An envelope on the FM modulator produces time-varying timbre with no filter

On filter-less FM synthesizers, time-varying timbre is achieved by applying an amplitude envelope to the modulating operator rather than to a filter cutoff. The modulator’s envelope controls the modulation depth (index) over time, and since index sets spectral brightness this is the FM equivalent of filter-envelope modulation in subtractive synthesis: a slow attack gives a ‘filter-opening’ swell, while a fast attack with slow decay gives a bright-then-dark transient that settles into a purer tone. A separate envelope on the carrier controls amplitude (loudness), so the two operate independently — which lets even a two-operator FM voice produce complex, evolving timbres. This time-varying FM is responsible for classic electric-piano and bell sounds, whose bright attack and pure sustain come from the decaying modulation depth.

Examples

DX7 two-operator patch (Op.1 carrier, Op.2 modulator): give Op.2’s EG a slow attack and decay for a gradually brightening then darkening tone; adjusting Op.2’s EG Rate 1 changes how quickly the brightness appears (‘E.G. 2 controls timbre change’). In VCV Rack: VCO-1 (modulator) → VCA (controlled by ADSR) → VCO-2 FM input (carrier); adjust the FM amount and both VCO FREQs (the C:M ratio) — the attack is bright, the sustain pure.

Assessment

On a two-operator FM patch, which envelope controls brightness over time and which controls loudness? What is the difference between constant and envelope-controlled modulation depth, and what real instrument does time-varying FM emulate especially well? What happens if both envelopes are set to instant attack and no sustain?