Without modulation, sounds tend to be uninteresting and fatiguing to the ear. They also sound synthetic, rather than natural, in the absence of some type of sonic modulation. Vibrato is a type of modulation commonly used by orchestral string players to add animation to their instrument pitch.
To make sounds less static, you can use a range of synthesizer controls to modulate basic sound parameters. To this end, many synthesizers, including ES1, ES2, and EXS24 mkII sampler, provide a modulation router. Alchemy and Sculpture provide further unique modulation options.
The router enables you to direct, or route, one or more modulation sources (the parameter or control that is modulating another parameter) to one or more modulation targets (the parameter being modulated).
You can affect modulation targets, such as oscillator pitch or filter cutoff frequency, by using modulation sources that include the following:
Velocity modulation: You can modulate a target in different ways with the impact of your keyboard playing (harder or softer). The most common example of modulation controller use is a velocity-sensitive keyboard, set to control the filter and level envelopes. The harder you strike the notes, the louder and brighter the sound is.
Key scaling: You can modulate a target in different ways by adjusting the position you play on the keyboard (low or high notes). Keyscale modulation is often used to control filter cutoff, resonance, or both; higher notes sound brighter than low notes. This emulates the behavior of many acoustic instruments.
Controls: You can use controls such as the modulation wheel, ribbon controllers, or pedals attached to your keyboard. The modulation wheel is most commonly used for pitch bends during performance.
Automatic modulation: You can use envelope generators or LFOs to modulate signals automatically. The most common LFO modulations are control of the pitch or level of a sustained note, resulting in a vibrato or tremolo.
Modulation sources can be—and often are—triggered by something you’ve done, such as playing a note on the keyboard or moving the modulation wheel.
The modulation wheel, pitch bend ribbons, foot pedals, keyboard, and other input options are referred to as modulation controllers, MIDI controllers, or just controllers.