From: mark-r@ee.man.ac.uk (Mark Robinson (JO PhD))
Newsgroups: comp.music
Subject: Re: ___ Envelope, envelope ___
Date: 8 Jan 93 10:28:45 GMT
Reply-To: mark-r@spec0.ee.man.ac.uk (Mark Robinson (JO PhD))
Organization: Dept Electrical and Electronic Eng, Manchester University, UK

In article <747@vercors.frec.bull.fr> heritier@delta4.frec.bull.fr (Franck HERITIER) writes:
>Yet an other question, about dynamic variation of a sound.
>I'd like to know how are named the diferent phases of the envelope
>of a sound (or a wave in general). I know some terms like attack, 
>sustain, duration, tempo, release, delay, decay, reverberation...

Being brought up on analogue synths (my first computer music was
performed using 1V/oct CV signals) ADSR envelopes are very real to
me.

The envelope of a sound determines how the volume of the note
varies over its duration. Early synths used AD or ADR envelopes,
but as they became more sophisticated ADSR was adopted to model
natural sounds better. The initials stand for Attack, Decay, Sustain
and Release and refer to the distinct parts of the sound envelope. On
analogue machines there is usually a slider pot to control each
term individually. The meanings are as follows:

Attack: The time constant of the rise in volume of the note when
  a key is first pressed. A fast attack gives a percussive or
  piano-like sound, a slow attack is more 'stringy'. We recognise
  a particular sound largely from its attack portion, a feature
  which Roland exploited in its LA synthesis (sampled attack,
  synthesised decay/release - although IMHO it produces abysmal
  pianos but good brass and strings).

Decay: Once the note reaches its maximum volume it enters the
  decay phase, where the volume falls off at the rate set by the
  decay control. If the key is still pressed when the level falls
  to the that set by the sustain control, the sustain phase is entered.

Sustain: during this phase the volume of the note is held at the
  sustain level until the key is relesed. Note that unlike the
  AD and R controls, the sustain control is a level not a time
  constant. Somtimes a very small amount of decay is added to
  the sustain (ie the level falls slowly) for more realism.
  Often this was just natural decay of voltages off capacitors
  in analogue days, now it has to be 'simulated'. Once the key
  is released the dacay phase is entered.

Decay: This determines the time taken for a note to die away after
  the key is released. A longer decay produces sustained notes,
  and 'Sustain' buttons usually increase this time constant, 
  although some seem to just increase the sustain level. On
  monophonic machines a long decay would result in one note
  cutting off abruptly when another one started, because there's only
  one VCO, and it can't do do jobs at once.
  If the key is released during the attack or decay phase, then
  the release phase is entered early.

Let's see if the limitations of ASCII graphics and the vi editor
allow me to draw an ADSR curve.


                        /\ D
                    A  /  \     S
                      /    \________
                     /              \ R
                 __ /                \____
                    ^               ^
                 key pressed      key released

hmm, best I could do I'm afraid.

Note on some (most?) synths the envelope generator could be
patched to the VCO(s) and LFO as well as the VCA, allowing it to
modulate the pitch of the note and any vibrato/tremolo effect
throughout the sound's duration. Patching it to the VCF(s) allowed
a multitude of subtle effects to be produced, by changing the
timbre of a note throughout it's duration. It was the ability
to be able to predict the effect of the VCF(s) that made a good
analogue synth programmer. Some machines had a seperate AD or even
another ADSR generator for more versatility in this area.

>Thanks to help me, and please apolozige for my poor english (I'm
>just a poor French man).
>
No problem, your English is better than my French.
>Frank.

Mark
mark-r@spec0.ee.man.ac.uk
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