Staggering harmonic attack times in additive synthesis simulates a low-pass filter sweep

In additive synthesis on the DX7, each of the six carrier operators can have its own envelope generator. By setting EG Rate 1 (attack speed) to progressively slower values for higher harmonics — so the fundamental enters first, followed by overtones in sequence — the sound mimics the effect of a low-pass filter opening over time. The listener hears progressively brighter harmonics appearing, which is perceptually similar to a filter cutoff rising from low to high. Addsynth 4 achieves this with harmonics fading in ‘smoothly but quickly … one by one over a short span of time.’ This reveals an unexpected functional equivalence between additive envelope sequencing and FM modulator EG.

Examples

Addsynth 4: Ops 1-6 have EG Rate 1 descending by a factor of 10 as harmonic ratio ascends. Op. 1 (ratio 1) enters first; Op. 6 (ratio 6) enters last, producing a brightening sweep.

Assessment

Explain how staggered operator attack times in a six-operator additive patch produce an effect similar to a low-pass filter sweep, and name one way the two approaches differ in timbral character.