Frequency modulation refers to using the output of a low-frequency oscillator to continually alter (modulate) the frequency of another oscillator. This example provides the user control of the amplitude and frequency of both the "carrier" oscillator (the one we hear directly) and the "modulator" oscillator (the effect of which we hear indirectly).
In music the term vibrato (Italian for “vibrated”) means small repetitive fluctuations of pitch and loudness in a tone. Singers and instrumentalists use vibrato intentionally to add interest and expressivity to their sound.
This example shows interference between two sine tones that have nearly the same frequency, causing a beating effect.
In electronic music parlance, the word "modulation" means change, specifically the continuous or cyclical change caused by using one signal to control another. The controlling signal, known as the modulator, is used to control some property of the sound signal we're listening to, called the carrier.
This patch demonstrates how one might make an audio patch that can serve as a voice in a polyphonic synthesizer. It's quite similar to the FMtone~ patch shown in “Generating a simple 2-operator FM tone”, but with some modifications that make it suitable for use with the poly~ object so that it can be used polyphonically.
In order to enable and disable portions of an audio program easily, and to be able to reuse them multiple times, you will probably want to encapsulate an entire audio-generating or audio-processing procedure inside a single patch with inlets and outlets so that it can be used as a subpatch object in some other patch. This patch shows an example of a simple 2-oscillator frequency modulation tone generator that could easily be used as a subpatch in some other patch.