CHAPTER 6
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Applying Stereo, Pitch, and Delay Effects
Using the Convolution effect
The Convolution effect multiplies every sample in one wave (the impulse) by the samples
contained in another waveform. (An “impulse” is the data by which every other sample in a
waveform is multiplied. For instance, if the impulse is a single sample of a full volume “click”
sound then the convolution of that impulse with any audio data is just that audio data itself.
If that click is at half volume, then the convolution is the audio data at half volume.) In a
sense, this effect uses one waveform to “model” the sound of another waveform. The result
can be that of filtering, echoing, phase shifting, or any combination of these effects. That is,
any filtered version of a waveform can be echoed at any delay, any number of times.
For example, modeling someone saying “Hey” with a drum track (short, full-spectrum
sounds such as snares work best) results in the drums saying “Hey” each time they are hit.
You can build impulses from scratch by specifying how to filter the audio and what delay
rate to apply, or by copying audio directly from a waveform.
With the proper impulses, you can simulate any reverberant space. For example, if you have
an impulse of your favorite cathedral, and you convolute it with any mono audio (for which
the left and right channels are the same), the result sounds as if that audio were played in that
cathedral. You can generate such an impulse by going to the cathedral, standing where you
want the audio to seem to be coming from, generating a loud impulsive noise (like a “snap”
or “click”), and recording the noise in stereo. If you use this recording as an impulse, convo-
lution with it sounds as if the listener is at the exact location of the recording equipment, and
the convoluted audio is at the location of the snap or click.
If several ticks descend in amplitude over time, such as one tick every 100 milliseconds,
with each tick half as loud as the previous one, then the resulting convolution with audio
has 100 milliseconds between each echo, and each echo is half as loud as the previous one.
To get a feel for Convolution, open and play with some of the sample Impulse (.imp) files
that come with Adobe Audition. You can find them in the Imps folder within the folder for
Adobe Audition and on the Adobe Audition CD.
Use convolution to sustain a sound for any length of time. For example, the sound of a
person singing “aaaaaah” for one second can be turned into thousands of people singing
“aaaaaah” for any length of time by using dynamically expanded white noise. Also, to send
any portion of an unprocessed “dry” signal back out, simply add a full spectrum echo at
0 milliseconds. The Left and Right volume percentages are the resulting volume of the dry
signal in the left and right channels.
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