Amplitude modulation is the process of changing the volume of one wave according to the pattern of another wave (Volume is used here as a simple way to understand amplitude. The two are not exactly the same in practice.)
Changing the volume of a sound results in a tremolo effect when that change happens relatively slowly. When we speed up that change, we no longer perceive a simple change in volume. That periodic change itself becomes an audible pitch. In fact, when two simple sine waves are combined in this way, two audible pitches appear. These pitches are called “sidebands” and their frequencies turn out to be equal to the sum and difference of the two original waves.
For example, a sine wave ad 150 Hz modulating another sine wave at 100 Hz produces a sideband at 250 Hz (150 + 100) and another sideband at 50 Hz (150 -100).
That’s great, but what does it mean for synthesizing sounds? In the podcast we play a few samples so you can hear the effect. You’ll notice the sound has a very distinct character which may not be appropriate a lot of times. However, when you need something with a little extra abstract flavor, try some amplitude modulation.
Okay, so you’ve seen both terms: amplitude modulation (AM) and ring modulation. Amplitude modulation keeps one of the two original fundamental waves while ring modulation doesn’t. So, ring modulation may sound more hollow and strange. If your synth only has one or the other, you may be able trick it by manually mixing in (or mixing out) the original fundamental.
Many classic synths offered either amplitude modulation or ring modulation on board. The ARP 2600 and Yamaha CS-80 both offered them, and the Arturia V Collection includes these features. The Ensoniq ESQ-1 offers amplitude modulation, and the Korg microKorg and MS-20 offer ring modulation. These are just a few.
If you’re looking for ring modulation outside of the synth itself, take a look at the DOD Gonkulator Ring Modulator effect pedal.
But if you’re truly adventurous, try building one yourself. Refer to the Delptronics Ring Modulator as a guide.