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Technique : Effects / Processing
Restoration software such as Izotope's RX2 can breathe new life into damaged audio — with the right moves from the user!
Izotope's RX2 software makes available some of the most powerful restoration tools around, at an affordable price, and as a result has become a very popular package. In this article, I'm going to share some power-user hints and tips that will help you get the best from it.
If you're new to RX, a good place to start would be checking out Sound On Sound's original review in July 2008 (/sos/jul08/articles/izotoperx.htm).
Since that review was published, Izotope have released a new and
significantly expanded version called RX Advanced, in addition to the
basic RX. RX Advanced has a number of extra modules, and some of the
modules that appear in both versions have extra features in the Advanced
release. Both variants can be run as stand-alone applications or as
plug-ins for your favourite Mac OS or Windows DAW: Audio Units, VST,
RTAS and AAX Native plug-in formats are supported. However, running RX
as a plug-in means that its processes have to operate in real time. This
makes some features unavailable and limits the effectiveness of others,
so I tend to export files that need processing to the stand-alone
version of RX and re-import them into my DAW once processed. (For this
reason, my number one feature request would be for Izotope to improve
the links between DAWs and the stand-alone application, perhaps in the
same way as Synchro Arts have done with Revoice Pro.)
The basic version of RX has five main restoration
modules. Declip is for repairing clipped and distorted audio, while the
Declick & Decrackle module is intended for restoring recordings from
vinyl records, although it is also good for dealing with digital
clicks. Remove Hum can eliminate low-frequency noise such as mains hum,
along with up to seven harmonics. Denoise removes broadband noise that
is relatively static in profile; it is effective both on electrical
noise such as hiss, and acoustic noise such as air-conditioning.
Finally, Spectral Repair is designed to remove occasional random sounds
that have interrupted a recording, whether these come from the
instrument being recorded or from external sources. Beeps, car horns,
mic pops and mouth clicks are all grist to its mill.
Supplementing these are a selection of 'fix it' modules
such as Gain & Fades and Channel Ops, which can help with all kinds
of routing and phase-related problems. There is also a Spectrum
Analyser module to help track down exactly where the problems are. In RX
Advanced, there are more 'fix it' modules you wouldn't necessarily
expect to find in a restoration software package, such as Resample for
downsampling audio files, Dither for reducing the word length of audio
files, and pitch-shifting and time correction using Izotope's Radius
technology. The Advanced version also operates as a VST and AU plug-in
host in stand-alone mode, and boasts an intriguing Deconstruct module,
where you can separate and adjust the tonal (pitched) and broadband
(unpitched) elements of an audio file.
To illustrate some of the techniques involved in
getting the best from the various RX modules, I'll work through some
examples of audio files that have needed some work. I will describe how
things happen in the stand alone application, but most of what I am
covering could be undertaken in the plug-ins within the limits of
real-time processing.
Peak Distortion
Digital audio is not forgiving when it comes to peak distortion: if your signal exceeds 0dBFS, you will experience clipping, with anharmonic distortion that is usually very obvious and unpleasant.
My first screenshot
1: The stand-alone RX Advanced. A stereo audio file is loaded that has severe clipping distortion. shows an audio file where the tops of the waveform have been chopped off, and the challenge is to restore this audio to its former glory using the Declip module. Either adjust the Clipping threshold by eye so that either the red lines on the audio waveform are below the clipped-off peaks, or click the Compute button in the module window and adjust the Clipping threshold control until the red line is just before the white line in the module window display. Next, adjust the Makeup gain. This is actually an attenuation control, and it is needed because once RX has reconstructed the missing peaks on the audio, its new peak level will be around 6dB higher — hence the default setting of -6dB. If you are already close to 0dBFS, you might want to consider reducing this further.
2: RX's Declip module. 'Makeup gain' is a misnomer, as it's actually attenuation that is required following the de-clipping process.
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