An inverted modulator envelope creates a metallic release sound used in FM harpsichord and bell patches

A standard ADSR envelope starts at zero and peaks at the attack. An inverted envelope does the opposite: it starts and ends at maximum amplitude, with a dip in the middle. When applied to an FM modulator, the inverted shape causes the sound to begin bright, grow darker during sustain, then return to brightness on release — a characteristic ‘metallic’ tail audible in DX7 harpsichord presets and many bell sounds. The low sustain variant is particularly useful: the sound is indistinguishable from a normal envelope until the key is released, when the modulation jumps back up and produces a characteristic high-frequency metallic tail. This asymmetry is impossible to achieve with a single filter EG.

Examples

DX7 harpsichord presets use inverted envelopes on modulators. FM Demo 2-B: sustain is kept low so the inverted behavior is hidden until key release, then a bright metallic click appears.

Assessment

Describe the audible result of applying an inverted envelope to a modulator operator versus a standard envelope. In which part of the note (attack, sustain, or release) does the difference become most apparent?