DIGITAL AUDIO

Digital Audio is often thought of as a part of MIDI, but generally this refers to the actual recording and editing of sound waves in digital form. The most common wave formats for this recording is AIFF (Audio Industry File Format) and WAVE files. Both formats are compatible with OSX and Windows Platforms. The addition of acceptable compression formats has made sound files and music easily exchanged on the Internet and the Wide World Web. The most popular compression format is mp3.

Digital Recording: Audacity

Introduction

We will explore the world of Digital Audio by using Audacity which is a free application that may be used on all formats.. You may download Audacity at: http://audacity.sourceforge.net/.

You should also download a plugin called Lame from this same website. Lame is used by several sound recording applications to convert wave and aif soundfiles to mp3 files. Once you have installed Audacity place the Lame plugin into the Audacity Folder. The first time you try to export an mp3 file, you will be asked to locate this plugin. Browse until you find it and click on it. You won't need to do this again. Audacity can now export mp3 files,

This is an Introduction to Audacity. Complete Tutorials are available on the Audacity website. Audacity is a program that digitizes acoustic sounds. It operates like a tape recorder, except that you are recording onto a hard disk. The sampling rate will effect how much disk space you will need to record a selection. The higher the sampling rate, the more disk space is needed. Sound is recorded either with a microphone, from other sound sources such as a CD, a cassette tape, or other such sources. You may need to select sound sources (microphone, line-in, etc.) from your computer's System Preferences or Sound Control Panel You can also access the sound input sources directly from Audacity's Preferences. In the Menu Portion of Audacity, you will find a small window for selecting a source. On the Mac, it will indicate the "Default Input Source" which has been selected from the Sound Preferences, but on Windowns you will find the sources are directly selectable.

You might begin by opening a track from a CD. To do this you could select an aif file from iTunes. iTunes is especially helpful in importing soundfiles from CDs and for managing and converting soundfiles. When you select an aif file to open, Audacity will begin importing it:

You will see a picture of the sound. You will use this picture to edit the sound. In this case it will show a stereo file. The left and right tracks have been linked as a stereo file. We can use the editing features of our operating system to select, cut, copy, and paste.

Digital Audio Effects

Digital Audio programs contain many effects such as those used in recording studios (echo, delay, filters, flangers, equalizers, etc.). Audacity has a number of effects. In order to apply an effect, you must select all or a portion of the sound. Select the effect and it will be processed. Then listen to the effect by playing it. If you don't like it, select "undo" from the edit menu. After launching Audacity, you will see a menu at the top for the commands and processes. You must select a portion or all of the soundfile for the effects to become available.

For our first edit, let's select everything after two minutes and delete or cut it so that we have only the opening two minutes remaining:

The Effects Commands are comprised of traditional sound processing treatments such as reverberation, echo, delay, flanging, etc. You must have a portion or all of the music selected in order to apply any of the effects. You should play around with the effects to see how the sounds are affected. You can choose "Undo" for effects that you don't like. Don't forget that you can use the traditional editing tools as well to affect sounds such as cutting and pasting, as well as copying and pasting from other files and older materials.

Do fade in and fade out effects for this opening section. Select aboujt the firwst 15 seconds and then go Effect Menu. Select Fade In:

Select the last 15 seconds or so, and perform a Fade Out from the Effect Menu. Your file will now look like this:

Here is a brief description of the Effect Menu effects from the Audacity Manual..

Repeat Last Effect - selecting this command is a shortcut to applying the most recent effect with the same settings. This is a convenient way to quickly apply the same effect to many different parts of a file.

Amplify - changes the volume of the selected audio. If you click the "Don't allow clipping" checkbox, it won't let you amplify so much that the audio ends up beyond the range of the waveform.
BassBoost - enhances the bass frequencies
Change Pitch - changes the pitch/frequency of the selected audio without changing the tempo. When you open the dialog, the starting frequency is set to Audacity's best guess as to the frequency of the selection. This works well for recordings of singing or musical instruments without background noise. You can specify the pitch change in one of four different ways: musical note, semitones, frequency, or percent change.
Change Speed - changes the speed of the audio by resampling. Making the speed higher will also increase the pitch, and vice versa. This will change the length of the selection.
Change Tempo - changes the tempo (speed) of the audio without changing the pitch. This will change the length of the selection.
Compressor - compresses the dynamic range of the selection so that the loud parts are softer while keeping the volume of the soft parts the same. You can optionally apply gain, resulting in the entire piece having higher perceived volume.
Echo - very simple effect that repeats the selection with a decay, sounding like a series of echos. This effect does not change the length of the selection, so you may want to add silence to the end of the track before applying it (using the Generate Menu).
Equalization - Boost or reduce arbitrary frequencies. You can select one of a number of different curves designed to equalize the sound of some popular record players, or draw your own curve.
Fade In - fades the selection in linearly
Fade Out - fades the selection out linearly
FFT Filter - similar to Equalization, lets you enhance or reduce arbitrary frequencies. The curve here uses a linear scale for frequency.
Invert - Flips the waveform vertically, reversing its phase.
Noise Removal - This effect lets you clean up noise from a recording. First, select a small piece of audio that is silent except for the noise, select "Noise Removal", and click on the "Get Noise Profile" button. Then select all of the audio you want filtered select "Noise Removal" again, and click the "Remove Noise" button. You can experiment with the slider to try to remove more or less noise. It is normal for Noise Removal to result in some distortion. It works best when the audio signal is much louder than the noise.
Normalize - allows you to correct for DC offset (a vertical displacement of the track) and/or amplify such that the maximum amplitude is a fixed amount, -3 dB. It's useful to normalize all of your tracks before mixing.
Nyquist Prompt - for advanced users only. Allows you to express arbitrary transormations using a powerful functional programming language. See the Nyquist section of the Audacity website for more information.
Phaser - the name "Phaser" comes from "Phase Shifter", because it works by combining phase-shifted signals with the original signal. The movement of the phase-shifted signals is controlled using a Low Frequency Oscillator (LFO).
Repeat - repeats the selection a certain number of times. This operation is quite fast and space-efficient, so it is practical to use it to create nearly-infinite loops.
Reverse - This effect reverses the selected audio temporally; after the effect the end of the audio will be heard first and the beginning last.
Wahwah - uses a moving bandpass filter to create its sound. A low frequency oscillator (LFO) is used to control the movement of the filter throughout the frequency spectrum. Adjusts the phase of the left and right channels when given a stereo selection, so that the effect seems to travel across the speakers.

Any items which appear after these built-in effects are VST, Ladspa, or Nyquist plug-ins. It is possible for a poorly written plug-in to crash Audacity, so always save your work before using a plug-in effect.

When recording, make sure the meters for sound level are active . If you have selected "Play Through" in the recording options, you should see this green light reflect the audio levels of the sound. In general, try to have the green light reflect as a high a recording level as you can without going into the red (red indicates distortion or clipping).

You record by clicking on the red record button on the Control Palette. The Control Palette is above the recording area and has the traditional transport controls along with sound source and record and playback nmetters for sound levels, When you have finished the selection click on the square stop button. You can play the selection back by pressing the space bar or clicking on "play button " on the Control Palette. An indicator at the bottom of the Audacity window will show you how many minutes of recording time you have and how much is left as you record.

One you have recorded your first track, you can continue adding tracks from the Project Menu. As you add them, you will find a panning control on each track that lets you locate the sound of the track in relation to right and left speakers.

Once recorded, there is much you can do to edit the sound. The editing is done with the effects and traditional editing devices such as deleting and copying material.

Once you have made a recording, use "Save As," and save it as an AIFF file with a name in your folder. I recommend that you adopt a suffix of "aiff" so that you know the file type. Always select the file type when saving and be sure your file name matches what you have selected (for example, "somalia.aiff" reminds you of the format you have used to save the song.)

The process described here is generally the same process you will use with virtually any digital audio program. Digital Audio Recording on the the computer is very much the same, regardless of platform. Our comparable program in the Windows Operating System is Sound Forge. The programs are quite similar even though the visual look of the programs are slightly different. However, a major difference in the programs is that SoundEdit 16 is a multi-track program while Sound Forge is a stereo recording and editing program limited to two channels.

Copyright by Prof. John V. Gilbert

Send feedback or questions to john.gilbert@nyu.edu