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171 lines
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171 lines
6.0 KiB
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original version by: Nikos Drakos, CBLU, University of Leeds
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* revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan
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HREF="node71.html">Additive synthesis: Risset's bell</A>
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HREF="node68.html">Examples</A>
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HREF="node69.html">ADSR envelope generator</A>
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<H2><A ID="SECTION00882000000000000000">
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Transfer functions for amplitude control</A>
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</H2>
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<P>
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Section <A HREF="node60.html#sect4.curved">4.2</A> described using ADSR envelopes to control
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amplitude, for which exponential or quartic-curve segments often give more
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natural-sounding results than linear ones. Patches D03.envelope.dB.pd and D04.envelope.quartic.pd (the latter shown in Figure <A HREF="#fig04.15">4.15</A>)
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demonstrate the use of decibel and quartic segments. In addition to amplitude,
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in Example D04.envelope.quartic.pd the frequency of a sound is also controlled, using linear
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and quartic shapes, for comparison.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig04.15"></A><A ID="4770"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.15:</STRONG>
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Linear and quartic transfer functions for changing amplitude and
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pitch.</CAPTION>
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<TR><TD><IMG
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WIDTH="349" HEIGHT="369" BORDER="0"
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SRC="img382.png"
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ALT="\begin{figure}\psfig{file=figs/fig04.15.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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Since converting decibels to linear amplitude units is a costly operation
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(at least when compared to an oscillator or a ramp generator), Example D03.envelope.dB.pd uses table lookup to implement the necessary transfer function. This has the
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advantage of efficiency, but the disadvantage that we must decide on the
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range of admissible values in advance (here from 0 to 120 dB).
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<P>
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For a
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quartic segment as in Example D04.envelope.quartic.pd no table lookup is required;
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we simply square the <TT>line~</TT> object's output signal twice in succession.
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To compensate for raising the output to the fourth power, the target values
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sent to the <TT>line~</TT> object must be the fourth root of the desired
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ones. Thus, messages to ramp the frequency or amplitude are first unpacked to
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separate the target and time interval, the target's fourth root is taken
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(via two square roots in succession), and the two are then sent to the
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<TT>line~</TT> object. Here we have made use of one new Pd object:
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<P>
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<BR><!-- MATH
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$\fbox{ $\mathrm{unpack}$}$
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-->
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<IMG
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WIDTH="69" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
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SRC="img383.png"
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ALT="\fbox{ $\mathrm{unpack}$}">:
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<A ID="4907"></A>unpack a list of numbers (and/or symbols) and distribute them to separate
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outlets. As usual the outputs appear in right-to-left order. The number of
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outlets and their types are determined by the creation arguments. (See also
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<TT>pack</TT>, Page <A HREF="node34.html#pdpack"><IMG ALIGN="BOTTOM" BORDER="1" ALT="[*]"
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SRC="crossref.png"></A>).
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<P>
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The next two patches, D05.envelope.pitch.pd and D06.envelope.portamento.pd, use an ADSR envelope generator
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to make a pitch envelope and a simple <TT>line~</TT> object, also controlling
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pitch, to make portamento. In both cases exponential segments are desirable,
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and they are calculated using table lookup.
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<BR>
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<B> Next:</B> <A ID="tex2html1591"
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HREF="node71.html">Additive synthesis: Risset's bell</A>
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<B> Up:</B> <A ID="tex2html1585"
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HREF="node68.html">Examples</A>
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<B> Previous:</B> <A ID="tex2html1579"
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HREF="node69.html">ADSR envelope generator</A>
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<B> <A ID="tex2html1587"
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HREF="node4.html">Contents</A></B>
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<B> <A ID="tex2html1589"
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HREF="node201.html">Index</A></B>
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<ADDRESS>
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Miller Puckette
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2006-12-30
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