounds are not obscuring the desired signal completely. For example, this mode can be used to bring noises like chair squeaks down to a level where they are inaudible and blend into background noise.

The Replace mode can be used to replace badly damaged sections where there are gaps or breaks in the audio, while the Pattern mode is suitable for badly damaged audio with background noise, or for audio with repeating parts. It searches surrounding areas for a similar piece of audio and blends it into the selection. Finally, the Partials+Noise mode is an advanced version of the Replace mode that restores harmonics of the audio more accurately: it links detected harmonics by synthesizing them through the selection, and interpolates the rest of the signal using the Replace method.

Zooming in vertically clearly shows the horizontal bands representing the fundamental and harmonic frequencies of the tape motor buzz.As well as the removal of unwanted noises, Spectral Repair is capable of tackling many of the same problems that the other plug–ins are designed to handle. To try out some of the basic features, I opened up a section of audio in the Attenuate mode and highlighted a section across both time and frequency. This area actually represented a word in the 'Tape Motor' test file. Once I hit Repair, the plug–in removed the word but left the background sound intact — absolutely amazing! Next, I zoomed into the display near the bottom, which revealed the hum and its harmonics as horizontal bands every 50Hz. You could use Spectral Repair to eliminate these, but since you have to highlight each band separately and remove it individually, and the plug-in only handles four seconds at a time, it would be a painstaking process.

Spectral Repair can be used for conventional restoration tasks such as removing clicks from vinyl records.Finally I tried the 'Vinyl Record' test file in Replace mode, to see how Spectral Repair would handle the clicks. The two screens, left, show the original and the cleaned–up version, and the latter sounds as good as it looks — brilliant! Again, however, the Spectral Repair module is a lot easier to use as a stand–alone application than as a plug–in, especially when you get into the more advanced features.

Conclusion

The stand–alone RX Advanced suite is undoubtedly an excellent product that is ideal for forensic audio and serious cleaning–up work. However, it seems as though iZotope have had to make compromises to get the application to work as a plug–in, and if you need the features of the Advanced version I think you will have to work in the stand–alone application. However, if you are happy with the standard version of RX, the plug–in option should work well for you and is very competitively priced.