Simple FM extends into a family of richer techniques via feedback, multiple operators, and modulated parameters

Simple FM (one sine carrier, one sine modulator) is the base case of a larger family. It extends by: feedback (an operator’s output modulates itself or the modulator, adding density toward noise); multiple-carrier FM (MCFM), where several carriers share one modulator to sculpt formants; multiple-modulator FM (MMFM), which raises partial count via parallel or serial modulators; non-sine operators (used sparingly, as rich modulators overcrowd the spectrum); exponential FM for analog-style pitch tracking; and envelopes on the modulation index to make timbre evolve over a note. The DX7’s six operators in configurable algorithms are all instances of this family.

Examples

DX7 uses 6 operators in 32 configurable algorithms, all variants of MCFM/MMFM. Feedback at high levels creates metallic noise. An ADSR on I recreates piano, bell, and bass brightness envelopes.

Assessment

Describe the timbral effect of a decaying envelope on modulation index I. Then explain why multiple non-sine modulators in series are usually avoided.