Cleaning up your act – A look at izotope RX Audio Restoration Software

We all know from experience that it is not always possible to capture the best quality sound when filming. Sometimes things go wrong. Fortunately, many recording problems can now be overcome in post production by using specialist sound restoration software – but not all!

There is a limit to what can be ‘recovered’ for a professional soundtrack but you can often clean up unwanted interference or background noise which otherwise would make your soundtrack unusable – or at least unprofessional. You may even be able to ‘erase’ unwanted sounds which occur underneath your audio or even reconstruct the audio where there is a short glitch in the soundtrack or a brief overpeaking of the signal.

 

What can we do?

Let’s first look at some of the audio problems we might encounter – and probably have! One of the biggest problems, particularly if you are a one man camera operator, is overpeaking. With analogue recording this was slightly less critical because the signal distorted progressively – but with digital recordings the signal is destroyed quickly when you exceed maximum level. You can’t retrieve something that has been totally destroyed. But you can often ‘recover’ very short duration over-peaks by using audio waveform reconstruction techniques – which is where modern software can help.

 

Other things that can plague us are unwanted background noises and interference – things like noise from fans or lights or the dreaded hiss and hum generated by electronic equipment or the hiss from a favourite microphone.

Fortunately these are some of the easiest things to remove from recordings since they are fairly constant low level sounds – that is, sounds that cover the same frequency band and are fairly constant in volume. Of course, it is far better to get rid of the unwanted noise before you start filming, but this is not always possible – and, yes, sometimes we don’t realise that we have this unwanted background noise until we get back to do the edit!

 

If someone drops a spoon or chinks a cup during a concert you may even be able to remove that sound whilst leaving the music more or less unaffected – or even get the software to reconstruct a very short section of musical sound to replace the unwanted noise. There are of course limitations to this and it will only work if the sound to be ‘reconstructed’ has a uniform pattern – because the program has to sample the last bit of good sound and the first bit of sound after the unwanted event and reconstruct the sound graph. It works well with a decaying note or applause.

 

It is also possible to remove most of the clicks and pops from a recording of an old vinyl record, either manually or automatically using sound

restoration, and get rid of background hiss and crackle – although the more you remove the more it will affect the remaining sound, which can become a little muffled unless you use an equalizer to put back a bit of punch.

 

Audio Restoration – iZotope RX

 

iZotope is the company that produces the technical software that is licensed to well-known software program producers to run their signal processing ‘behind the scenes’. Some of iZotope’s processing algorithms are to be found in Sony’s Acid Pro, Sound Forge, Adobe Audition and in Thompson’s Edius 5 video editing program, to name but a few. So if you specifically want to get to grips with audio restoration then it seems sensible to have a look at the programs marketed by the developers themselves. That is how I came across iZotope RX.

 

On the face of it the program is quite simple – but underneath it employs very sophisticated programming which took four years to develop. On the one hand it is automated but intelligent, and on the other it is controllable in a very clear and graphical way so that you can generally see what you are doing and, most importantly, hear the changes you make in real time. You can also save your settings as presets so that you can call them up to process the same problem later without having to re-establish your parameters.

 

So what is included in RX?

Basically there are five processing modules: a Declipper for getting rid of overpeaking and minor distortion; a De-clicker for getting rid of clicks pops and scratches; a Hum Removal tool that you can adjust to filter out frequencies up to about 15kHz; a De-noiser for filtering out hiss and general background noise; and a tool called Spectral Repair – which magically recalculates the audio across a small missing or defective section of your recording (up to 4 secs).  There is also a 4-band

Parametric Equalizer with additional High and Low-Pass filters, and a simple gain control to enable you to quickly change the overall gain settings of a track.

 

The iZotope Difference

Well, many programs can do some or all of these things – so how does iZotope RX differ? The main difference is in the quality of the processing and the simple user-adjustable settings. Whenever you process audio you tend to get unwanted side effects or artefacts. Obviously if you want to reduce or eliminate a continuous noise you normally have to reduce the frequencies that make up that noise – but if the audio you wish to keep contains the same frequencies at an audible level then they will be reduced or eliminated as well – so your remaining audio will have something missing. Now this may not be important or noticeable if the audio you wish to keep only has these frequencies at a very low level – but if they make up a significant part of your wanted sound then you will notice the difference if they are taken out, and this can cause some very strange effects.

 

Too much processing can cause what is sometimes called the ‘drain pipe’ effect. You manage to take out the unwanted noise but you are left with some strange noises in the background – like someone talking into a watery drainpipe. So iZotope has also provided you with manual controls to counter unwanted side effects.

Let’s look at the main features of the program.

 

Spectrogram and Waveform Display

This is probably the best feature of iZotope RX. It takes the simple sound graph, which we are all familiar with on our video timeline and places underneath it a ‘Spectrogram’ display. This is essentially a picture of the sound track over time which shows where each element of sound is on the vertical frequency scale at any point in time and, more importantly, how loud that sound is at that frequency. The brighter the colour, the louder the sound. So as you play the soundtrack and watch the conventional sound graph showing the loudness peaks, you can also see where each element of sound is located on the spectrogram. You can then stop playback and highlight on the spectrogram any piece of sound at any frequency and process it with any of the processing tools – and you can listen to the sound you are taking out as well as the sound you want to keep. This means that you can now isolate, say, the sound of that spoon chinking a cup during a concert, draw a box around it and delete it or more likely apply one of the more subtle processing tools to get rid of it whilst preserving the character of the remaining audio. You can also view the Spectrogram separately and zoom in on any part of it to identify the specific bit of sound you want to deal with.

 

Background Noise – The De-noiser

As in most programs with a noise reduction feature you start by taking a sample of the noise you want to eliminate – usually a small section your soundtrack that contains only constant background noise – although the program can attempt to analyse the track and work out what it thinks is unwanted background noise if you do not have a section of noise alone. iZotope also provides separate controls for Tonal Noise (what we hear as sounds) and Broadband Noise – the general hiss you get in the background – and you can adjust the threshold at which these controls operate and vary the amount of noise reduction that is applied – and then correct any artefacts with the Musical Noise Suppressor control.

 

Hum Removal

Again this is an easy control to apply but this time you work primarily from a graph. There are pre-sets for normal mains hum frequencies, but you also have the freedom to slide the node on the graph over the frequency scale to establish the frequency where your problem sound disappears. You can then adjust the spread of the frequency that you are taking out, so that you do not take out too wide a frequency band which would affect your wanted sound unnecessarily. I was able to use this to identify the frequency of a hum generated in one of my microphone/preamp set-ups and saved the settings as a pre-set to use in future. You can also process up to 8 harmonics if you have a more complex problem.

 

Over-Peaking – The De-clipper

This is a useful and simple tool for getting rid of intermittent over-peaking that causes unpleasant distortion – but it is not magic and does have its limitations! To work effectively you really need to process each problem area separately, although you can apply it to the whole file if you wish. You simply highlight the overpeaked area, click ‘Calculate’ to show the results on a simple graph, and move the threshold line up the graph to set the clipping threshold. You then apply some make up gain to restore the sound levels to match the adjoining sound. In effect the tool recalculates the overpeaked section and redraws the sound graph below peak level.

 

Scratched Records – The De-clicker

This will operate in automatic or manual mode. It is really intended for removing the clicks from recordings of vinyl records, although you can use it for similar noises on other recordings. Again, there are a number of pre-sets for analogue and digital, including dusty records, scratched records, etc., as well as the ability to remove single clicks. You can adjust the sensitivity of the process and set the maximum click width it will identify – so that it does not take out short sounds you intended to keep.

 

Replacing Sound – Spectral Repair

This is the magic calculator that can replace tonal sounds where there is a short break in your soundtrack or where you need to delete a very short intrusive noise such as a cough, a scraping chair or a spoon chinking a cup. You can apply this across the whole frequency spectrum (i.e. across a slice of the timeline) or draw a box on the Spectrogram around the specific element of sound you wish to process. In simple terms it will analyse a defined section of sound before the unwanted event and a section afterwards and then predict the sound that is missing. You can set how much emphasis the calculation will place on the sound just before the event compared with the sound just after – so it is possible to get some seamless results if the sound gap is not too long and you take the trouble to audition a number of processing settings. You can either attenuate the unwanted event or totally replace it with the calculated sound. But with this tool it is important to experiment and select the correct processor – there are four to choose from!

 

The Parametric Equaliser

A parametric equaliser enables you to select a particular frequency you wish to get rid of or attenuate and then adjust how far at either side of that frequency point your process will affect before it falls away to the original unaffected sound. This is shown clearly on an adjustable graph. When you have found the spread of frequencies you need to cover you then adjust the amount of cut (or gain) you need. The standard version of RX has four adjustable frequency points plus a high-pass and low pass control at either end of the frequency graph. You can activate any or all of the control points depending on what you are trying to achieve.

 

What’s Useful and what’s Not?

Well, all the tools are useful depending on what job you need to do. For general video work I have found the De-noiser the most frequently used – it’s very easy to apply, very effective and if used sensibly does not create drainpipe artefacts that you cannot get rid of by simply adjusting

a slider as you listen.

The Hum Removal tool comes a close second – although you can usually get equally good results by removing hum with the Denoiser. The Hum Removal tool is more sophisticated, though, and lets you control unwanted harmonics as well – and you can also see exactly which frequency is causing you a problem. So if it it’s not 50kHz UK mains frequency, you know you have a different problem with your equipment and set up. You can filter it out here and then set about finding the root cause of the interference in your kit or location set up. But if you need to get rid of a particular sound like a cough, it is the ability to see that bit of sound in the Spectrogram and isolate it by drawing a box around it, that can save you a lot of time when cleaning up a soundtrack, whether you are using attenuation, deletion or Spectral repair. You can also use the De-clipper to good effect but if you have over-peaked considerably throughout your recording there is nothing much you can do to recover it satisfactorily. There is certainly no instant magic solution as there is for low level hum, hiss and buzz – but for those short over-peak spikes it does a good job.

 

The De-clicker is really for cleaning up records – so there is not much use for that in recovering normal video soundtracks. Nevertheless it is there if you need it, particularly if you need to include old archive sound in your production.

 

Conclusions

I think everyone should have some form of audio recovery program on hand to deal with those unforeseen problems that arise from time to time. Programs like RX are not cheap but they do a good job. You can get by with cheaper software, which can often get rid of background hiss and hum and suppress pops and crackles – but if you need to take control and get the best out of a bad job then there is no substitute for getting a higher spec package.

 

The modules in iZotope RX are generally easy to use at the basic level but there are many controls if you wish or need to go deeper. And there’s even more in the advanced version – but that is about three times the price! Check out the link below – I think you’ll be impressed.

Chris North M.M.Inst.V.

Article courtesy of iov.com

 

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