What’s FM?
WHAT'S FM?
A complete discussion of FM Synthesis is beyond the scope of this manual. Here we will cover the basic ideas
of FM, and briefly explain each TX81Z voice parameter. For a more detailed study, see “FM Theory &
Applications: By Musicians for Musicians”, by Dr. John Chowning and David Bristow, published 1986 by
Yamaha Music Foundation.
FREQUENCY
MODULATION
FM stands for Frequency Modulation. In FM, one waveform modulates another
waveform, creating a new, complex waveform, as shown below. So, to do FM
synthesis, we need two oscillators. The upper one is called the “Modulator”
and the lower one (which goes directly to the output of the synthesizer) is
called the “Carrier”. The complexity or “brightness” of the resulting waveform
will depend on the output level of the Modulator.
EG (Envelope Generator) By varying the output level of the modulator, we can produce all kinds of
sounds. However, sounds in the real world usually change as time goes by. This
“shape in time” is called the Envelope. A synthesizer simulates this using an
Envelope Generator to change the output level of each operator. If the output
level of the modulator changes, the tone will change. If the output level of the
carrier changes, the volume will change.
OPERATOR
In Yamaha FM synthesizers, each oscillator has its own Envelope Generator
(EG) to vary its output level over time. This “package” of osci EG is
called an Operator. An operator can be either a modulator or carrier. The only
difference is how it is used.
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