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Restoring Audio with iZotope RX™ iZotope with Nat Johnson

Tools, tips and techniques.

© 2008 iZotope, Inc. All rights reserved. iZotope and iZotope RX are either registered trademarks or trademarks of iZotope, Inc. in the United States and/or other countries. Other product or company names mentioned herein may be the trademarks of their respective owners.

INTRODUCTION ..................................................................................................4

Intended Audience for This Guide .................................................................................. 4

WHAT IS AUDIO RESTORATION?......................................................................5

What are the Goals of Good Audio Restoration? ........................................................... 5

WHAT IS IZOTOPE RX? ......................................................................................7

GETTING SET UP TO RESTORE AUDIO ...........................................................8

Equipment....................................................................................................................... 8 Transferring media to the computer ............................................................................ 10 Recording from analog formats .................................................................................... 10

SUGGESTIONS FOR THE RESTORATION PROCESS....................................13

HOW TO IDENTIFY AUDIO PROBLEMS...........................................................14

Using the Spectrogram and Waveform Display............................................................ 14 How to Spot Audio Problems........................................................................................ 17 Changing Spectrogram Settings.................................................................................... 21

BROADBAND NOISE REDUCTION...................................................................24

What is Broadband Noise? ........................................................................................... 24 Basic Techniques for using RX’s Denoiser..................................................................... 24

HUM AND TONAL NOISE ..................................................................................31

Finding and Removing Hum.......................................................................................... 31 Using Denoiser for Tonal Noise Removal ..................................................................... 35

REMOVING CLIPPING.......................................................................................36

How does RX’s Declipper work? ................................................................................... 36 Repairing Clipping ......................................................................................................... 36

REMOVING CLICKS AND POPS.......................................................................39

Using RX’s Declicker ...................................................................................................... 39

REMOVING INTERMITTENT NOISES AND GAPS ...........................................43

Using RX Spectral Repair............................................................................................... 45

GENERAL RX TOOLS........................................................................................47

Comparing Settings....................................................................................................... 47 Batch Processing ........................................................................................................... 48 EQ and Gain .................................................................................................................. 49 Working with Presets.................................................................................................... 50 Keyboard Shortcuts and assigning your own................................................................ 51

EXPORTING AND DELIVERING AUDIO ........................................................52

iZotope MBIT+™ Dithering Panel (RX Advanced only).................................................. 54 iZotope 64‐Bit SRC™ Panel (RX Advanced Only)........................................................... 56

SUMMARY .........................................................................................................58

APPENDIX: REPAIRING THE INCLUDED AUDIO FILES.............................60

Example 1: Removing Broadband Noise from a Concert Recording ............................ 61 Example 2: Using Denoiser and EQ on a Shellac Recording of Spoken Word .............. 63 Example 3: Restoring an Historical Speech: Making Voice More Intelligible .............. 64 Example 4: Cleaning up a Phone Interview with Spectral Repair................................. 65 Example 5: Removing Clicks and Pops from a Concert on Record ............................... 67 Example 6: Removing Clipping from a Phone Interview .............................................. 69 Example 7: Removing Guitar String Squeaks with Spectral Repair .............................. 71 Example 8: Filling a Gap in Audio with Spectral Repair ................................................ 73

INTRODUCTION

Today, audio and video recordings can be made by nearly anyone. From the simple camcorder to sophisticated studios and sound stages, a growing number of recordings are produced daily. And as our world becomes increasingly and audibly congested, so the rate of ruined recordings is rising! Human error, unexpected electrical or mechanical interference and unwelcome intrusions from aircraft, cell phones, pets, people and Mother Nature regularly impact even the most seasoned professional. Whether you’re a professional or just getting started, you have probably run into difficult audio situations like this and wished you could correct what seemed like an impossible dilemma. The purpose of this guide is to help you develop practical and effective ways to remove noise and fix audio problems using iZotope RX™. We hope this guide will help you understand the basics (and beyond) of audio restoration and help you to apply them to your own projects. Intended Audience for This Guide

• If you don’t know anything about audio restoration and don’t have iZotope RX, we still hope this guide will help you. Feel free to download the trial version of RX to follow along and try out these concepts for yourself.

• If you don’t understand audio restoration but do have RX, you’re in luck. RX gives you a complete

range of audio restoration tools and this guide shows you how to use them.

• If you have RX and know the basics of restoring audio, this guide will still show you tricks or techniques that are possible in RX. Just say “yeah, I knew that” when appropriate for the other parts.

Try It For Yourself! Be sure to download our RX Restoration Guide audio examples and try your hand at restoring them. At the end of this guide you’ll find an Appendix that will walk you through how to do the repairs. It’s a great way to apply what you’ll learn from reading this guide.

WHAT IS AUDIO RESTORATION? Audio restoration is the process of removing noise and other imperfections from sound recordings. Audio restoration can address problems including:

• ambient background noise, including fans, air‐conditioners and heating systems • electrically‐generated interference like buzz from fluorescent lights • hum from bad ground connections or induced from power sources • background hiss from reel‐to‐reel tape, audio and video cassettes • surface ticks and pops from vinyl, shellac 78 or other phonograph recordings • audience noise including coughs, pagers and cell phones • overload distortion (or clipping)

When you hear the word “restoration,” you might be inclined to think of dusty vaults filled with aging master tapes and records. Of course, audio restoration includes taking old recordings and reviving them, but it is hardly limited to antiquity. Every time you switch on a USB recorder, camcorder, digital camera, voice recorder, or cell phone you may encounter a variety of unexpected and unwelcome audio “guests.” For example, we recently read a review of a concert pianist whose debut at Carnegie Hall was marred by the ringing of a cell phone. The reviewer assumed that,” this and other intrusions will no doubt be magically erased when this concert is issued on CD.” While there is no magic button for removing cell phone noise (yet), the latest generation of audio restoration tools like RX can be just as useful for cleaning up 21st century problems as they are for restoring the earliest phonograph recordings. What are the Goals of Good Audio Restoration? The goal of good audio restoration is to render the best possible sound with the least audible human intrusion. In essence, your intervention in the original recording should be transparent and not introduce new artifacts that distract the listener. It’s useful to remember that no recording is truly perfect. What we hear in playback, no matter how well recorded originally, represents only a facsimile of the actual audio event. In the early 1980’s, when compact‐disc players became readily available and affordable, commercial tape vaults turned into gold mines. But strangely, during the early days of CD restoration, whole armies of purists became outraged when the record companies sought to change the character of certain, reverenced recordings (no matter how old) from their original, boxy‐quality and dull, wooly ambience. Today, we have fresher ears that seem to be more

objective. Whatever your tastes and even as times change, the basic intention of restoration should remain the same: render the best possible sound with the least obvious interference.

WHAT IS IZOTOPE RX? iZotope RX is designed to be the definitive audio restoration application. RX features innovative new processing technology for removing noise and repairing audio. In addition, its unique workflow features are designed to help you get the best results when working on challenging projects. What can RX do?

• Remove hum without sacrificing low‐end frequencies • Reduce ambient hiss without compromising upper partials • Eliminate pops and clicks leaving no audible artifacts • Remedy distortion and even gaps in the audio recording • Repair audible clipping with perfect precision • Replace damaged parts with inaudible patches • Visualize your repairs before and after processing with our revolutionary spectrogram

utility • Record and recall an unlimited number of precise presets within each module • Use a variety of application modules in any order in virtually unlimited combinations

Where can I use RX? RX is ideal for audio restoration and archiving, recording and mastering, broadcasting and podcasting, video production, forensics, and any application that demands flawless audio. In addition, RX features the most advanced spectrogram available, plus a wide array of convenient workflow features designed to make the process of audio restoration easier – with superior results.

GETTING SET UP TO RESTORE AUDIO Equipment Editing or Mixing Software In addition to iZotope RX, we recommend an audio editor or DAW (Sony Sound Forge, Adobe Audition, Digidesign Pro Tools, Cakewalk SONAR, Steinberg Cubase, MOTU Digital Performer, etc.). RX does offer basic editing and a fine parametric EQ, but programs like these will give you the tools you need to record, apply fades, assemble and deliver your projects. Computer Requirements To make your restoration experience more rewarding (and far less frustrating) you’ll want a fast computer with lots of memory and ample hard drive storage. No need to break the bank! Hardware costs are continually dropping and today, you can purchase a very decent desktop computer with a large monitor and professional sound card for less than a good laptop. Sound Card/Audio Interface You can use the sound card that came with your computer to monitor audio when doing restoration work, but we recommend getting a higher quality sound card or audio interface designed for professional‐level audio recording and playback. This in most cases will let you monitor audio with less noise, will give you higher quality inputs for transferring from analog media to your computer, and will include professional standard outputs such as XLR or balanced quarter‐inch phone jacks, which will let you connect to audio reference monitors with well shielded cables. Check your favorite audio retailer and you’ll see a wide range of interfaces at a wide range of prices. Choose one that fits you and has features that fit the way you want to work. For example if you will be transferring a lot of recordings from vinyl, some interfaces include features like a ground screw and built in phono preamp. Connector Cables Poor connector cables are often the weakest link in recording and transfer. If you’re connecting your computer to your speakers with a 1/8th inch cable that came with your commercial CD player, don’t be surprised if your cables actually add noise, making it harder to hear what you’re trying to remove. Spend a few extra dollars and buy higher quality, well shielded cables. You’ll thank us. Video Display The larger the video monitor, the easier it will be to perform quick and accurate sound decisions. RX provides a high‐resolution spectrographic display to visually aid in the restoration process and, with a large LCD screen, you’ll actually be able to see the problems that you hear.

Using two monitors, you can double your visual workspace by displaying your main work‐screen with spectrogram on one monitor while viewing the audio processing modules on the other.

Monitoring Audio Don’t be a cheapskate when it comes to speakers! Reliable audio monitoring, as you might imagine, is essential to successful restoration. We suggest a full‐range speaker system that suits your needs, as well as your working space. It’s a good idea to frequently check your work on other systems as well. As with mastering, the most important advice we can give with monitors is to learn the unique characteristics of your pair and how they sound in your room. The best way to “learn” your monitors is to listen to lots of recordings you know well to calibrate your ears. Room Acoustics You can have the best reference monitors money can buy, but if you’re working in a tiny square room with no acoustic treatment you will probably not hear the “flat” representation of frequencies that critical audio restoration work requires. Even small details like where your speakers are placed can change your speakers’ frequency response drastically. We recommend researching room acoustics online. Even if you’re on a budget, you can come up with a solution that will dramatically improve the sound of your room. Headphones Occasionally, you may want to listen to low‐level details that may not be obvious on loudspeakers. Obviously, we need to be careful of sound pressure levels on headphones and it’s wise to use them sparingly.

Transferring media to the computer When you sit down to start restoring an audio project, you might be lucky enough to have a digital file, such as .wav or .aiff, that has either already been digitized, or started in a digital format. However, many audio restoration tasks require you to get an old recording from tape, vinyl or other sources to your computer. Here are some tips for getting the most usable digital transfers of your audio. Start with the master copy It can’t be stressed too strongly: always utilize original, unprocessed source when possible. Files that have already been copied from record to tape, or digital files that have been converted to MP3 or another compressed format like lossy ATRAC, will be more difficult to repair. You’ll be faced with more audible artifacts introduced during the copying or compression process. Similarly, audio that someone has already attempted to do noise reduction and audio repair on may be much harder to get good results with than an original copy. Input levels Whatever your source – tape, disc, analog or digital – if you’re recording audio into the computer it’s important to keep input signals at a moderate level. Too high a level may result in inadvertent clipping at the soundcard or audio interface. Too low a level and you may not have adequate headroom to accommodate changes in level as you apply certain processing functions during restoration. Since the noise floor of an analog to digital converter is fixed, if you record at too low a level, you will have a lower signal to noise ratio.

Some CD and DAT players feature digital outputs, and many sound cards/audio interfaces have their own digital inputs. Taking advantage of this can be the perfect way to get digital audio directly into your computer without having to go through conversion back to analog and then back to digital.

Recording from analog formats Vinyl LPs and 45s Once you transfer a record to the computer you can do a lot to remove noise and improve the quality of the recording. However getting a good transfer from vinyl records can be tricky. Keep these things in mind:

• The vast majority of turntables don’t output a line level signal, they require a phono pre‐amp. Phono preamps not only boost the level of audio coming from the turntable, they also apply special equalization that is part of the RIAA standard for creating vinyl records. Without this gain and EQ stage, your transfer will just plain sound bad.

• Most audio amplifiers designed to work with turntables have phono pre‐amps built in. So do professional DJ mixers (many of these also provide excellent balanced outputs suitable for connecting to your audio interface). As mentioned earlier, some audio interfaces even have built in phono pre‐amps.

• Some turntables now include direct USB outputs, getting around the pre‐amp problem. However, keep in mind that many of these are very inexpensively made, and that you will get what you pay for when it comes to audio quality. Make sure you check the specs before buying. A good turntable/audio interface combination may give you better quality.

• Make sure your turntable is grounded! Mixers and amplifiers with phono inputs will

usually have a thumb screw that allows you to connect the ground wire from your turntable. Use it! If you don’t you will probably be adding electrical hum to your transferred file.

• Clean your records! Removing dust with a vinyl brush and/or using a vinyl cleaning

product can make a huge difference in the quality of your vinyl transfer. Why spend hours removing dust pops in the computer? You can get rid of most of them beforehand by simply cleaning your record.

• Use a good stylus. If your record player has had the same needle for 30 years, chances

are it’s time for a replacement! Using worn out needles can both affect audio quality and actually damage your records. Some turntables are now hard to find replacement needles for. However there are plenty of companies including Shure, Stanton and Ortofon that make a wide range of replacement styli. Check the internet or a local hi‐fi shop.

• When you have a choice, choose a stylus that is made for archiving. Many available

solutions are tailored to scratch DJs and sacrifice sound quality for tracking and other characteristics.

Shellacs, Cylinders and older phonograph formats There are a wide range of formats out there that preceded the modern vinyl LPs and 45 singles that are so commonplace today. In general, use caution when trying to transfer these types of recordings yourself without the proper equipment. For example, many schellacs require different, non‐standard stylus tips because of their varying groove wall widths. Some formats, like the Edison Diamond discs, have grooves that captured audio through vertical motion (called the hill & dale method) rather than lateral movement used by modern vinyl records. If you don’t have the right equipment, consider finding someone that specializes in formats like this to do the digital transfer for you.

While we could write an entire book on these rarer formats, that is out of the scope of this guide. If this is an area you’re interested in, you’ll find numerous web sites offering useful information and good tips on 78 RPM history, availability and playback equipment. Here are two websites with very good information:

The 78rpm Record Home Page ‐ http://78rpmrecord.com Tim Gracyk’s Phonographs and Old Records ‐ http://www.gracyk.com

Magnetic tape There are endless recordings out there made on cassette recorders and reel to reel machines. Whether you’re trying to restore the Watergate Tapes, working on reel to reel masters, or just cleaning up those old four track cassette recordings of your high school garage band, there are a few things to keep in mind when transferring:

• The quality of your playback equipment will make a difference. If you connect your old ghetto blaster’s RCA outputs to your computer, you’ll not only introduce more noise into the transfer, you might have other unwanted problems like speed fluctuations.

• Slight changes in a tape’s speed (called wow and flutter) can cause problems when

you’re trying to remove noise, because the noise can modulate along with the tape playback speed. This can make tonal and broadband noise difficult to remove because it will change over time. Work with a well built and well calibrated tape machine if possible.

• Tape heads can pick up debris and become magnetized over time, leading to poor

playback quality. Cleaning your deck’s tape head, capstan and rollers can help ensure you’re getting the best quality recordings. You can find deck cleaning/demagnetizing kits online and in specialty stores.

• Old mylar/polyester tape can stick to itself over time, causing tapes to self destruct

upon playback. If you’re pulling an old reel to reel tape out of a dusty archive, you may want to consult with an expert before trying to play it. Some tapes can be baked in an electric oven to remove moisture and revitalize the tape’s adhesive, but we don’t recommend you try this at home.

SUGGESTIONS FOR THE RESTORATION PROCESS On the subjects of recording and restoration, opinions are many and varied. Here are few general guidelines to keep in mind for the process of restoring audio:

1. Back up your work. Always make a backup of the original audio file before you begin attempting to restore it.

2. Keep the ears rested and the mind open. In doing audio restoration work, you will spend a lot of time focusing on details. Taking breaks will help you return with a fresh mind and see the big picture.

3. Make multiple versions. Sometimes it helps to try doing the same audio repair more than once with different settings and then compare the results. RX has a great Compare Settings tool that helps A/B results as you go. Also, you may come back to a version you tried a few days earlier when you were tired and now find it sounding worse than ever. This happens to all of us! See suggestion #2.

4. Begin work on the most obvious or obnoxious defect you hear first. There is no right or wrong order to use RX’s audio restoration modules in, but some processes will make sense to do before others in certain cases. For example, a loud hum might prevent you from hearing other details that need to be fixed, or clipping might make other flaws hard to hear—peeling away that first layer may make the next step more obvious to you. Take restoration one step at a time and don’t be afraid to try different combinations of tools to get the result you want.

5. Keep good notes. Accurate record‐keeping is invaluable, particularly when multiple options for the restoration process present themselves. Write down your settings, the technique you used and the name of each file you applied that method to, no matter how minute the different details may seem. Saving presets within RX can save you the trouble of writing out all of the parameters.

6. Back up your work. The first and last rule of any audio restoration project! You never know when a hard drive or your original analog master might fail. Again, always back up your work!

HOW TO IDENTIFY AUDIO PROBLEMS As with medical diagnostics, the key to successful audio restoration lies in your ability to correctly analyze the subject’s condition. This can be a life‐long, never‐ending quest—constantly honing the ear to distinguish that which is normal and that which needs to be corrected.

RX has many different tools for removing noise and other audio problems, and it also includes a spectrogram and waveform display that help you see and zero in on noise. To get started, it’s important to identify the problems with your file and identify which RX tool or tools will give you the results you want. Let’s briefly look at how to use the spectrogram and waveform display tools to examine your audio, then we’ll move on to specific examples of how to identify audio problems.

Using the Spectrogram and Waveform Display You’ve listened to the material in question but you may not be sure exactly what the problem is or how often it occurs. The spectrogram displays the individual frequencies of the audio over time. If you’re used to using the wave form display, it takes a bit to get your head around this unique way to “see” the audio. As a start, lets look at a few simple pieces of audio. Sine Wave Here is a picture of a sine wave moving up in pitch from 60 to 12,000 Hz as seen in RX’s waveform view. The waveform view can be enabled by moving the slider below the Spectrogram Display all the way to the left.

One thing you’ll notice when looking at the waveform display is that it is good at showing amplitude (loudness) of audio, but not very effective at showing what is happening at different

frequencies. For example, we can easily see here that the sine wave is the same loudness for the entire duration of the file. However, we can’t tell much about how the pitch or frequency changes over time. Now let’s look at the same file in RX’s spectrogram display:

Now it’s very obvious what is happening to the pitch of the audio: it is moving up! In the spectrogram display, the horizontal axis shows time, just like in the waveform display. But now, the vertical axis shows us frequency in Hz—that is, the pitch of the event that is happening. We can see how loud events are by how bright the image is. The black background is silence, while the bright orange curve is the sine wave moving up in pitch. Human Voice Now lets look at something more complex: the human voice. Here is a short spoken phrase as seen in RX’s waveform view:

Now what we’re seeing is the amplitude of the spoken words over time. If we switch to the spectrogram view, we’ll see many things we can’t see in the waveform view:

First of all you can now see that the human voice is much more complex than it might seem from looking at the waveform view. Each word is made up of a fundamental frequency (at the bottom of the spectrogram) harmonics that extend above that frequency, sibilance (“S” sounds) that begin or end words, and more. And of course, you can now more clearly see the noise that is surrounding the voice. This is why having a detailed spectrogram display is so important to doing audio restoration. It helps you clearly see the problems you are trying to fix. Now we’ll move on to some specific examples of noise and other audio problems and how to visually identify them.

How to Spot Audio Problems While your ears are great for locating problems in audio, nothing beats being able to simply glance at a file and immediately get a sense of what you need to fix. Better yet, if you’re working with a 2‐hour long live concert, you can learn to quickly zero in on problems like clipped sections, intermittent noises and more without having to listen to the entire recording first. Visual analysis will never replace your ears, but it can certainly compliment them in many ways. Let’s take a look at some common audio problems in RX’s waveform and spectrogram displays. Learning to identify these problems by sight will help you greatly as you learn to use RX’s audio restoration modules.

Hum Hum is usually the result of electrical noise somewhere in recorded signal chain. It can usually be heard audibly as a low frequency tone, usually based at either 50Hz or 60Hz depending on whether the recording was made in North America or Europe. If you zoom in to the low frequencies using RX's Spectrogram Display, you will be able to see hum as a series of horizontal lines, usually with a bright line at 50 or 60Hz and several less intense lines above it at harmonics. See below:

In situations like this example, which has only a few harmonics, RX's Hum Removal module is a good tool for the job. Hum Removal lets you set the primary frequency of the hum and control suppression of up to 7 harmonics above the low frequency. RX’s Denoiser is also a logical choice to use for removing hum, you may want to try both to see which gives you the best results.

Buzz In some cases, the electrical noise will extend up into higher frequencies, this will often manifest itself as a background buzz. See the example below:

In situations like this example, where the hum's harmonics extend into high frequencies, try using the Denoiser module in RX. Denoiser can work with this type of noise as well and features a Tonal Noise suppression control, which can often control high frequency buzz more effectively than Hum Removal.

The above screenshot shows RX’s Spectrogram Display in a higher time resolution mode that makes s settings. eeing tonal noise easier. Read on to learn how to adjust spectrogram

Hiss and other Broadband Noise Unlike hum and buzz, broadband noise is spread throughout the frequency spectrum and not concentrated at specific frequencies. Tape hiss and noise from fans and air conditioners are good examples of broadband noise. In RX's Spectrogram Display, broadband noise usually appears as speckles that surround the program material. See image below:

RX's Denoiser module is the go‐to tool for removing noises of this type.

Clicks, Pops and other short impulse noises Clicks and pops are common on recordings made from vinyl, and can also be introduced by some kinds of digital errors, including recording into a DAW with improper buffer settings. These short impulse noises appear in RX's spectrogram as vertical lines. The louder the click or pop, the brighter the line will appear. Below is an example of clicks and pops from a recording made from

vinyl:

RX's Declicker includes both automatic and manual tools for removing clicks and pops.

Clipping Clipping is a common problem that occurs when a loud signal distorts on input to a sound card/converter, mixing console, field recorder or other piece of equipment. The result is overload distortion. This clipping can usually be seen most easily from RX's waveform display (move the slider below the spectrogram to the left to superimpose the waveform display). As you will see in the image below, the clipping appears as "squared off" sections of the waveform.

RX has a dedicated Declipper tool which can in many cases rebuild the squared‐off peaks caused by clipping and restore the recording to a natural sounding state. Not every kind of distortion can be removed, but RX’s Declipper works well on a range of analog and digital clipping.

Sometimes heavily limited audio will also appear “squared off” when zoomed out – zoom in by time to see if individual waveform tops are clipped.

Intermittent Noises Intermittent noises are different than noises like hiss or hum, they may appear infrequently and may not be consistent in pitch or duration. Examples include coughs, sneezes, footsteps, car horns, cell phone rings, etc. The pictures below represent two different examples of these

noises: drinking glasses clinking together, and an electronic beep from a car ignition.

Because noises like this are often unpredictable, they usually need to be removed manually. RX's Spectral Repair tool provides a number of different modes that allow you to select and remove intermittent noises and replace them with re‐synthesized content based on the surrounding audio.

Gaps and Drop Outs Sometimes a recording may include short sections of missing or corrupted audio. These are usually very obvious to both the eye and the ear! See below:

RX's Spectral Repair can be used to resynthesize gaps in audio, sometimes of up to a half second long or more, by using information around the gap to fill in the missing information using patterns and advanced re‐synthesis.

Changing Spectrogram Settings In addition, there are countless ways to alter the spectrogram and how files can be displayed. Sometimes changing spectrogram settings can show audio in a different light, helping you to identify problems and zero in on noise. For example, just changing the colors of RX’s spectrogram can bring different events to the forefront. RX also has the unique ability to save your Spectrogram preferences as presets for easy recall later.

RX’s Spectrogram Display showing vinyl clicks and hiss with four different color settings

Spectrogram Settings iZotope RX offers advanced spectrogram options under the View | Spectrogram Settings menu. Here is an in‐depth description of the Spectrogram Settings controls:

Spectrogram Type ‐ RX offers some different methods for displaying time and frequency information in the spectrogram. RX has advanced spectrogram modes that allow you to see sharper time (horizontal) and frequency (vertical) resolution at the same time. There is always a trade off of display quality versus processing time, so keep in mind that some modes will take longer to draw on the screen than others.

• STFT (short‐time Fourier transform) ‐ This refers to the method that's used to transform the audio data into the spectrogram display. This type of spectrogram is the most common one and can be found in other editors. It has a fixed uniform time‐frequency resolution. This is the simplest mode in RX.

• Auto‐Adjustable STFT ‐ This mode automatically adjusts FFT size (i.e. time and frequency resolution of a spectrogram) according to the zoom level. For example, if you zoom in horizontally (time) you'll see that percussive sounds and transients will be more clearly defined. When you zoom in vertically (frequency), you'll see individual musical notes and frequency events will appear more clearly defined. This is RX’s default mode.

• Multi‐resolution ‐ This mode calculates the spectrogram with better frequency resolution at low frequencies and better time resolution at high frequencies. This mimics psychoacoustic properties of our perception, allowing the spectrogram display to show you the most important information clearly.

• Adaptively Sparse ‐ This mode automatically varies time and frequency resolution of a spectrogram to achieve the best spectrogram sharpness in every area of a time‐frequency plane. This often lets you see the most details for a thorough analysis, but it's the slowest mode to calculate.

FFT size ‐ FFT is a fast Fourier transform, a procedure for calculation of a signal frequency spectrum. The higher the FFT size, the greater the frequency resolution i.e. notes and tonal events will be clearer. However, choosing a larger number here will make time events less sharply defined because of the way this type of processing is done. Choosing "Auto‐Adjustable" or "Multi‐resolution" modes allow you to get a good combination of frequency and time resolution without having to change this setting as you work.

Enable reassignment ‐ This control enables a special technique for spectrogram calculation that allows very precise pitch tracking for any harmonic components of the signal. When used together with Frequency overlap / Time overlap controls, this option can provide virtually unlimited time and frequency resolution simultaneously for signals consisting of tones. Window ‐ The Window control lets you choose different weighting windows that are used for

the FFT analysis. This gets into some complex mathematics, but trying each of these will have different spectrum smoothing properties and prevent "spectral leakage" in different ways. Hann is RX’s default window setting. Frequency Scale ‐ Using different frequency scales can help you see useful information more easily. Different scales have different characteristics for displaying the vertical (frequency) information in the spectrogram display.

• Linear ‐ this simply shows frequencies spread out in a uniform way. This is most useful when you want to analyze higher frequencies.

• Logarithmic ‐ this scale puts more attention on lower frequencies.

• Mel ‐ the Mel scale (derived from the word Melody) is a frequency scale based on how humans percieve sound. This selection is one of the more intuitive choices because it corresponds to how we hear differences in pitch.

• Bark ‐ the Bark scale is also based on how we perceive sound, and corresponds to a series of critical bands.

Frequency overlap (a.k.a. zero padding factor) ‐ controls the amount of oversampling on the frequency scale of spectrogram. When used together with the Reassignment option, it will increase the resolution of the spectrogram vertically (by frequency). Time overlap ‐ This controls the time oversampling of the spectrogram. In most cases, overlap of 4x or 8x is a good setting to start with. However using higher overlap together with the Reassignment option will increase the time resolution of a spectrogram, making transient events appear more clearly. Color Map ‐ RX's spectrogram display allows you to choose between several different color schemes. There is no right or wrong color setting to use‐we recommend you try them all. Sometimes certain color modes will make different types of noise stand out more clearly. Experiment! High quality rendering ‐ turning this control off makes spectrogram rendering slightly faster, but you'll lose some detail and clarity in the spectrogram image.

Reduce Quality Above ‐ RX's spectrogram uses very accurate rendering letting you see audio problems, such as clicks, even at zoomed‐out state. However doing such rendering for long files can be somewhat slow. So, when the length of the visible spectrogram is above the specified number of seconds, the spectrogram calculation is changed to a fast and less accurate preview mode. When the user zooms in, the spectrogram calculation becomes accurate again.

BROADBAND NOISE REDUCTION What is Broadband Noise?

Broadband noise is generally defined as noise that is spread across the frequency spectrum. Typical examples of broadband noise include air conditioning, tape hiss, camera noise and other types of interference that hardly changes in level or spectral shape throughout the recording. Visually, it appears as random speckles or a haze over the audio file – similar to the static you see on your TV when there is no signal. In general, the concepts in this chapter can also be applied to other kinds of stationary noise and even hum and buzz—we think you’ll find RX’s Denoiser to be a very versatile tool. Below are screenshots of a noisy sound file recorded in 1914 – the voice of Andrew Carnegie, philanthropist, who endowed many renowned institutions (notably Carnegie Hall in NY).

The far left hand side of these shots illustrates how RX visualizes the background surface noise (notice the white arrows pointing to the green granules in the left image) and the results of RX’s Denoiser (right image). Notice that after being processed with Denoiser, the visual “static” will be decreased in level, or will appear dimmer. This is a good visual indicator of how much noise you’ve removed. Also, we’ve selected a portion of voice component for display on the RX spectrum analyzer – another extremely useful visual instrument in the RX toolbox (you can find this tool under View | Spectrum Analyzer, or just hit Alt + R). Keep these visualizations in mind as you read on. Basic Techniques for using RX’s Denoiser

Training RX’s Denoiser works by “learning” what the noise in your file is from a section of noise‐only material. By examining this section, Denoiser creates what is called a “noise profile.” Once you’ve taught Denoiser what noise is, it uses this noise profile to intelligently suppress noise that

is mixed in with other sounds. This technique lets you keep desired audio while reducing the volume of unwanted noise.

To train Denoiser, identify a section of the recording that contains only noise, without any useful audio signal. Often these places are at the beginning or end of a file, but may also be during pauses or breaks.

Selecting just noise and hitting “Train” will capture a noise profile

Select the longest section of noise you can find, then hit the Train button. This will "teach" Denoiser the noise profile of your file. In some cases you may not have a section of just noise, or you may want RX to find a quiet section for you. If this is the case, open the options menu next to the Train button (+) and choose Auto Train.

Denoising After the noise profile is captured, adjust the Noise Reduction slider and hit Preview to listen to the result. Moving the Noise Reduction slider up and down will control how aggressively RX is reducing the noise. When you’re happy with your settings, you can hit Apply to process the entire file. If you wish to just process a certain selection (by time or by frequency or both), select an area using RX’s selection tools then hit Apply.

By making a selection in your project, you can preview just that selection. While in preview mode, you can also have RX loop through a selection of audio by turning on the loop feature on RX's transport.

Make sure that the “A (realtime)” algorithm is selected if you want to preview the result in realtime—the other modes may yield higher quality but they take much longer to process. Starting with the “A” algorithm is a good way to quickly get a sense of how the noise reduction will sound. In many cases you won’t need to use RX’s more powerful “B” and “C” algorithms to get a great sounding result.

Output noise only – This useful check box outputs the difference between the original and processed signals (suppressed noise). This is a tremendously useful feature in Denoiser because it allows you to hear instantly how effectively noises are being removed and how much, if any, program content is being impacted by your settings.

How much noise should I remove? Sometimes having a little bit of broadband noise is a good thing. It can help gel recordings together and make them sound more natural‐‐some audio engineers actually add broadband noise subtly to their mixes to simulate the forgiving characteristics of tape’s noise floor. And sometimes, for interviews, movies and other situations, leaving a bit of noise adds important atmosphere to the recording.

When attempting to remove background or broadband noise from a recording, it’s preferable to identify and isolate annoying and obtrusive elements – high hiss levels, disc noise, air‐conditioning, etc. When you’ve successfully removed that which is annoying, listen to what’s left. Hopefully, you’ll still hear some kind of room‐tone or natural ambience. If not, you may have removed too much.

To get a usable, natural sounding result, you won’t normally have to apply more than 15db of Noise Reduction to get a huge difference. We encourage you to try a wide range of settings by using the Compare button (more on this feature later). Keep in mind that turning Noise Reduction all the way up isn’t usually the best sounding option!

Smoothing and Musical Noise Reduction Especially when doing very drastic noise reduction, you may begin to hear artifacts left over from the noise reduction process. These are often chirpy, watery noises which in technical terms are referred to as “musical noise.”

The Smoothing (Basic tab) and Musical Noise Reduction (Advanced tab) controls supress artifacts that result from removing broadband noise. Increase the Smoothing (Basic view) or Musical Noise Reduction (Advanced view) parameter to reduce these artifacts. RX’s musical

noise reduction incorporates cool new technology based on digital imaging to give you very natural sounding results with less artifacts.

Choosing an Algorithm RX's Denoiser offers three algorithms that range in processing time. It is a good idea to start with Algorithm A, as it works very quickly. This algorithm is optimized for realtime playback, so it’s best to use this setting when you’re using Denoiser as a plug‐in. If you're not happy with the results in this mode, try B and then C, which will take longer to process but they use more complex techniques that may result in an even better fix.

Algorithm B uses a different Musical Noise Reduction technique for smoothing out artifacts which can give you more natural sounding repairs. Algorithm C uses multi‐resolution processing to preserve transients better than other noise reduction tools. While they sound excellent, Algorithms B and C are fairly slow, and may not be able to preview in realtime. This is a good place to use the Compare Settings window to save time when comparing these options (see the General RX Tools chapter for more details).

About the Noise Spectrum Display After you train RX’s Denoiser, you’ll immediately see the Noise Spectrum Display change. This shows the “profile” of the noise you’ve trained RX with. You can see where the noise you’re trying to remove sits in the frequency spectrum, and how loud it is. The Noise Spectrum Display offers useful information both during playback and when the noise reduction process is applied. The four curves shown display different information:

• The Magenta curve – shows the spectrum of the input audio signal

• The Yellow curve – shows the spectrum of the denoised (output) audio signal

• The Blue curve – shows the noise threshold. This curve represents the learned noise profile added to the threshold elevation control

• The White curve – shows the desired noise floor after denoising, This can be controlled by Reduction and Residual whitening controls (advanced mode only)

Scrolling and Zooming ‐ by grabbing and moving the horizontal and vertical rulers, you can adjust what part of the graph is shown. In addition, holding the pointer above a ruler and using your mouse wheel will zoom the display in and out. Running the mouse over the graph displays corresponding db/frequency characteristics for each curve in the upper part of the window.

Denoiser’s Advanced Controls Both iZotope RX Advanced and the basic version of RX feature an “Advanced” Denoiser panel that you can switch to if you want more control over the noise removal process. Here are some deeper features you can use to fine tune Denoiser:

Tonal and Broadband noise reduction controls RX Denoiser’s Simple mode gives you just one Noise Reduction slider. The advanced panel gives you two: one for broadband components of the noise and one for tonal components. This allows you to tweak the amount of noise removed for both types of noise separately.

Why use this? Sometimes there will be two types of noise in your audio file that are of varying intensity. For example, you may have a very loud buzz (tonal noise) as well as a quieter tape hiss (broadband noise). By fine tuning these controls, you can remove the buzz more aggressively while gently removing the tape hiss. This can lead to more natural sounding repairs with less residual noise and artifacts.

The numerous slider controls found in both Simple and Advanced mode offer fine‐tuning of your settings. Holding the CTRL key as you move the slider with your mouse allows more exact management of the slider motion.

Threshold Controls The advanced panel also features threshold controls for broadband and tonal noise. These controls are useful for fine tuning the noise suppression. For example, if you feel not enough noise is being “caught” by Denoiser, try raising the threshold slightly. If you feel Denoiser is damaging audio you don’t want to remove, try lowering the threshold slightly.

In many cases you will never need to touch the threshold control to get the result you want. We recommend you start with a threshold of zero.

Musical noise suppression This control adjusts the reduction of artifacts known as "musical noise". This control is the advanced equivalent of the Smoothing control found in the Basic panel.

Residual Whitening This feature modifies the amount of noise reduction applied at different frequencies to shape the spectrum of the noise that’s left over.

When residual whitening is zero, noise suppression is the same at all frequencies, and the suppressed noise has a similar spectral shape to the original noise. When residual whitening is at its highest setting, the desired shape of suppressed noise floor is made closer to white noise, which is usually more neutral sounding. This control in combination with the Musical Noise Suppression control can make a huge difference in controlling residual artifacts.

More Advanced Mode Controls (RX Advanced Only):

MNS algorithm ‐ Musical noise artifact suppression algorithm

• The Simple algorithm performs independent noise gating in every frequency channel of FFT. Release time of sub‐band gates is controlled by the Release control. This is a fast algorithm with small latency (delay) that is suitable for real‐time operation.

• The Advanced and Extreme algorithms perform joint time‐frequency analysis of the audio signal which results in better quality and less "musical noise" artifacts. These algorithms have higher latency and computational complexity.

FFT size (ms) In RX, we control FFT window size for specific requirements: longer windows mean lower time resolution, but better frequency resolution. Good frequency resolution (longer windows) allows for better separation of tonal signals and noise, but it also reduces time precision of Denoiser which can lead to softened transients and less noise reduction around transients. Switching to Multi‐Resolution mode will disable this control.

Multi‐Resolution The multi‐resolution processing selects the best time‐frequency resolution for every portion of the signal to minimize smearing of transients and at the same time achieve high frequency resolution where it is needed. The overall quality of noise reduction is increased, but at the cost of much higher computational complexity. The FFT size control does not have any affect in multi‐resolution mode because FFT resolution is selected automatically.

Whenever FFT size is changed, it's recommended you retrain Denoiser because the old noise profile was taken at a different FFT size and therefore becomes inaccurate.

Knee sharpness Adjusts the sharpness of the noise gates used in the noise reduction process (1 is soft knee, 8 is sharp knee).

Release Selects the release time of the sub‐band noise gates in milliseconds. Longer release times reduce musical noise more effectively, but can produce echoes of noise after loud signals.

Psychoacoustic suppression Enables a psychoacoustic model that dynamically controls suppression amount. This allows RX to use softer suppression where noise is subjectively inaudible to the human ear. The position of the slider controls the influence of psychoacoustic model on suppression levels.

Harmonic enhancement This control allows for softer suppression of harmonics of the signal that could be buried in noise. It can make the resulting signal brighter and more natural sounding, but high values of harmonic enhancement can result in high‐frequency noise being modulated by the signal so use it sparingly.

HUM AND TONAL NOISE

Hum can originate from bad ground connections or inducted from power sources such as AC mains or transformers and can usually be heard audibly as a low frequency tone, usually based at either 50Hz or 60Hz. Hum be easily eliminated using the RX Hum Removal module, which we’ll describe in this chapter.

Tonal Noise such as buzz from fluorescent lighting, is not as easy to trace back to a primary frequency and may extend up into high frequencies that are out of the scope of what RX’s Hum Removal module can handle. Tonal noise varies from broadband noise in that it tends to be concentrated at certain frequencies. We’ll also discuss removing tonal noise with RX Denoiser in this chapter. Finding the Hum’s Primary Frequency To begin, select a section of the recording where the hum is prominent. Sometimes there will be silence (or near silence) at the beginning or end of the program material that will contain noise but not any other audio. Otherwise, try choosing a quiet passage of the recording where hum is obvious. Next, select the section of audio where the hum is most clearly audible. Choose the Loop Playback button. This will allow you to set Hum Removal's parameters as audio plays back.

When attempting to remove hum, you first need to find the hum's primary frequency. The two most common base frequencies for hum are 50 Hz (Europe) and 60 Hz (U.S.) due to those regions’ difference in electrical delivery. It’s a good idea to try the 50 and 60 Hz presets included with RX first. If you hear the hum disappear or get much quieter, you’ll know you’re on the right track.

In some cases, for example a recording made from analog tape that is not precisely at its original recorded speed, you may need to choose the "Free" Frequency Type. Selecting this option unlocks the Base Frequency control and allows you to manually find the Hum's root note. With Preview engaged, move the slider up and down until you find the point where the hum lessens or disappears.

Using the Spectrogram

For even more precise settings in free mode, use RX's Spectrogram Display to zoom in on the project's low frequencies. You can hover over the low‐frequency part of the scale on the right and move your mouse wheel up to zoom in. Under View | Spectrogram Settings, choose the Adaptively Sparse spectrogram type to see pitch with razor sharp accuracy. Dragging the ruler up and down will re‐position the spectrogram at different frequencies. Remember, hum usually appears as a bright horizontal line, sometimes with many less bright lines above it (harmonics).

Zooming in on the vertical axis shows that this hum tone is at 110 Hz

Adjusting Filter Depth You can adjust how aggressively Hum Removal attacks the primary frequency by pulling down the first frequency node. The deeper the cut, the more hum will be removed, but you may also adversely affect wanted audio in that frequency range. Try selecting the Linear Phase Filters check box when you’re making very deep filter cuts in full frequency range content, they will help preserve surrounding audio due to their exceptional steepness. This mode is more CPU intensive and will introduce some latency, so keep this in mind (especially when using the Hum Removal plug‐in).

Adjusting Filter Width (Q) You can also adjust the width of Hum Removal’s filters. The wider the filter, the more overlap there will be with surrounding frequencies. Grab the handles on either side of the Hum Removal filter node and drag them apart to widen the Q or together to narrow it.

Attenuating Hum's Harmonics Because higher frequency harmonics often result from hum, RX's Hum Removal module has controls for attenuating these overtones.

Using the Number of Harmonics control, you can select up to 7 harmonics above the primary hum frequency. Again, the spectrogram display in many cases makes it easy to identify the number of hum harmonics in your project.

After selecting number of harmonics, RX engages new filters at set harmonic points above the primary frequency. Use the Harmonic Slope control to how aggressively the higher harmonics are being cut. The Filter Q control adjusts the width of the hum filters.

Using the Residual Output control By selecting the Residual Output checkbox, RX will boost the signal at the points where you are attempting to remove hum. This is useful for fine tuning your settings. Play through a section of your file where the hum is mixed with other material, and select Residual output mode and then hit Preview. Now you can adjust parameters like the Filter Q (width) control and the Harmonic Slope control to maximize hum removal while minimizing the affect on the program material. If you hear wanted program material leaking through in this mode, you may want to try narrower filters or Linear Phase mode.

Low and High Pass Filters RX’s Hum Removal module includes low and high pass filters that allow you to gently or steeply roll off the highest or lowest frequencies in your project. This is not as precise a tool as the steep notch filters described earlier. However if you have a simple low hum and rumble below a voice for example, you may wish to simply enable a high pass filter that cuts all frequencies below 65 Hz.

Filter DC Offset This removes the DC (direct current) offset that is caused by the imbalance that sometimes occurs in A/D converters. DC offset is exhibited by the waveform appearing above or below the zero line, and is undesirable because maximum audio levels cannot be effectively achieved. DC offset may also cause audible artifacts depending how the signal is being processed.

Using Denoiser for Tonal Noise Removal In some cases, electrical noise will extend up into higher frequencies; this can manifest itself as a background buzz. See the example below:

As we mentioned, RX’s Hum Removal tool is optimized for low frequency hum. For addressing that upper frequency buzz we recommend you use RX Denoiser, which is capable of removing this type of tonal noise. See the Broadband Noise Reduction chapter for details on how to use Denoiser.

REMOVING CLIPPING

Live concerts and on‐location interviews frequently become the victims of signal overload or clipping. This is often the result of too little time available for set‐up or sound check. In music recording, overly‐enthusiastic singers and drummers are frequently the worst offenders! iZotope RX features a Declipper tool which can save recordings from some types of both analog and digital clipping or overload distortion.

How does RX’s Declipper work?

RX features a unique Declipper which uses advanced interpolation techniques to intelligently rebuild the peaks of clipped audio recordings. This tool can be invaluable for fixing recordings that you may have thought were ruined! However, we should note that some distortion simply cannot be fixed. There is always a point at which a digital converter or analog tape has been pushed so hard that there is not enough useful information left for RX to make a good repair. You may find old records that have been played so much they have groove wall distortion from record wear—this can be virtually impossible to fix. In some cases, if Declipper does not work for you you may be able to use RX’s Spectral Repair tool to fix short corrupted segments. See the Spectral Repair chapter for more details.

Repairing Clipping

Choosing a Declipping Mode In most cases, it is best to start with the Declipper module's Single‐band mode. For trickier clipping, you may need to use Multi‐band mode (or Multi‐resolution mode in RX Advanced). We’ll discuss this later in the chapter.

Using the Histogram to set the Clipping Threshold The most important part of setting Declipper is figuring out at what level in the recording the clipping is happening so you can set the Clipping Threshold control. Depending on how the recording was made, whether the clipping is analog or digital in nature, and other factors, clipping can appear at different dB levels.

The histogram meter in the Declipper module allows you to analyze clipping in your file. Select a section that is clipped, and hit the Compute button (if you’re using the plug‐in version of Declipper, the meter will automatically update during playback). A graph will appear showing the average levels of the audio over the selected time. This is called a histogram.

If clipping is present in the selection, it will usually appear as a horizontal line in the histogram that extends all the way across the meter. This is where the peaks in the file are concentrated, usually indicating clipping.

To set the module's threshold, move the red line in the histogram, or the Clipping Threshold slider, until the red line in the histogram is just below the line where clipping is concentrated.

if the clipping point is below ‐8 dB FS, the histogram will be empty. Your file may need to be normalized. You can also raise the overall gain of the file using RX's Gain module.

Makeup Gain This control accommodates the increased peak levels caused by the declipping process. When RX rebuilds the clipped peaks, they will often go higher than zero. If the rebuilt peaks go above zero dB in RX, your recording will clip again when you export it. The make up gain control lets you set the resulting gain of the declipping process so that the waveform is brought down below zero.

When using Declipper, start by setting make‐up gain at around ‐6.0 dB. If you're removing clipping during a short selection of a file, you may want to set this control higher to prevent a noticeable volume drop in the program material during the declipped section.

Choosing a quality mode There are three quality modes in RX's Declipper: Low, Medium, and High. Low quality mode processes very quickly; High quality mode processes slowly but in some cases can achieve better results. In many cases you will find that "Low" quality mode gives you great results. To save time, always start by previewing the Low quality modes first.

When using Declipper as a plug‐in, the higher quality modes may introduce a lot of latency or cause drop outs. If this happens, you can switch to a lower quality mode for realtime use, then change to high quality mode when you’re ready to bounce down or render your track.

To avoid fluctuations in volume, you may want to apply Declipper to your entire file rather than just short clipped sections. Single Band mode will not adversely affect the quality of the non‐clipped material.

When to use Multiband and Multi‐resolution modes

Usually, Single‐band mode is a good start for any audio file, but if the quality of single‐band declipping is insufficient, you may want to try other modes, such as Multi‐band or Multi‐resolution declipping. Both Multiband and Multi‐Resolution modes have similar parameters to Single Band mode. However they also allow you to select the number of frequency bands RX uses to perform interpolation. A higher number of bands does not necessarily mean higher quality, you may need to search for a good sound with this parameter as it is extremely situation‐dependent. For example, multi‐resolution mode can be used in certain scenarios like high female vocals with reverberation that have only a moderate amount of clipping. We recommend you try multiple modes and preview the results in the Compare Settings window.

REMOVING CLICKS AND POPS Clicks and pops can occur at almost any stage in the recording process and can appear in nearly any medium: surface noise from mechanical media (especially discs), static electricity, power‐line interference, inadvertent physical contact with a microphone (including those on camcorders) or telephone handset and/or bad audio connector cables. Occasionally, digital errors will result in a few, unexpected clicks as well. Your challenge will be to reduce or remove clicks and pops without disturbing program content. Thankfully RX provides some easy to use, high quality tools for doing just that.

Using RX Declicker

RX’s Declicker repairs or reduces clicks, pops and other impulse noises in the audio waveform. It has two modes: Manual and Automatic. In the Manual mode, you can manually select a click or other impulse noise on RX’s Spectrogram or Waveform. In Automatic mode, the program automatically detects and interpolates clicks in a long selection.

As with other modules discussed earlier, we recommend you begin with factory settings or presets as you explore the best way to eliminate clicks. And as with other operations in RX, it’s easy to Undo and Redo before committing your changes to the file itself.

Automatic Mode When using the automatic feature, it’s best to preview the offending clicks in the waveform view in order set the proper width via the Maximum click width control. The waveform view is more accurate than the spectrogram for determining click width.

Before applying Declicker, click on preview to hear the result of your settings and while audio is in playback, fine tune the sensitivity and click width sliders to remove the most clicks and pops. The ideal setting should remove the most clicks without damaging transients in the program material. For example, drum hits may be mistaken for clicks at high Width and Sensitivity settings.

Output Clicks Only Ticking this check box will let you hear just the clicks and pops that RX is removing. This is extremely useful for checking to make sure you’re only removing what you want to. Hit preview with this box checked and if you’ve set the Width and Sensitivity controls correctly you will hear just your clicks and pops without the content you want to keep.

If you want to create amazingly realistic lo‐fi effects, try leaving Output Clicks Only on when you apply the Declicking process. You’ll be left with just the clicks and pops, which you can export as a sample that you can use as an effect elsewhere.

Quality The Low and Medium quality modes offer fairly good sound quality with low latency and CPU usage. These modes are ideal for using in time critical applications or when you’re using the plug‐in version of Declicker in realtime in your audio host. The high quality settings will allow you to remove more ticks with less residual damage to the audio.

You may find the lower quality settings are helpful for previewing the declick process in real‐time. Preview in low quality mode then apply the process in high quality mode. This can be especially useful for long recordings with a lot of small clicks

Manual Mode

Automatic mode is great for a quick fix, but many situations require a “custom” setting in order to totally clean the odd click or pop from your file. Find a prominent click or pop in the file you’re cleaning and after selecting it with the mouse, zoom in on the selection and examine it in the spectrogram.

Once zoomed in, you’ll want to carefully reselect the noise you need to eliminate including only the noise spike that’s visible. While the spectrogram will help with finding clicks, the waveform display will help you accurately select click boundaries. When you hit Apply, the click will be removed using interpolation from the surrounding audio data.

The Declicker can only process selections with 4000 samples or less in Manual mode. Should your selection be wider than 4000 samples, you may need to remove the problem with Spectral Repair (which we’ll discuss in the next chapter).

If the result is not satisfactory, try adjusting Interpolation Order or other parameters. If the click or pop is very long, Declicker may cause a minor drop in level. You may want to try the RX Spectral Repair tool instead.

Processing on one channel only In the example below, we wanted to remove a click, present in the left channel alone, without disturbing the sound in the right channel. The fragile program content (a boy choir singing a cappella in an empty cathedral) required precise and delicate processing. RX can isolate either the left or right channel and process in just that channel when required. In a stereo program, you can select either left channel alone, right channel alone or both together when choosing a portion of your file for processing. Select the left channel alone by dragging the mouse at the top of the display, the right channel alone by dragging at the bottom of the display or both channels by selecting in the middle. You can also de‐select a channel by clicking the gray rectangle to the left of the channel in the display.

There are many tricks of the trade and the more you try, the easier and quicker declicking will become. For example, with material containing clicks and pops of varying sizes try running Declicker in several passes. Begin by repairing short clicks, and then continue on to longer ones. Also, you might try first applying the automatic declicker and then moving on to manual declicker for those clicks missed in automatic mode (or vice versa).

REMOVING INTERMITTENT NOISES AND GAPS Intermittent noises can include a wide range of intrusive sounds that don’t fall into the category of broadband or tonal noise. This can include cell phone rings at a violin recital, squeaking door hinges interrupting an interview, or traffic noise that mars dialog recording for a film. Gaps, dropouts and short sections of corrupted audio are also a common audio problem. These can come from everything from a loose mic cable to digital errors. There are several things that make these noises especially hard to fix:

They can be wildly unpredictable in frequency and timing. Unlike broadband noise or clicks, noises like this can’t be removed with an automated

process. They can be very time consuming to fix. Most traditional noise reduction tools and audio editors cannot effectively remove them

without leaving many artifacts or damaged audio. Thankfully, iZotope RX includes advanced Spectral Repair tools that allow you to fix these difficult problems. In this chapter we’ll explore using the Spectral Repair module to remove intermittent noises and fill gaps. We strongly recommend you try the Spectral Repair examples included in the Appendix and audio demo files that accompany this guide.

Selecting by time and frequency to repair noises

The process of spectral repair will require making precise selections, either along the timeline or by frequency. This will require interacting with RX’s Spectrogram with different selection tools. If you’re using the Spectral Repair plug‐in that comes with RX, it also includes a built in spectral editor window that works in a similar way to RX’s interface.

If you didn’t know how to read a spectrogram display when you started this guide, you probably now have a better sense of what information one can help you see. When going after tricky intermittent noises, RX’s Spectrogram Display not only allows you to see the problem, it allows you to selectively choose a time and frequency range to apply Spectral Repair to. This is the essence of Spectral Repair—it allows you to focus in and repair only certain parts of the frequency spectrum.

Let’s take a look at the RX spectrogram and the all‐important time‐frequency selector controls:

Time Selection – this tool allows you to select a segment of your project along the timeline. Time‐Frequency Selection ‐‐ allows you to make rectangular selections in the Spectrogram

Display to isolate sounds by time and frequency. Frequency Selection ‐‐ lets you select by just frequency (vertically in the Spectrogram). The Zoom Selection tool, when enabled, zooms into time and frequency selections as you make them. Most often when removing intermittent noise you will be using the Time Selection tool to select a short slice of audio along the timeline, or using the Time‐Frequency selection tool to draw a box around an offending sound buried in your audio file. Here’s an example of what making a Time‐Frequency selection will look like in RX’s Spectrogram Display:

In this case we’ve drawn a narrow rectangular selection around a very clearly defined sound: an electronic beep. This is a good example of a sound that occupies only a certain frequency range that must be surgically removed to avoid damaging the surrounding frequencies. In some cases you will want to select all frequencies over a range of time and apply Spectral Repair. Usually this works best for short sections of audio that are completely corrupted or unusable, gaps and dropouts, and where a noise extends from the top to the bottom of the frequency spectrum. Previewing the selected sound Once you’ve drawn a selection around a specific time and frequency range, click the Preview Selection button in the transport. This special Play button will play back only the selection, allowing you to hear what is in the box you’ve selected. This is great for making sure you’ve

accurately selected only what you want to fix.

Decided you want to change your selection slightly? You can simply grab the edges of a selection box to resize it.

Choosing a Spectral Repair Mode Now that you’ve carefully selected the noise you want to repair, open the Spectral Repair tool. The first thing you’ll need to do is choose from one of Spectral Repair’s four processing modes. Each of these modes uses a different technique for making a repair. You may want to try each of these in turn on the same noise to see which works the best. Here are some general guidelines on how to use each:

Attenuate This mode is great for blending noises into the background of a recording and making them inaudible. For example, an event like the beep shown above, a cell phone ring, a car horn or a squeaky door are good candidates for this mode. Attenuate looks at the audio around the selected event and uses that information to blend the offending sound into the recording’s background. This mode is suitable for recordings with background noise or where noise is the essential part of music (drums, percussion) and should be accurately preserved. It's also good when unwanted events are not obscuring the desired signal completely.

• The Strength control adjusts how aggressively Spectral Repair attenuates the sound you’ve selected. If you set strength to a low setting, some of the noise may still be audible. If you set it too high, you may hear a “hole” in the audio at the point of the repair. Finding a good balance between the two is the key to getting great results with this tool.

• Direction determines which direction the Spectral Repair algorithm will look when it is figuring out how to repair the sound. In Vertical or Horizontal modes it will look at the material above and below, or to the left and right, of the selection. In 2D mode, it looks around the event in all directions. Trying each of these modes can give you dramatically different results. Try them all to see which one works best on your repair. We recommend using Vertical mode for long narrow repairs, and horizontal mode for short time selections that cover a wide frequency range.

Replace Unlike Attenuate mode which gently blends the selection into the background, Replace mode completely resynthesizes the selected area. Replace mode works very well when there is a sustained musical part or other tone that is interrupted by an unwanted sound, and can also be useful when removing noises from dialog. This mode can also be used to close up gaps, dropouts and corrupted sections of audio.

Pattern This mode searches for a pattern in the surrounding material and uses it to replace the selection

you’ve made, seamlessly blending the repair into the surrounding audio. Pattern mode is particularly useful for gaps and badly damaged or corrupted sections of audio. Try using Pattern mode on drop outs in rhythmic parts like drum tracks—it can work wonders.

Partials+Noise This mode can be thought of as an advanced version of Replace mode. It is also a good choice for sustained musical material and tones. When used carefully it restores harmonics of the audio more accurately. It can even track changes in pitch between the beginning and end of a gap. Partials+Noise mode links detected harmonics by synthesizing them through the selection, and interpolates the rest of the signal using the Replace method.

• Harmonic Sensitivity adjusts the amount of detected and linked harmonics in Partials + Noise mode. Lower values will detect fewer harmonics, while higher values will detect more harmonics and can introduce some unnatural pitch modulations in the interpolated result.

Common Controls Spectral Repair’s modes share several common controls that affect the way the selection is repaired.

• Surrounding Region Length lets you choose how far before and after the selection Spectral Repair will look when figuring out how to make the repair. Trying different settings will help you find the best repair for your particular problem.

• Before/After Weighting determines whether Spectral Repair uses more information from before or after the selection to make the repair. For example, you may need to repair a sustained section of audio right before another event on the timeline. To avoid smearing of that other event into your repair, set the Weighting control all the way to the left to only use audio before the selection to make the repair.

• Number of bands selects the number of frequency bands that RX uses to make the repair. A higher number of bands can provide better frequency resolution, but will also use a wider surrounding area.

• Multi‐resolution (RX Advanced only) this mode allows for better frequency resolution at

low frequencies and better time resolution at high frequencies simultaneously.

GENERAL RX TOOLS

Comparing Settings The ability to easily, repeatedly and accurately compare settings, visually and aurally, within the five RX modules is a valuable tool as well as a true timesaver in whatever project you undertake. You’ll find the Compare Settings feature to be an extremely useful reference when contrasting a wide range of choices with the original. Taking notes on your settings and saving your own presets can be an excellent path toward efficient and effective audio restoration procedure. Sending settings to the Compare Settings Window Find a setting in the module you feel works for a specific situation and then click on the Compare button. RX automatically opens the Compare Settings window while processing the result of your settings in the background. View and change your settings and click again on Compare. RX will add a new item to the list each time you click on Compare. By clicking on Original with your new Settings, you’ll have an instant comparison with every Setting listed in the window. Repeat this step with other control settings or processing modes in the module. These will be listed with default names, but you can use the Rename button to choose your own names for the items in your Compare list. Notice that the Spectrogram display, as well as the sound, changes each time you select a Setting in the Compare Settings window.

Batch Processing RX offers numerous time savers and Batch Processing is one of the best. Simply defined, Batch Processing permits the automation of processing on groups of files.

Use of Batch Processing allows automation of a long list of files to be run and processed in the background while you do other things: get a snack, walk the dog, take a nap.

Apply Batch Processing to files

Begin by selecting File| Batch Processing. Click the "Add Files" button to add one or more file for batch processing, then choose the processing module you wish to use. To view the settings for a particular batch processing job, click the View Settings button. After making changes, to grab a snapshot of the module's current settings to use for batch processing, click the Update Settings button.

Running a Batch Process

Once you are satisfied with the batch processing jobs, click the Run button to run all of them. You will see a progress dialog while RX runs each job. To cancel the current job and all subsequent jobs, click the Cancel button.

After batch processing, the results will be presented to you in the batch processing window. If a job has failed or been cancelled, you can click the Run button to attempt these jobs again.

Setting the Batch Processing output settings

To configure the output file format for all batch processing, click the Settings button.

RX includes a 4 band parametric equalizer module with adjustable notch filters and low‐pass/high‐pass filters. This can be a handy tool for cutting or boosting certain frequencies, or for quickly removing unwanted low or high frequency sounds like rumble or hiss.

The EQ module comes in two flavors or EQ Types:

Analog EQ is a non‐linear phase filter. The Linear‐phase EQ uses a FIR (finite response) filter.

The analog filter gives a very different (some say “warmer”) character than the linear phase filter which is very precise and designed to minimize phase shifts in audio. You can easily switch between the two filters to hear which best suits your project. As with all the modules in RX, you have the option of Preview and Compare plus the ability to create and save your own settings (presets) in EQ.

Working with Presets Groups of settings in each of RX’s modules and plug‐ins can be saved and recalled as presets. Each module in RX comes with its own set of factory presets that provide a good place to begin. Once you’ve made modifications to the settings in a module, you can name the preset and save it to its own filename. Moreover, you can export your presets to any file folder or storage area on your computer for backup or to share with other RX users.

It may be useful to separate and associate the filename of each preset for export with the type of module. For example, if I’ve found a useful preset for Hum, I’ll name the preset Roosevelt Oval 1940 Side A and export the file as Hum RoosOval1940_A. When saved, RX will add the extension (.pluginpreset). Whatever you name your exported file, RX automatically knows which preset goes with which module.

Keyboard shortcuts and assigning your own

Like presets, RX includes default keyboard shortcuts but you can customize them to your preference. The keyboard shorcuts menu is quite straightforward and like the presets tool, saves time and increases efficiency.

Presets menu ‐ save groups of key assignments with this tool (see paragraph on Presets)

Show commands containing ‐ lets you search by keyword for a command you want to assign to a keystroke

Shortcuts for selected command ‐ shows if there are any keystrokes assigned to the command selected in the above menu

Remove ‐ removes the currently assigned keystroke from a command

Press Shortcut Key ‐ to assign a new keystroke to a command, select the command from the menu, then click in this field and press a key or combination of keys

Assign ‐ assigns the entered keystroke to the current command

Shortcut key currently used by ‐ lists commands to which the current keystroke is assigned

Reset ‐ returns settings to defaults

EXPORTING AND DELIVERING AUDIO

Once you’ve completed the restoration process, you will want to save your file for the next procedure – mastering or perhaps some fine tuning in your favorite editor or DSP application. To save files as a copy or in another format go to File|Save As (Ctrl+Shift+S). You can also select just a portion of your project and export that segment as a new file by going to File|Export Selection (or Ctrl+Shift+E).

This would be a good time to save your original work as a back‐up file, either in a separate folder or an auxiliary storage device such as a flash drive. Having a duplicate copy of your originals is always a smart idea!

Exporting to different formats

RX supports the import of certain audio formats ‐ WAV, Broadcast WAV, AIFF, MP3, WMA and can export to WAV and AIFF with varying types of dither and degrees of bit‐depth or word‐length.

Dithering

Dither is routinely introduced into the processing of digital audio in order to prevent quantization error – an unpleasant audio artifact generated during rounding or truncation of digital values. Put simply, Dithering is used to attain higher quality when exporting to a lower bit depth (for example from 24‐bit audio to 16‐bit for delivery on CD). Your choice of dithering algorithm will depend largely on program content and what you perceive to be the best sounding result. If you’re interested in reading more about the concept of dithering, you can find iZotope’s Dithering Guide at www.izotope.com/support.

To access the Output Options and dithering controls, click on File|Export (or Ctrl+Shift+E).

Check the Preserve non‐audio data box when you export your file if you need to preserve additional data in your audio, such as cue markers or ACID‐ized WAV information.

Dither Modes:

• None (Truncate) ‐ converts to a new bit depth without applying dither, instead truncating or removing the extra bits.

• White Noise (TPDF) ‐ standard quality dithering

iZotope MBIT+™ Dithering Panel (RX Advanced only) The Dither module applies iZotope's MBIT+ dithering technology to improve the quality of your audio files whether you are converting to 24, 20, 16, 12, or 8 bits. MBIT+ uses proprietary noise shaping method for improved quality.

While the basic RX application does have one simple MBIT+ mode, RX Advanced gives you an entire panel for customizing the dithering settings for optimal results.

As with many of the modules in RX, the MBIT+ options menu allows for saving and importing of your own settings as Presets.

New bit depth ‐ This sets the target resolution (bit depth) of the audio file.

Noise shaping ‐ By shaping the dither noise, it is possible to provide more effective and transparent dithering by shifting the dither noise spectrum into the least audible frequency bands. You can control the aggressiveness of this shaping, ranging from None (no shaping) through Ultra (roughly 14 dB of audible noise suppression)

Dither Amount ‐ The dithering amount can be varied from None (noise shaping only) to High. No dithering or Low dither amount can leave some non‐linear quantization distortion or dither noise modulation, while higher settings completely eliminate the non‐linear distortion at the expense of a slightly increased noise floor. In general, the Normal dither amount is a good choice.

Auto‐blanking ‐ This option instructs RX to completely mute dither output (i.e. dither noise) when the input signal is completely silent (0 bits of audio).

Limiting of noise peaks ‐ Dither noise is random in nature and exhibits very low amplitude. However, after noise shaping, especially in aggressive dithering modes, the high‐frequency dither noise is significantly amplified, and the overall dither signal can show spurious peaks up to ‐60 dBfs, (an abbreviation for decibel amplitude levels in digital systems which have a maximum available level). If such high peaks are undesirable, you can enable this option to effectively suppress the spurious peaks in the noise‐shaped dither.

Harmonics suppression – If even slight dithering noise is undesirable, simple truncation remains the only choice. Truncation results in harmonic quantization distortion that adds overtones to the signal and distorts the timbre. In this case you can enable Suppress Harmonics option to slightly alter the truncation rules, moving the harmonic quantization distortion away from overtones of audible frequencies. This option doesn't create any random dithering noise floor. Instead it works more like truncation, but with better tonal quality in the resulting signal. This option is applicable only in the modes without dithering noise and without aggressive noise shaping.

iZotope 64-Bit SRC™ Panel (RX Advanced Only) RX's Resample module allows you to convert an audio file from one sample rate to another. Sample Rate Conversion (SRC) is a necessary process when converting material from one sampling rate (such as studio quality 192kHz) to another rate (such as CD quality 44.1kHz). It is common to record and edit in high sampling rates since higher rates allow higher frequencies to be represented. For example, a 192kHz audio sample can represent frequencies up to 96kHz whereas a 44.1kHz audio sample can only represent frequencies up to 22.05kHz. When reducing the sampling rate, or down‐sampling, it is crucial to remove the frequencies that cannot be represented at the lower sampling rate. Leaving frequencies above this point causes aliasing. Not unlike quantization, aliasing can be heard as the frequencies in an inaudible range are shifted into an audible range causing distortion and noise. With iZotope SRC's users can completely avoid the common aliasing artifacts while maintaining the maximum frequency content.

New Sampling Rate ‐ This setting chooses the sampling rate you want to convert to. Choose a sample rate from the drop down list, or click on the field to type in another sample rate Filter steepness ‐ Controls the steepness of filter cutoff. Steeper filters perform better in frequency domain, i.e. pass more in useful range and suppress aliasing better, but worse in time domain, i.e.have stronger ringing. Ringing of digital filters is smearing of transient energy in processed signals near the Nyquist frequency. It's disputable whether this affects sound quality at all, but some suggest that excessive ringing negatively affects stereo imaging and bass perception. That's why it may be important for some users to decide how precise they really need to be in frequency domain and how much ringing they can tolerate in the time domain. Cutoff shift ‐ Cutoff scaling shifts the filter cutoff frequency up or down to fine‐tune the frequency response of the resampler. Shifting cutoff down suppresses more useful pass‐band signals, but also rejects more aliasing components in the stop‐band. It's possible to shift the cut‐off to completely suppress any aliasing in the stop‐band (by about 180 dB or more). It's

disputable whether a small amount of aliasing near the Nyquist frequency is audible, but some experiments by Bruno Putzeys suggest that it can significantly affect the result. So, this control sets the trade‐off between aliasing suppression and pass‐band range. Pre‐ringing ‐ sets the ratio between pre‐ringing and post‐ringing in the SRC filter and controls its phase response. When pre‐ringing is 1, the filter is linear‐phase, i.e. has equal pre‐ and post‐ringing. When pre‐ringing is 0, the filter is minimum‐phase, i.e.has no pre‐ringing (only post‐ringing), and its phase response is non‐linear across all the frequency range. Intermediate values of pre‐ringing adjusts ratios of pre‐ and post‐ringing, and also linearize the filter's phase response in the pass‐band as much as possible.

SUMMARY Thank you for reading Restoring Audio with iZotope RX. We hope that this guide has given you the background you need to successfully restore audio with a variety of problems. Now that you’ve read the guide, be sure to download our audio examples and move on to the Appendix, where you’ll find descriptions of how to repair real world audio problems using what you’ve just learned from the Guide.

About the Authors In addition to the hard work of iZotope’s development, QA and content development teams, we also wish to extend our thanks to Nat Johnson whose writing, restoration expertise and audio examples contributed greatly to this guide. Nat Johnson While still a music student in Boston, Nat Johnson began a career in radio broadcasting and sound recording when he joined the Concert Network as an announcer and music programmer. During this period, he was offered the opportunity to record a season of live performances for Sarah Caldwell’s Opera Company of Boston. Soon thereafter, Nat joined the staff at WGBH‐TV and Radio and was assigned the unique, triple‐role of announcer, producer and audio engineer and where he became engaged with the recording and mixing of multi‐track recordings, including broadcasts by the Boston Symphony and Pops Orchestras. While in England producing an album for WGBH at the BBC Studios in London, Nat received an invitation from Ray Dolby to tour Dolby Laboratories at Clapham and later, as a producer for RCA Records, continued his associations with Dolby in New York developing Pro‐Logic surround remastering projects for CD. In 1999, while supervising classical and film‐music reissues at BMG/RCA, Johnson initiated a unique audiophile series of high‐end CD’s employing Weiss 24/96 A/D converters and Sonic Solutions’ High Density editing system. In 2000, Nat was nominated for a Grammy as Producer of the 94‐CD Rubinstein Collection for BMG/RCA. In 2004, Nat founded his own company, Rockport Restoration Studios in Rockport, Massachusetts, specializing in the restoration and preservation of early sound recordings. At present, he is heading a project to restore all known extant recordings containing the speeches and White House conversations of President Franklin D. Roosevelt.

Appendix: Repairing the Included Audio Files

Example 1: Removing Broadband Noise from a Concert Recording DETAILS: Player‐organ recording in Boston: Vierne Organ Symphony No. 1‐ Allegro (STEREO). Originally recorded on Ampex 440‐B, ¼” analogue tape, no noise‐reduction used at session.

COMMENTS: “The principal problem was a steady, low‐level background noise resembling tape‐hiss. The challenge was to reduce the noise without sacrificing upper register harmonics during quiet passages in the music. Thus, the settings in Denoiser (particularly threshold) I used were conservative.” ‐Nat TIP: Zooming in to selected quiet passages with the spectrogram, examining the waveform and checking your program aurally while comparing against the original can be a check that your program material isn’t being compromised during processing. For an easy comparison, I’d recommend opening two instances of RX simultaneously – one with the original audio and one with your processed version. TIP: Leaving a little noise behind is preferable to hearing no background sound at all! GOALS:

• Train RX’s Denoiser using a sample of just noise. Then use Denoiser to reduce background hiss and low‐level hum.

• Try not to damage the musical quality of the performance – sometimes leaving some noise behind is better than taking the life out of the recording! You may want to be conservative with the Noise suppression controls, and turn the Smoothing or Musical Noise Reduction controls up to prevent artifacts.

Train Denoiser with the just‐noise signal present before the organ starts playing

Before and After noise reduction

Example 2: Using Denoiser and EQ on a Shellac Recording of Spoken Word DETAILS: This example comes from Eugene Field's poem "Departure,” read by George Broderick. The first numbered record of Eldridge R. Johnson's Consolidated Talking Machine Company, the disc was assigned catalog number A‐1. Recorded November 1900, take 3. Original was a 7” shellac – transferred directly from museum copy. (MONO)

COMMENTS: “This very early disc, manufactured with primitive shellac pressing material, was intolerably noisy and until RX Denoiser became available, there was no known way to reduce this degree of overwhelming background interference without losing some of the voice as well. I used a threshold of only 4.6 on Denoiser which was sufficient to drive down the background grunge and lift up the voice. Preview and Output Noise Only were most useful tools during my experimentation with different settings.” – Nat Johnson Background information on the recording plus the entire RX restoration can be found on‐line at

http://www.gracyk.com/johnson.shtml. GOALS:

• Use Denoiser to get rid of the heavy background noise. Try to improve the intelligibility of the voice without obscuring the poem with noise reduction artifacts.

• Using RX’s EQ will get rid of the high frequency distortion without damaging the voice in the recording. Using a lowpass filter that cuts everything above about 4,000 Hz will do nicely.

The Spectrogram Display before and after Denoiser and EQ are applied

Example 3: Restoring an Historical Speech: Making Voice More Intelligible DETAILS: This file contains the voice of famed philanthropist Andrew Carnegie, recorded In 1914 (Internet source) (MONO) COMMENTS: “Some of the words in this speech, recorded by Carnegie in 1914, are somewhat difficult to decipher owing to a pronounced Scottish burr, made even more muffled by someone’s rudimentary attempt at noise reduction. In addition, there was steady‐state background noise. I began with RX EQ to raise the voice out of the mud using a slight boost roughly between 1K and 9K and then ran Denoiser. I left the EQ compensation in following Denoising as this left the speech a bit clearer.” – Nat Johnson GOALS:

• Use gentle linear phase equalization to lift the voice out of the background noise.

• Use RX Denoiser to reduce steady‐state background noise

Using a tag-team approach of EQ and Denoiser can make “buried” dialog much more

intelligible

Example 4: Cleaning up a Phone Interview with Spectral Repair DETAILS: Interview with Francis Ford Coppola, recorded directly to DAT from phone patch (MONO) COMMENTS: “These problems were caused mainly by mechanical contact with the telephone handset. Prior to the development of RX Spectral Repair, there was no truly successful method of removing long noises (longer than 10ms or so) from on top of program signal without corrupting desired content. The RX Spectral Repair modules, along with the RX spectrogram and zoom controls, allowed extremely precise isolation of each problem. By accurate removal (or reduction) of the disturbance followed by interpolation, spectral repair was able to fix nearly every disturbance in this interview.

“For instance, at 11.62 seconds into the supplied Coppola example where he speaks the words ‘motion picture business,’ there is a loud click. Because the click is over 4000 samples, Declicker won’t work – in either automatic or manual mode. Instead, the noise was successfully removed by using Spectral Repair using Replace.” – Nat Johnson SUMMARY:

• Using RX Spectral Repair’s Replace and Attenuate modes, remove phone handset noise from the interview.

• Using Replace mode, get rid of handset noises.

• Use attenuate to remove low frequency “thumps” left behind.

Each of the phone handset noises is comprised of a vertical spike and a horizontal

Removing the vertical spikes with Replace mode still leaves a thump …

A quick pass with Attenuate mode gets rid of the low frequency sound

Example 5: Removing Clicks and Pops from a Concert on Record DETAILS: Kings College Choir, Cambridge, England. Source: 33 1/3 RPM LP (1965 recording, STEREO) COMMENTS: “I found this long sought‐after disc in a second‐hand record shop and wanted to transfer it to CD. Fortunately, the only problems we encountered were mostly light vinyl clicks and heavy room ambience from inside the massive chapel at Cambridge University. Declicker was employed using the automatic function using High Quality Declicker and one or two spots in the manual mode. Generally, I recommend using High Quality Declicker whenever possible. The rumbling room tone was reduced by Denoiser, again using a threshold of 0.0.” – Nat Johnson GOALS:

• Use RX Declicker in automatic and manual modes to remove clicks and pops

• Use RX Denoiser to remove background noise. Try to retain the musical character of the performance while removing as much of the offending noise as possible

The recording before running Automatic Declicker

Clicks (vertical lines) are now completely repaired

Example 6: Removing Clipping from a Phone Interview DETAILS: This phone interview for an iZotope Podcast with producer Morgan Page was recorded with phone line breakout box and USB audio interface. COMMENTS: This is a classic example of input gain being set too high resulting in nasty clipping. In this example the distorted sections can be clearly heard, but looking at them with the Spectrogram Display and Waveform overlay shows them even more clearly. Begin by selecting a section of clipped audio then hitting Compute below Declipper’s Histogram Display. This helps you see where the clipping is occurring. Set the Threshold control so that the red line is below the clipping in the Histogram (clipping will appear as a horizontal line, usually with no information above it in the display). Hit apply to see the result. GOALS:

• Use RX Declicker in automatic and manual modes to remove clicks and pops

• Use RX Denoiser to remove background noise. Try to retain the character of the voice while removing as much of the offending noise as possible.

Analyze a clipped area with the “Compute” button and Histogram window

After running Declipper, Waveforms show a more natural shape and harmonic distortion disappears

Example 7: Removing Guitar String Squeaks with Spectral Repair DETAILS: This recording by guitarist Jamie Robertson contains a few loud squeaks—the result of sliding one’s hand up and down the fretboard quickly. COMMENTS: In situations like this, it’s always a judgement call. Should you remove all minor blemishes if your tools allow you to? Or should you leave things like these squeaks in the final product to keep the recording’s character intact? Sometimes removing only the most offending events is the best tactic, but we’ll let you decide how far to go! Identify the squeaks using the Spectrogram Display. Using Replace Mode in Spectral Repair, select the squeaks carefully and apply the process. HINT: Because these squeaks often occur right before a new chord or note is played, you may want to set the Before/After weighting control all the way to the left. This will ensure that the repair is made using sustained material from the earlier chord only. GOALS:

• Use RX Spectral Repair’s Replace mode to replace squeaks with natural guitar sustain.

• Use “Before/After Weighting” to use sustained material from the previous chords only when making a repair.

• Aim for a realistic result that will not sound unnatural to the listener.

String squeak selected

String squeak interpolated using material from before the event

Example 8: Filling a Gap in Audio with Spectral Repair DETAILS: This is a sweeping square wave with a .2 second section of missing audio. COMMENTS: RX’s Spectral Repair has a mode called “Partials + Noise” which is suited to fixing sections of missing audio like this one that have changes in pitch. This extreme example demonstrates how RX can re‐synthesize missing sections of audio, including harmonics above a note. GOALS:

• Use RX Spectral Repair’s Partials + Noise mode to fill the gap

• Try to create the most natural sounding repair possible – when the settings are correct, the replaced audio should be audibly perfect

• Experiment with the area you select—this can make a big difference in the results of the processing

Select the gap and use Spectral Repair’s Partials + Noise mode

When settings are applied correctly, Spectral Repair will fill the gap convincingly

izotope with nat johnson - tacsystem.comrestoring audio with izotope rx™ page 6 of 74 ©2008...

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