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<H1><A NAME="SECTION00810000000000000000"></A>
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<A NAME="sect4.adsr"></A>
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<BR>
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Envelope Generators
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</H1>
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<P>
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An
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<A NAME="4571"></A><I>envelope generator</I> (sometimes, and more justly, called a
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<A NAME="4573"></A><I>transient generator</I>) makes an audio signal that smoothly rises and
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falls as if to control the loudness of a musical note.
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Envelope generators were touched on earlier in Section <A HREF="node12.html#sect1.synth">1.5</A>.
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Amplitude control by multiplication (Figure <A HREF="node10.html#fig01.04">1.4</A>) is the most direct,
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ordinary way to use one, but there are many other possible uses.
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<P>
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Envelope generators have come in many forms over the years, but the simplest
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and the perennial favorite is the
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<A NAME="4577"></A><A NAME="4578"></A><I>ADSR</I> envelope generator. "ADSR" is an acronym for
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"Attack, Decay, Sustain, Release", the four segments of the
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ADSR generator's output. The ADSR generator is turned on and off by a control
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stream called a "trigger". Triggering the ADSR generator "on" sets off its
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attack, decay, and sustain segments. Triggering it "off" starts the
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release segment. Figure <A HREF="#fig04.01">4.1</A> shows the block
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diagram representation of an ADSR envelope generator.
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig04.01"></A><A NAME="4583"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.1:</STRONG>
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ADSR envelope as a block diagram, showing the trigger input (a
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control stream) and the audio output.</CAPTION>
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<TR><TD><IMG
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WIDTH="83" HEIGHT="174" BORDER="0"
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SRC="img356.png"
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ALT="\begin{figure}\psfig{file=figs/fig04.01.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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There are five parameters controlling the ADSR generator. First, a
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<I>level</I> parameter sets the output value at the end of the attack segment
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(normally the highest value output by the ADSR generator). Second and third,
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the <I>attack</I> and <I>decay</I> parameters give the time duration of the
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attack and decay segments. Fourth, a <I>sustain</I> parameter gives the level
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of the sustain segment, as a fraction of the level parameter. Finally, the
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<I>release</I> parameter gives the duration of the release segment. These five
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values, together with the timing of the "on" and "off" triggers, fully
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determines the output of the ADSR generator. For example, the duration of the
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sustain portion is equal to the time between "on" and "off" triggers, minus
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the durations of the attack and decay segments.
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig04.02"></A><A NAME="4593"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.2:</STRONG>
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ADSR envelope output: (a) with "on" and "off" triggers separated;
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(b), (c) with early "off" trigger; (d), (e) re-attacked.</CAPTION>
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<TR><TD><IMG
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WIDTH="332" HEIGHT="530" BORDER="0"
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SRC="img357.png"
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ALT="\begin{figure}\psfig{file=figs/fig04.02.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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Figure <A HREF="#fig04.02">4.2</A> graphs some possible outputs of an ADSR
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envelope generator. In
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part (a) we assume that the "on" and "off" triggers are widely enough
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separated that the sustain segment is reached before the "off" trigger is
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received.
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Parts (b) and (c) of Figure <A HREF="#fig04.02">4.2</A> show the result of following an
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"on" trigger quickly by an "off" one: (b) during the decay segment, and (c)
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even earlier, during the attack. The ADSR generator reacts to these situations
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by canceling whatever remains of the attack and decay segments and continuing
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straight to the release segment. Also, an ADSR generator may be retriggered
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"on" before the release segment is finished or even during the attack, decay,
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or sustain segments. Part (d) of the figure shows a reattack during the
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sustain segment, and part (e), during the decay segment.
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<P>
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The classic application of an ADSR envelope is using a voltage-control keyboard
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or sequencer to make musical notes on a synthesizer. Depressing and releasing
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a key (for example) would generate "on" and "off" triggers. The ADSR
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generator could then control the amplitude of synthesis so that "notes" would
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start and stop with the keys. In addition to amplitude, the ADSR generator
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can (and often is) used to control timbre, which can then be made to evolve
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naturally over the course of each note.
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