Intro to Synthesis, Part 6: How to Use LFOs to Modulate Filters, Pitch, and Pulse Width

Our Intro To Synthesis series has already taken us through many of the basics of synthesizers, from the oscillators that generate foundational sounds, the filters that shape and add character to your tone, and the envelopes that can be applied to your synth’s amplifier or filters.

Now, in the sixth and last episode of our first season, we'll learn how to use modulation to affect other parts of the synth. Reverb’s Justin DeLay begins with a basic definition what modulation is: the use of "signals generated within your synthesizer to change or shape various aspects of the sound that you’re producing over time."

The process often starts with an LFO, or a low-frequency oscillator. Like any other kind of oscillator, an LFO is generating a constant signal, but instead of sending it out to a speaker, we’re keeping its waveforms inside the synth, using it as a control signal that will command the audible signal.

One way to use an LFO is to set it to generate a sine wave, the simplest waveform, that will then modify a filter. For example, if you raise the LFO's frequency, the filter will open and close faster as it travels along the sine wave.

A standard control setup that can be found on many synths, including Justin’s Korg Mono/Poly, will have the LFO connected to a mod wheel, which allows you to increase or decrease how much the LFO is influencing the filter cutoff.

Korg Mono/Poly Analog Synthesizer

LFOs, like other oscillators, are capable of producing various kinds of waveforms, such as ramp-down saw waves, ramp-up saw waves, and square waves, which will all modulate a filter, pitch, or pulse width differently.

While a sine wave at a slow speed is great to add a little bit of movement over time to your sound, a square wave at a faster rate can emulate the sound of a sequenced bassline or even create a kind of delay effect on the tail. But you can experiment with any waveform at any speed.

Another classic use that Justin demonstrates is an emulation of pitch vibrato—starting with a pure note and adding vibrato at the end, like a guitarist during a solo or a singer at the end of a line. In this video, Justin starts with a sine wave LFO with a relatively quick speed and then uses the mod wheel to increase the effect at the end.

You can also use an LFO to create pulse-width modulation, which will change vary the shape of a square wave. Various LFO waveforms here will affect the square wave differently. A sine wave LFO at medium speed will replicate something like a chorus effect, especially on higher-register sounds. A slower speed will just add a subtle interesting timbre.


We’ll have new seasons of Intro To Synthesis in the future, but until then, be sure to watch all of the first season’s previous videos below.


Full Intro to Synthesis Series

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