9

Using the LFO to Modulate Sounds
A modulation source found on nearly all synthesizers is the LFO (low frequency oscillator).
This oscillator is used only as a modulation source and does not generate any audible
signals that form part of your actual synthesizer sound, because it’s too low to be heard.
It can, however, affect the main signal by adding vibrato, filter sweeps, and so on.
LFO Controls
The LFO generally offers the following controls:
• Waveform: Allows you to choose the type of waveform (triangle waves and square
waves are seen most often). Triangle waves are useful for filter sweeps (slow changes
to the filter cutoff frequency) or when simulating an ambulance siren (slow changes
to the oscillator frequency). The square waveform is useful for rapid switches between
two different pitches (vibratos or octaving, for example).
• Frequency/Rate: Determines the speed of the waveform cycles produced by the LFO.
When set to low values, very slow ramps are produced, making it easy to create sounds
such as ocean waves rolling in (when white noise is chosen as the waveform in the
main oscillator).
• Sync mode: Allows you to choose between free running (a user-defined LFO rate) or
synchronization with an external tempo source (such as a host application).
LFO Envelopes
The LFO can also be controlled with an envelope generator in some synthesizers. As an
example of where this might be useful, imagine a sustained string section sound where
vibrato is introduced a second or so into the sustained portion of the sound. If this can
happen automatically, it allows you to keep both hands on the keyboard.
In certain synthesizers, a simple envelope generator is included for this precise purpose.
Often, this envelope consists only of an attack parameter, or occasionally it includes decay
or release options. These parameters perform in the same fashion as the amplitude
envelope parameters (see Envelopes in the Amplifier Section), but they are limited to
control of LFO modulations.
Using Envelopes for Modulation
The main envelope generator of the synthesizer not only controls levels over time, but
it also is often used to modulate other sound parameters when you press or release
keyboard keys.
The most common use of envelope modulation is to control the filter cutoff and resonance
parameters with the keyboard velocity or keyboard scaling modulation sources (see
Modulation Routing).
Global Controls
This section covers global controls that affect the overall output signal of your synthesizer.
365Appendix Synthesizer Basics