The syNth Degree: LFOs

As mentioned in the envelopes post, in synthesizers, samplers and other virtual instruments, there are many different parameters (like pitch, volume, filter frequency, etc.) that can be dynamically controlled. One way to control these is through modulators. A modulator is something that affects a change in something else (think about modulating from one key to another). LFOs are one form of modulation.

LFO
LFO stands for low frequency oscillator. An oscillator generates a waveform. You may be familiar with the concept of oscillation from your oscillating fans at home (unless you have central air of course). These fans swivel to create a "waveform" of air blowing through the room, and are therefore oscillators. LFOs are not audio generating oscillators, they generate control signals that allow modulation of different parameters. The difference between envelopes and LFOs as modulators, is that an envelope modulates a parameter for the entire duration of a note or phrase, with one attack, one decay, and one release, and an LFO repeats the attack and decay cycle.

LFO components
Shape
LFOs create a waveform shape, determining the character of the modulation. Does the volume rise and fall gently, as with a typical tremolo effect? This is caused by a gently sloping sine or triangle wave. Does the pitch bounce back and forth between two distinct notes? This is caused by a pulse wave LFO affecting pitch. Typical LFO shapes are sine, pulse (or square), triangle, sawtooth (allowing ramp ups or ramp downs), and random sloping or stairstep.






The following are audio clips from a synthesizer with an LFO modulating the volume.
 
sine wave at a rate of 2 Hz (2 cycles per second, or 1/4 note at 120 beats per minute)
 
sawtooth wave at a rate of 2 Hz
 
square or pulse wave at a rate of 2 Hz


random wave at a rate of 8 Hz (8 cycles per second, or 1/16 note at 120 beats per minute)


Rate
The rate determines the frequency of the waveform, or the speed of the modulation. Rate can be measured in hertz (Hz) or cycles per second, or the rate can be synced to tempo and be measured by a notation value (1 bar, 1/4 note, 1/8 triplet, etc.). The faster the tempo in your sequencing software, the faster your 1/4 note LFO will modulate. LFOs measured in Hz can typically go from as slow as .1 (or one LFO cycle over 10 seconds) to 100. Modulation faster than 100 Hz gets into the realm of frequency modulation, or FM synthesis, which we will discuss in another post.



Here is an audio clip from a synthesizer that employs a sine wave LFO modulating the pitch. The LFO's rate is automated from .1 Hz (or 1 cycle per 10 seconds) to 100 Hz, while the LFO's amount is to a constant of multiple octaves.






Amount
Amount determines the strength of the waveform. In audio waveforms, rate determines pitch, and amount determines amplitude or volume. Amount is usually determined by percentage.

Here is an audio clip from a synthesizer that employs a sine wave LFO modulating the pitch. The LFO's amount is automated from 0 cents to multiple octaves over 4 seconds, while the LFO's rate is at a constant 2Hz (or two cycles per second, or at 1/4 note at 120 beats per minute).