Written 1999/03/03
When two waveforms of equal and opposite amplitude meet, the result is a cancellation of both waveforms. Silence. Okay, I know what you're thinking. "How is that useful in making synthetic sounds?" A certain amount of cancellation can yield some interesting results.
For the purposes of this article, let's assume you have a sampler at your disposal (wavetable synths, alas, seldom have the option of inverting waveforms and thus will be excluded). Now suppose you have isolated a sample that can sustain for a long time. It can either be just a really long sample or a loop. Create a copy of this individual waveform that will play along with the original. If you play any key, you will basically hear the original sound except very loud. This is because the copy waveform is reinforcing the amplitude of the original. This is the opposite of what we want for our little experiment.
This is when you need to invert the copied waveform. This means the positive amplitudes will be turned negative and vice versa. If your sampler doesn't have such a feature, many audio editing programs for computers can perform this task, such as Syntrillium's Cool Edit or Creative's WaveStudio. Hopefully you have some way to convert or transfer the sounds back and forth from your computer to your sampler. There are sample-conversion programs for many brands, or if you've got the same format of digital input/output on both machines you're set. Now, whether you simply found the option on your sampler or you had to resort to the alternate kinds of trickery and witchcraft described above, you should have an inverted copy of the original waveform. And after all the fuss I've put you through so far you're probably itching to hear something cool. But before you discover that the steps thus far only produce total silence, and you swear I will feel your wrath for this, let me just say we're not quite finished yet.
The last step is modulation. Any difference in pitch, pan, or volume to either or both of the waveforms will undo the cancellation to various degrees. For example, try detuning one of the waveforms. You should hear sort of a phasing effect coupled with a slow attack. You could also set an envelope to change the volume of one wave, or pan the waves slowly in opposite directions (note that the panning method only works in stereo!). Maybe put on a subtle pitch LFO. Any modulations you can think of that will offset the two waveforms in time, amplitude, or stereo space will work.
Using inverted waveforms can make for great pad sounds, unique phasing-type effects, or breathe life into once sterile single-cycle loops. As usual, play with those parameters and see what magic you can work.
