Among musical instruments, synthesizers are wholly unique in that they are a platform to create your own instrument rather than being instruments in and of themselves. Sure, many come with loads of fun presets that could provide a lifetime of sounds. But the real appeal is when you break them down to their component sections (OSC, VCF, ADSR, LFO, etc.) and build up your own sounds through experimentation and design.
For classic subtractive synthesizers like the Moog Subsequent 25 or ARP Odyssey, every patch starts with one or more oscillators, which are then shaped by filters and modulators to create a seemingly endless array of sounds.
In the first of a four-part series, we’ll take a look at one of the most important modulators — the low-frequency oscillator (LFO) — and some simple ways it can be used when designing patches.
So, what is an LFO? Like the name implies, it’s an oscillator, but it operates below the audible spectrum (i.e., below 20Hz). LFOs can be used to modulate parameters on your synthesizer, such as an oscillator’s pitch, the cutoff frequency of a voltage-controlled filter, and pulse-width modulation. We’ll take a look at those three uses, but, depending on your synthesizer’s capabilities, an LFO can be assigned to affect numerous parameters.
For a more in-depth look, check out this video, “Choosing LFO Shapes and Parameters,” from Sweetwater’s resident synth guru Daniel Fisher’s Synth Clips series.
If this is already a little over your head, then think about it like this: by adjusting a knob or slider on your synth, whether it’s a pitch knob, filter cutoff, or anything else, your hand is modifying the sound. Alternatively, you can assign an LFO to that parameter, and then the LFO adjusts the knob for you in a repeatable and predictable way. After all, to modulate just means to modify or control something.
So, let’s explore some real-world examples of using an LFO to modulate parameters and create more interesting sounds!
Note: Audio samples were made with the Behringer Odyssey, the Softube Tube Delay plug-in, and the Softube TSAR-1R Reverb plug-in.
Modulating Pitch
One of the most common applications of an LFO when creating a patch is to modulate pitch to generate a subtle vibrato. This is particularly useful for leads and patches that emulate strings or woodwinds.
In our first example, we designed a very simple flute-like sound by using a single oscillator set to a square wave that’s using a cutoff filter to remove some of the high frequencies for a smoother sound. The unmodulated patch has a pleasant tone, but it’s static and unexciting.
However, by dialing in a small amount of LFO pitch modulation, we can mimic a real-life flute performance.
Furthermore, by using an expression pedal to add more modulation to the end of a phrase, our performance has even greater expressiveness. For best results, select a sine or triangle waveshape for the LFO when trying to emulate vibrato.
We can also use the LFO to make drastic pitch changes, both for effects and for musical applications. An alarm or siren sound is easy to accomplish by cranking up the amount of modulation and setting the rate according to taste.
Another interesting application of modifying pitch with an LFO is setting up rhythmic pitch intervals. Here we’ve created a very basic bass patch using a sawtooth waveform, and we’re modulating it with a square wave because it limits us to hearing just two pitches. We can select the interval by altering the amount of pitch modulation. In this clip, I’m manually increasing the amount of modulation.
It’s important to note that, because most basic LFOs are bipolar — meaning they move from a negative value to an equal and opposite positive value — when you apply a significant amount of pitch modulation, the note you play on the keyboard will no longer directly correspond to the notes you’ll hear played back. For example, if I play a C3 on the keyboard but set the pitch modulation to play an octave interval, the interval does not move between C3 and C4. Rather, C3 is the center of that interval, so the patch will move between F2 and F3, a perfect fifth above and below the note, as heard in this example:
Modulating Filter Cutoff
Another popular application of LFO is to modulate the filter cutoff. Electronic artists as far back as Kraftwerk have employed this technique to add movement and dynamics to patches.
With no resonance or only a small amount of resonance, the LFO will create a tremolo-like effect.
But, when you add in resonance, you’ll get a much more exaggerated response, similar to the wobble or “wub” bass sound that’s a fixture in dubstep.
The LFO rate plays a big factor here. In the previous audio examples, the rate is faster, generating a driving rhythm. But, at slow rates, the LFO will create a subtler filter sweep, which can make a patch, especially slow drones, breathe.
Modulating Pulse Width
A third common use for an LFO is to modulate the pulse width of a square wave. When we talk about pulse-width modulation (PWM), we’re referring to altering the length of a square wave’s duty cycle, or the high and low states of the waveform.
As the ratio between the high state of the cycle and the low state of the cycle decreases (imagine the square becoming a thinner rectangle), the timbre of the oscillator becomes reedier and brighter. PWM is a crucial tool for creating evocative string patches, though this usually involves using two or more oscillators.
In this example, we used PWM to make small, cyclical timbre changes to a single oscillator, giving it more dimension and texture and a cello-like sound.
Here’s the patch without PWM:
And here it is with PWM:
Conclusion
There you have it — a very simple guide to modulation with an LFO! But, as we said, LFOs are a vital and powerful part of synthesis worthy of some in-depth exploration, and they’re a lot of fun, too! In the next installment, we’ll discuss envelope generators along with sound examples of how they can be used creatively!