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Square One: Catch a Wave

1/1/2009

When you look at a graphic waveform display, you're seeing a frozen snapshot of a sound. Those squiggly shapes aren't just eye candy: if you understand them, they will help you spot problems and make mixing decisions.

FIG. 1: This screen shot shows a waveform display in Steinberg WaveLab. The blue shapes are the audio data. From the sharp peaks and rapid decays, experienced computer musicians can tell that this is a drum loop.

FIG. 1: This screen shot shows a waveform display in Steinberg WaveLab. The blue shapes are the audio data. From the sharp peaks and rapid decays, experienced computer musicians can tell that this is a drum loop.

When you look at a waveform display, it's like seeing the sound waves themselves. A display like the one shown in Fig. 1 is a two-dimensional graph that shows increments of time along the horizontal axis, or x-axis, and the sound's amplitude on the vertical axis, or y-axis. In general, the greater the waveform's amplitude, the more space it will occupy on the vertical axis. If you zoom in, a few milliseconds of audio data will fill the display and you might not see the full height (amplitude) of the waveform. If you zoom out, you'll see less detail but will get an overview of the entire waveform over many minutes. By viewing a whole song, for instance, you might be able to see a level dip in one section that you might not have noticed while listening. On the other hand, you might barely see the waveform of a hi-hat sound — even though the sound is plainly audible — but by zooming in, you can clearly view it.

When the waveform is zoomed out, the software doesn't display all of the detail in the data. Instead, it finds and displays the largest peaks. For faster operation, the peak profile of a wave file may be saved automatically in a much smaller file on your hard drive. The small file can be loaded quickly each time the file needs to be displayed.

Waveform Displays Rule

Waveform displays usually have a vertical ruler at the left edge and a horizontal ruler across the top. If you look closely at the vertical ruler, you'll see that zero is in the middle, not at the bottom. This zero-crossing is where the air pressure goes from negative (below normal) to positive (above normal). The waveform should be vertically centered on the zero-crossing; if the waveform is mostly above or below the zero line, either your audio system has introduced a direct-current (DC) offset or your audio software is producing one. This is a problem that you need to track down and fix.

Most waveform editors let you display the rulers in a variety of formats. You may be able to view the number of seconds (absolute time), the number of individual samples, or the metrical values (bars and beats) on the horizontal ruler. Setting the horizontal ruler to bars and beats can be very useful when you need to edit the audio so that it will match the tempo and phrasing of a song. The vertical ruler may show the amplitude in decibels (dB), as raw data (the actual content of the sample words), or as a percentage of the greatest amplitude the software can handle.

Hitting the High Spots

Here are some simple problems you can identify by looking at a waveform display:

Clipping

A waveform's shape reflects the sound's frequency content. If a signal's level is too “hot” for any stage of the signal path to handle, then the peaks (the parts of the signal where the amplitude is highest) are cut off — a form of distortion called clipping. In a waveform display, you can easily recognize clipped waveforms because they have flat tops. To eliminate clipping, figure out which stage of the signal path is overloaded and reduce the level at that stage, then rerecord the sound.

Inadequate level

If the loudest peaks in the signal use only a fraction of the display's vertical space, the overall level is probably too low and you're more likely to hear noise that would be masked by a hotter signal. Instead of rerecording the track, you might be able to normalize the signal. Normalizing raises the level of the highest peak to the maximum it can be before clipping, and then raises the rest of the signal by an equal amount.

A good audio-editor program will have many other tools for editing the wave data. After using some of the available commands, inspect the waveform display to see the results as you listen to the edited sound. You'll quickly gain an understanding of what you're seeing in the display.


Jim Aikin plays music and writes about music technology. You can visit him online at musicwords.net.

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