Running an envelope through a resonant filter (instead of a slew) adds an LFO-like movement to the envelope's sustain and release phases

Slewing an envelope (smoothing its transitions) usually has little effect because the stages are already gradual. However, a DC-coupled filter with resonance applied to an envelope produces a fundamentally different result: at the sustain stage, the filter’s resonance means the envelope doesn’t fully settle, creating a subtle oscillation. On release, the filter resonance causes a brief up-and-down movement rather than a clean linear decay. The result is a complex, partially self-oscillating movement — described as ‘kind of like adding an LFO to your envelope in a really unusual way.’ This technique is rarely taught and exploits the near-equivalence of filters and slews (both are low-pass operations on CV).

Examples

Patch a 100 ms release ADSR through a DC-coupled filter with resonance at 60–80%. Route the output to oscillator pitch. On release, instead of a smooth pitch fall, the pitch wobbles briefly before settling — an organic, unusual effect.

Assessment

Why does a filter with resonance produce a different result from a slew when applied to an envelope? What property does the filter have that the slew lacks?