Basic Audio Editing

Last updated: 22 January 2009
Published in: Digitising analogue media | Creating new digital media |
Tags: audio | audio editing |

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Summary

This document addresses some basic techniques used when editing digital audio within a DAE (Digital Audio Editor). It assumes little or no previous knowledge and experience of audio editing and offers advice on how and when to use common tools within the editing environment

Introduction

Editing audio files offers a multitude of possibilities for manipulating sound. From getting rid of unwanted noises to sonically reshaping a waveform beyond any recognisable form to the original, editing can be a very powerful tool. In the world of commercial audio, you can guarantee that any radio program you listen to, or and music CD you hear will more than likely have been extensively edited. Beginning to edit audio files can be a very daunting task and it is worth taking time over as you will find that you improve with practice. You may also find that some editing techniques are more difficult to execute correctly than others, and on occasion a waveform will ‘look' like its been edited correctly but will still ‘sound' problematic. In these cases the best advice is to take a short break and come back to your work later. This helps rest your ears and give you a fresh perspective in time.

The editing window will display your selected waveform and allow you to zoom in and out for reference and to allow you to see the waveform in microscopic detail. A selection of editing tools will be made available to you which will vary in sophistication depending on the level of software you are using. Below is a list of the more common tools you will come across within DAEs.

Common editing tools

Should I ‘top and tail' my audio files?

A technique common to all editing projects is ‘topping and tailing' audio files. Put simply, this is the process of removing unwanted silence or noise before and after the desired audio section within a file. It is considered good practice to top and tail all your audio files. Even when archiving files, there may be a period of time of capture before or after the analogue file was played or stopped which would need editing out.

Diagram 1

a)

b)

Diagram 1b is the resultant region after the silence at the beginning and end of the region shown in 1a has been removed.

This method of removing unwanted noise and silence should be also be applied to the overall track within a project. Consideration may need to be given if you plan to deliver multiple tracks on a medium where they are to be heard directly one after another, as often a short silence in between tracks creates a natural effect. It is therefore good practice to leave any space at the end of one track instead of at the beginning of another. This is because if the listener wishes to access a track directly through an mp3 player menu, for example, silence at the beginning of a track is particularly undesirable and the effect of a break in sound is transformed into initial silence.

Cutting audio files at random positions can lead to unwanted audible clicks. See the section below on how to avoid this effect.

There are audible clicks where I have made cuts to my audio files. Why is this?

When you cut, or split audio files, the initial amplitude of the new waveform is not user defined by depends on the wave at the position of the cut. A wave starting in a state other than equilibrium is unnatural and often results in a ‘pop' or ‘click'. To avoid this, try to split the audio file as the wave crosses zero amplitude on the Y (horizontal) axis. Some DAEs have a built in feature which ca be turned on which automatically moves your cut to the nearest zero crossing to avoid this unwanted effect.

Diagram 2

The dark grey region highlights an area of the wave which begins and ends on zero amplitude, Y=0.

Another way to eliminate clicks at the beginning or end of audio files is to use fades. Read the section further in this document for more information on using fades effectively.

How can I remove unwanted noises from my audio files?

Imagine the situation. You are editing the recording of some spoken word and every now and then the microphone has picked up the sound of someone coughing, which you wish to remove. This occurs:

A. Inbetween times when the principal is speaking and...

B. At the same time as the principal is speaking.

Using the cutting tool to isolate and remove regions of unwanted audio, or applying silence over a specific time will, on the whole, rectify the problem raised in scenario A.

Scenario B, on the other hand, is more of a complex issue to resolve.

It is important to remember that attempting to change any one of a multiple of sounds that occur simultaneously in an audio recording, will directly affect the sounds that you are trying to preserve. Only with highly sophisticated software is it possible to have complete control over ‘sounds within sounds'.
However it is possible to minimise the effects of these unwanted sounds and accurate editing can certainly help. In this situation it is obvious that cutting the entire sound of the cough from the waveform will eliminate the voice of the lecture at that time which will result in a strange audible effect. If you were to locate and remove the initial transient of the cough (which is possibly the loudest part), less overall information will be removed and the loudest part of the noise will be silenced.

If the interrupting noise is a short impulse such as a click or bang, using the zoom tool to locate these impulses at a high resolution as possible is highly recommended. You will find that short impulses such as these can often be cut and removed without having an audible effect on the resultant sound, provided it has been repaired correctly (see next section).

Diagram 3

a)

b)

c)

Diagram 3a shows a waveform with an audible ‘click' to be removed. Diagram 3b shows the ‘click' zoomed in closely and indicates a region in dark grey which has been highlighted to begin and end at zero amplitude crossings. The wave in 3c shows us the original waveform once the region highlighted in 3b has been deleted.

You may find there are other types of undesirable noise in your audio files you wish to remove. Noise such as a hum from mains power, an electrical device, background noise from the street or environment, or digital interference, which you may not have noticed during recording, may be audible upon playback. Noise at a constant frequency or of a constant band of frequencies can be eliminated using a technique called filtering to filter out unwanted frequencies. Most DAEs come with built in filters for this purpose in the form of a plug-in. Further information on filtering can be found in the advice document Audio Processing techniques.

I have cut/split my audio files. How do I join two regions together and make it sound natural?

Creating a composite audio file that fools the listener into thinking they are listening to a continuous single file is a key challenge when editing. Depending on the differences of the files, this can be relatively simple or extremely difficult. Here are some aspects to consider when you are joining audio files together.

• Create zero-crossings. As previously mentioned, cutting regions at zero crossing will start the new file from silence instead of at a point within the sound.
• Use cross-fading. When overlaying beginning and ends of regions or placing one directly after the other a useful thing to do is to apply a cross-fade across the two files. A cross-fade fade-outs the volume of a file at the same time as fading in a second, the subtle effect of which sounds less harsh on the ears than one file stopping and another starting.
• Leave a natural gap in-between regions. Listen to the files in position and adjust according to what is sounds like, not what it looks like on the screen. Musical files need to keep to the tempo or feel of the piece and care should be taken when working on spoken work, allowing gaps where a breath might take place for example. 
• Adjust relative volumes. It may be necessary to adjust the volume of one region to match that of another. Although this can be done by visual comparison it is imperative to listen to any adjustments you make and base further decisions on the audible results.

Why should I use fades?

As discussed in the previous section, fades can be useful for restoring a natural blending effect to the otherwise harsh sounds of putting two separate sounds one after the other. A ‘fade' is a general term which means the audible change in volume from silence to a reference point, or vice-versa.

Fade-ins and fade-outs can be useful at the beginning and end of tracks and are often used in music to slowly introduce the listener to a song or to gradually end a song without the music actually finishing. Adding music and sound effects to spoken word projects can often benefit from being faded in or out so as not to distract the listener from what is being said. As with all these editing techniques, it is best to listen to the effects as you work and conduct changes based on the audible results and not necessarily what is displayed on the screen.

Diagram 4

a)

b)

4a displays two regions next to each other in an audio editor. 4b shows the same two regions once a cross-fade has been applied. The white lines indicate the volume levels. It can be seen that as one volume rises second falls, relative to the first ones rise.

"Audacity(R) software is copyright (c) 1999-2008 Audacity Team.
Web site: http://audacity.sourceforge.net/. [It is free software
distributed under the terms of the GNU General Public License.]
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Last updated: 22 January 2009
Published in: Digitising analogue media | Creating new digital media |
Tags: audio | audio editing |

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