Envelope Filters – Passing The Envelope

Digital musicians deal with envelopes and filters on a daily basis, but the terminology can seem a bit daunting for newcomers. So, here’s a brief answer to the question – what is a sound envelope anyway?

Something To Keep Your Signal In

Basically, an envelope determines how a signal behaves over time. The fundamental parameters of an envelope are Attack, Decay, Sustain and Release.

Changing these parameters can radically alter the characteristics of a sound, and an understanding of how they work is essential to sound synthesis. However, even if you simply want to tweak preset sounds rather than creating them from scratch, it’s good to know what’s going on under the hood.

An envelope can be applied to different things; an amplitude envelope controls how the amplitude of a signal will vary over time, whereas a filter envelope can change the filter frequency over time. However, the principles of the envelope remain the same, regardless of what arcane uses you may try to apply them to.

Attack Of The 50ft Envelope

The attack parameter determines how quickly the envelope is applied once it has been triggered; for example, when using an amplitude envelope, the attack will control the time it takes a note to reach its full amplitude after you hit a key on the keyboard.

Decay Club

The decay paramter determines how long it takes the envelope to fall from its highest attack level to its default level. This is nicely illustrated in the diagram above, which is taken from the Propellerheads’ series on teaching music with Reason (available for free on their website).

Sustained Performance

The sustain parameter is the only one of these envelope controls that is purely level-based rather than time-based. The sustain determines the level (or volume, in the case of an amplitude envelope) that the signal will stay at once the decay phase has passed. The signal will be maintained at this level for as long as the key is pressed.

Release Me

The release parameter is essentially the reverse of the attack parameter. It determines how long the signal takes to return to zero after the key has been…well, released.