Exploring the Audacity 2.1 audio editor

Core Competence

Audacity is particularly suitable for editing and cutting individual audio recordings in stereo. Version 2.1 does not attempt to be a multichannel recording studio. The support for multitrack recording is pretty solid in Audacity, but alternatives such as Ardour [7], Qtractor [8], or Bitwig Studio [9] do a far better job. The complexity of a fully equipped music studio is sometimes unnecessary, however. Audacity is particularly useful in radio production. The six to eight tracks used in a well-made radio broadcast with one or two interview partners are easy to achieve with Audacity.

Audacity is ideal for situations with limited time to set up the session. For example, Audacity shows and hides recordings in an unusually convenient and intuitive way with the Envelope Tool on the Tools toolbar. The simple mixer (Figure 6) available in View | Mixer Board might not be very impressive, but it does allow simple and intuitive operation.

Figure 6: Simple but effective: Audacity focuses on basic functions for tools with multitrack mixes.

You certainly notice Audacity's almost 20 years of experience from the tools for audio editing. You can edit quite precisely down to the level of individual samples. Integration is quick: If you know that the mouse wheel turns into a zoom tool when you hold down Ctrl, you have already discovered the most important secret.

Use the pen tool as soon as magnification reaches the point where individual samples appear as dots in the sound wave display. When drawing using this tool, automatic functions prevent you from generating a waveform that Audacity cannot properly process. When editing, the right part of the recording moves up; cut marks help you with editing (see the "Musical Notation" box). Audacity also sets the limits of sections to prevent clicking noises by damaged samples. All editing actions can be undone by pressing Ctrl+Z.

Musical Notation

Audacity lets you add label tracks to mark particularly important passages. You can create these tracks in Tracks | Add New | Label Track and then add content by clicking in the label and entering text. Alternatively, you can import a longer label in one go via File | Import | Labels. Save the label tracks in a file via File | Export Labels (Figure 7). The syntax could hardly be simpler: There are tabs between the positions specified in samples on the timeline and the text on the label; a newline ends a label.

The notes, which by themselves are useful, impress with qualities that are not immediately obvious: For example, a new label track can combine the underlying soundtracks into a group that can be synchronized separately. The label tracks also prove to be a useful tool when digitizing LPs and music cassettes. If you select File | Export Multiple, Audacity displays a wizard in which the labels can be used to split files and name file. Audacity then automatically numbers the individual tracks.

In the label tracks, Audacity distinguishes between labels for points and regions. A point label can be converted into a region label at any time by dragging it to the label symbol's angular brackets. Tracks | Edit Labels opens all the project's label tracks in an editor designed as a simple table. Alternatively, you can export the labels as simple text files, edit them with your favorite editor, and save the file as a new label track.

Figure 7: Marks for inserts, notes, lyrics: The text tracks in Audacity offer numerous practical applications. The texts can be exported as plain text files and edited in an editor.

Note that Audacity basically works destructively. Cutting actions have a direct effect on the audio file, not just on the display. If you cut a section out of a recording, you can only restore it by canceling the previous editing steps, which, in turn, means that all other changes to the corresponding section are lost. The same is true when applying effects: Audacity stores any manipulation in a linear way; the filter settings cannot be reverted after further editing steps.

This design has benefits along with disadvantages. For example, the CPU is only burdened with computing an effect once, whereas non-destructive systems such as Ardour must constantly recompute all the effects in a project. However, non-destructive programs offer a "freeze" solution for this problem: To this end, the software computes the track with your effects and saves the unedited material separately together with the effect settings. This allows the track to be "thawed" as required, and the filter is applied to the new soundtrack with other settings.

Such a function might not fit into Audacity's technical design, but it would certainly be a possible workflow variant: It is, for example, possible to keep a backup of unedited material on hand beyond the borders of the current session in order to undo an effect later.

Conclusions

Audacity has taken a giant leap forward with version 2.1. The program's simplicity requires far fewer compromises than the predecessor 2.0. Real-time support for effects works well. The revolutionary spectral mode for modifications stands out in particular among the changes (see the box titled "Sound Scalpel"), which once again confirms that Audacity is very much recommended as a tool for editing and restoring audio recordings.

Sound Scalpel

Audio editors offer the option of precisely selecting sections of recordings in chronological order and boosting quiet passages or damping louder ones; users apply subtle manipulations using equalizers and filters. But, what if you have a deep humming sound or a nasty crackle between the 14th and 16th seconds in what is an otherwise good recording? You will not want to change the volume for the flaw, but you need a way to eradicate it.

Some tools require very complicated settings and a very sophisticated equalizer to deal with this problem, and undesirable interference can often result. However, the procedure works very well in Audacity: First, switch the display in the drop-down list in the track header on the left from Waveform to Spectrogram. The logarithmic variant Spectrogram log(f) often appears more differentiated and is thus easier to read.

Recognizing the dynamics in the recording may now be more difficult in the spectrogram display, but you do get prominent frequencies displayed as color gradations at each point of the track. The colored stripes represent the intensity of frequencies read from the scale on the left. The selection tool, which only provides vertically continuous stripes in the waveform view, now draws rectangles with settable upper and lower limits. On the right, you will find a small tool that precisely displays the selection frequency in the bottom toolbar.

Paul Licameli, the main developer for this function, is also working on a number of plugins. The names of these three simple, but effective Nyquist modules, reached from the Effect menu, each begin with Spectral edit and provide various filter functions. Spectral edit multi tool [10] fulfills the tasks of a classic filter, which, depending on the selection, executes a high-pass, low-pass, or notch filter.

The filter module determines its settings from your selection in the output material and immediately gets started when accessing items from the effects menu. Selections starting at the top or bottom trigger a high- or low-pass. A selection that starts with 100Hz and ends with 300Hz effectuates the use of a notch filter at 150Hz. The other two modules, Spectral edit parametric EQ [11] and Spectral edit shelves [12] also use this simple and elegant method. These modules also let you adjust the amount of attenuation in a simple interface (Figure 8). These filters are missing in Arch Linux; if necessary, you can download them from the Audacity SVN and copy or link them to /usr/share/audacity/plug-ins/.

Comparable functions in Linux-compatible audio software only appear in the Snd audio editor [13] and Sonic Visualiser [14]. In Snd, such functions can only be accessed via a programming interface, for which you would have to write such operations in Scheme or Ruby yourself. Sonic Visualiser, as the home of the Vamp plugins also used in Audacity, provides additional and more flexible spectrograms; however, operating these features requires more expertise than with Audacity. Additionally, the program proved less stable than Audacity in our lab; Ardour does not yet even provide such functions. You will find more information and a few manuals about dealing with the spectral view in Audacity's online manual [15].

Figure 8: In the spectral view, it is possible not only to select audio material from the timeline but also to select and edit the frequency spectrum.

Further improvements to the new functions are due in future versions of Audacity. The project team is already working on real-time support and graphical interfaces for additional plugin formats such as LV2 and LADSPA. The option of applying spectral mode to all plugins is at the top of my wish list.

The Author

Hartmut Noack works in Hanover as a lecturer, author, and musician. He has always thought that free software and home-made music fit together brilliantly. When not sitting in front of his Linux audio workstation, he's hanging about on web servers. Some of his work licensed under Creative Commons and created using free music software is available for download on his website http://lapoc.de.

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