Holding FM modulation depth steady and enveloping separate spectrum bands gives more natural timbres than sweeping the index

A production principle from the source (Rob Hordijk): to get convincing natural-instrument timbres from FM, do not dynamically change the overall modulation depth too much, because a rapidly swept index shifts the whole spectrum in a way that does not match how real instruments evolve. Instead, build several separate ‘spectrum bands’ and give each its own envelope and keyboard/velocity scaling, letting the relative contribution of each band change over time while each band’s own modulation depth stays comparatively stable. This is exactly why the DX7 is designed with per-operator envelopes and key/velocity scaling rather than one global FM-depth control.

Examples

Less natural: put a big envelope on a single modulator’s index (0 -> high -> low) so the whole spectrum morphs abruptly. More natural: fix each modulator’s depth for its band and use per-operator envelopes so the fundamental, formant, and brightness bands rise and fall on their own timelines.

Assessment

Program a patch two ways: (A) one modulator with a large index envelope; (B) two or three fixed-depth modulator bands, each with its own amplitude envelope. Play both and explain which sounds more like an evolving acoustic instrument and why.