XO Wave: Waveform Display

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(b)

(c)

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Figure 1. Several waveforms and overviews from a segment of "Secret Heart" from Let It Die, written by Ron Sexsmith and performed by Feist. (a) shows the waveform overview zoomed out. Some peaks and troughs may not be visible at this zoom level. (b) shows the waveform overview zoomed in. The slightly blocky appearance signifies we have reached the resolution limit of the overview. (c) shows the waveform zoomed out. At this level, some peaks and troughs in the waveform may not be visible. (d) shows the waveform zoomed in. A great deal of detail is visible at this resolution.

Waveforms are graphical representations of audio. They are useful for sound editing because different sections of audio can be visually identified quickly and efficiently -- especially once you have a little experience working with waveforms. Before computers, recordings could be edited by painstakingly splicing tape together. This was difficult not only because of the labor involved, but also because everything had to be done by ear alone. Additionally, fixing mistakes in spliced tape could be expensive or impossible.

Today, software such as XO Wave displays high-resolution graphical representations of the audio, easily distinguishing beats, loud and quiet sections, and even parts of notes and speech, greatly aiding in the editing process.

Sound is fundamentally our perception of tiny fluctuations in air pressure, so it can be visualized as waveforms by graphing air pressure over time. Sounds usually cause the pressure to fluctuate above and below some average pressure. In general, waveforms from louder sounds extend further from the middle than softer sounds, because they cause more extreme changes in air pressure. Sounds with high frequencies change quickly, often exhibiting a jagged or rough appearance, while sounds with lower frequencies exhibit slower shifts up and down. Sounds with both high and low frequencies can often be identified as having both frequent rough edges and longer swings above and below the middle of the graph.

XO Wave has an advanced and flexible method of displaying audio waveforms. The general approach is similar to the approach taken by many other audio software packages, although XO Wave gives you greater flexibility. Most of the time, you won't need to understand the details, but if you want to know exactly how XO Wave displays waveforms, or if you want to change the way waveforms are displayed, this document will help.

How Audio Waveforms and Overviews Are Displayed

By default, waveforms are displayed when you are zoomed in and "waveform overviews" are displayed when you are zoomed out. The waveform overview, or simply "overview", is a low resolution approximation of the true waveform which is computed when you record or import a file (or when you select Rebuild All Overviews from the Region Bin menu). Overviews are saved in the session's AudioFiles directory, with the .ovrv extension. Because they are pre-computed, overviews allow for very fast display of audio data when zoomed out; however, because overviews do not contain complete information, it is important to remember that they are only an approximation of the real audio waveform.

By default, overviews are displayed as filled-in graphs, as shown in Figure 1a. As you zoom in, the overview acquires a blockier appearance, as shown in Figure 1b, indicating the resolution limit of the overview has been reached. Zooming in further causes the overview to be replaced by a waveform display as shown in Figure 1c. By default, waveforms are displayed as lines, which easily distinguishes them from the filled-in shapes of the overviews. Note that until you zoom even further in, such as in Figure 1d, the waveform may skip over some peaks and troughs in order to speed drawing. If this is a problem in your scenario, you may change the zoom level at which waveforms are used in lieu of overviews, to see the true waveforms sooner when zooming in.

Changing Waveform and Overview Display Options

The Preferences window allows you to change the way waveforms and overviews are displayed in XO Wave's Edit window. To adjust these settings, select XO Wave:Preferences... on Mac OS X or Windows:Preferences... on Linux. Then select the Edit tab, as shown.

The Preferences window provides several options, the results of which are illustrated in Figure 2, below.

• Draw Rectified Overview: Turning this feature on causes overviews to be displayed as a graph ascending from the bottom, as shown in Figure 2b, rather than a graph ascending and descending from the middle (Figure 2a). Some engineers regard this as superior because it effectively uses less screen-space and provides an additional visual cue that the overview and not the waveform is being displayed. However, most engineers find rectified overviews awkward to work with.
• Draw Rectified Waveform: Turning this feature on causes waveforms to be displayed as a graph ascending from the bottom, rather than a graph ascending and descending from the middle. However, unlike overviews, waveforms are not symmetrical about the middle of the graph, and so this view necessarily discards some information. You can get that information back by activating Use Inv Waveform Color, described below, to achieve something more like Figure 2f.
• Fill Overview: Turning this off causes the overviews graphs not to be filled, as shown in Figure 2c. On some platforms with slower graphics cards, this has been found to improve drawing speed slightly.
• Fill Waveform: Turning this on causes the waveform graphs to be filled in, as shown in Figure 2e. Some people find this easier to view, while others prefer to retain the obvious distinction between waveforms and overviews.
• Minimum Edit Length: Controls the number of pixels the mouse must move during an edit for XO Wave to consider an edit to have taken place. This setting provides a slight "snap" to the cursor's original location while editing. It may be useful to set this higher if you have shaky hands, a poor mouse, or a laptop trackpad.
• Show Region Name in Region: Allows you to show or hide the region's name in the Edit window.
• Show File Name in Region: Allows you to show or hide the region's source file name in the Edit window.
• Use Inv Waveform Color: Turning this on causes XO Wave to display the negative section of a waveform using a different color (specified in the Preferences window under the Color tab). This is most useful in conjunction with Draw Rectified Waveform, where it can be used to distinguish positive and negative waveform sections as shown in Figure 2f.
• Zoom Mag: Controls the zoom level at which XO Wave's display changes from overview to waveform. This is useful in case you want to avoid blocky overviews or you want to make sure that no peaks or troughs are missed when viewing a waveform.
  • .5: Use this setting if you want to guarantee that waveforms are never shown unless all peaks and troughs can be represented.
  • 1: Use this setting if you want a large degree of certainty that waveform displays will show all peaks and troughs.
  • 10: This setting provides a balance between fast & smooth and accurate display, favoring accuracy a bit more than the default setting.
  • 25: The default settings gives a balance between fast & smooth and accurate display.
  • 100: Use this setting to avoid seeing blocky overviews.

a) default overviews	b) rectified overviews	c) un-filled overviews
d) default waveforms	e) filled waveforms	f) filled, rectified, colored waveforms
Figure 2. A few different ways of viewing waveforms and overviews in XO Wave. (a) shows the default overviews. (b) shows overviews rectified. (c) shows non-filled overviews which draw slightly faster on some platforms. (d) shows default waveforms. e) shows waveform filled. (f) shows waveforms filled, rectified, and using the inverse waveform color feature.