Understanding Envelopes, Part 1

Understanding Envelopes, Part 1

11 February, 2008 | 3.57PM

• Section: Music News Topics: Guide To Synthesis, Technology
• Written by: francispreve

After a brief hiatus, Beatportal’s Guide To Synthesis has returned.

Now that we’ve covered the essentials of oscillators and filters — a.k.a. the audio signal path of subtractive synthesis — it’s time to delve into modulation.

The term “modulation” covers components like envelopes, LFOs and MIDI continuous controllers, to name a few.

These modulators allow sound designers to effectively automate various parameters and elements — such as filter cutoff and volume — within the audio chain.

But there’s a lot more to it than just those two parameters.

In this tutorial, we’ll begin by addressing the essential components of the envelope itself. 

What’s an envelope?

My favorite analogy for what an envelope does is as follows: An envelope consists of a sequence of events that occurs every time you press a key.

For some, this may be an oversimplification, but it’s actually fairly accurate… Especially when you consider that many modern software synths include sophisticated multi-stage envelopes that can even create rhythmic patterns each time a note is played.

For the purposes of this tutorial, we’ll focus on the most common type of envelope (pictured above): ADSR.

ADSR is an acronym for the four stages of a basic envelope: attack time, decay time, sustain level and release time.

Some envelopes – such as those on hardware synths or their modern software emulations — are controlled by a knob or slider type interface.

Newer softsynths – like those from Native Instruments, Rob Papen and Ableton – feature graphically editable envelopes that display the shape of the envelope itself, instead of purely numeric values.

The ADSR configuration

Attack time

The first stage of the envelope is the attack (pictured below).

This parameter determines how the sound begins.

When a key is first pressed, the attack segment is activated and proceeds to the envelope’s peak (maximum value), which is set via the envelope amount parameter and/or the “peak level” on some synths.

Fast attacks are used for percussive sounds like pianos, drums and plucked sounds.

Slow attacks are useful for softer sounds like woodwinds or strings, as well as long sweeps and swells that are often used during breakdowns.

Decay time

After the attack segment is completed, the envelope proceeds to the decay stage (pictured below).

This component determines how long it takes for the sound to transition to the sustain level, or if the sustain is set to zero, fade to silence.

Fast decays are appropriate for percussive sounds, whereas longer decays are useful for sounds that fade out over a longer time, like pianos and cymbals.

Sustain level

After the decay completes, the sustain parameter (below) determines the overall level or volume that the envelope will maintain if a key is held for a longer duration.

For example, long string parts or chordal pad progressions require sounds that remain at a specified level for several bars.

By setting the sustain level to any value higher than zero, the sound will continue indefinitely until you lift the key.

Release time

The release parameter (below) determines the amount of time it will take for the envelope to return to zero — or silence — after the key is lifted.

Long releases are great for cymbal crashes, transitional sweeps, and ambient textures.

Short releases are ideal for any sound you want to stop immediately when the key is lifted, such as stabs, percussive sounds and gated, rhythmic effects.

Additional envelope segments

Some envelopes are slightly more complicated than the ADSR type, while retaining the same basic properties.

Common additions to the ADSR envelope include the following segments.

Delay time

Delay is an additional segment that precedes the attack portion, causing the envelope to wait for a given period after the key is pressed before starting the attack.

This can sound a bit odd when used on a volume envelope, as it just feels like severe latency.

That said, it’s quite useful for modulating timbre and pitch parameters in unusual ways, like adding a filter swell long after the key has been pressed.

Peak level

As previously mentioned, the peak level parameter specifies the maximum level an envelope will reach after the attack portion before moving on to the decay segment.

Hold time

This segment is rather unusual but quite useful for adding a bit of punch to an envelope.

Essentially, the hold time parameter will cause the envelope to sustain for a bit at after the attack peak, before moving on to the decay segment.

Note: According to legend, early Moog synths had a non-adjustable hold time of approximately 30 milliseconds immediately following the attack.

This is often associated with the overall impact that characterized vintage Minimoog sounds.

Regardless of its veracity, if your envelope supports hold times, try adding a bit of this to percussive and lead sounds as it really does work.

Sustain time

Some envelopes also include a second embedded decay that reduces the sustain level over time.

Used creatively, this can create two consecutive decay segments for more complex envelope shapes.

Curve

Non-linear curves are another great way to add impact and enhanced percussiveness to a sound.

Some envelopes (pictured above), such as those found in Ableton’s Sampler device, allow the curve of each segment to be varied from linear to exponential to inverse exponential, simply by dragging the blue Bezier dot on each segment.

While this sounds extremely math intensive and obtuse, it’s actually quite simple to manipulate and immediately hear the results.

At first listen, the effect can be quite subtle, so it’s useful to create a short repeating sequence and tinker with these parameters as the sound plays.

Next week, we’ll discuss more sophisticated envelope options and some interesting applications beyond volume and filtering.

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