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<H2><A NAME="SECTION00884000000000000000">
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Additive synthesis: spectral envelope control</A>
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</H2>
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<A NAME="4814"></A>
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<P>
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The next patch example, D08.table.spectrum.pd (Figure <A HREF="#fig04.18">4.18</A>), shows a very different application of
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additive synthesis from the previous one. Here the sinusoids are
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managed by the <TT>spectrum-partial</TT> abstraction shown in Figure
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<A HREF="#fig04.19">4.19</A>. Each partial computes its own frequency as in the previous
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patch. Each partial also computes its own amplitude
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periodically (when triggered by the <TT>r poll-table</TT> object), using a
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<TT>tabread4</TT> object. The contents of the table (which has a nominal range of
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50 dB) are converted to linear units and used as an amplitude control in the
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usual way.
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<P>
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A similar example, D09.shepard.tone.pd (not pictured), makes a Shepard tone using
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the same technique. Here the frequencies of the sinusoids sweep
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over a fixed range, finally jumping from the end back to the beginning and
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repeating. The spectral envelope is arranged to have a peak at the middle of
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the pitch range and drop off to inaudibility at the edges of the range so
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that we hear only the continuous sweeping and not the jumping. The result
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is a famous auditory conundrum, an indefinitely ascending or descending
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tone.
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig04.18"></A><A NAME="4821"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.18:</STRONG>
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Additive synthesis for a specified spectral envelope, drawn in a
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table.</CAPTION>
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<TR><TD><IMG
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WIDTH="472" HEIGHT="394" BORDER="0"
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SRC="img391.png"
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ALT="\begin{figure}\psfig{file=figs/fig04.18.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig04.19"></A><A NAME="4894"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.19:</STRONG>
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The <TT>spectrum-partial</TT> abstraction used in
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Figure <A HREF="#fig04.18">4.18</A>.</CAPTION>
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<TR><TD><IMG
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WIDTH="550" HEIGHT="406" BORDER="0"
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SRC="img392.png"
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ALT="\begin{figure}\psfig{file=figs/fig04.19.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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The technique of synthesizing to a specified spectral envelope can
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be generalized in many ways: the envelope may be made to vary in time
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either as a result of a live analysis of another signal, or by calculating
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from a set of compositional rules, or by cross-fading between a collection
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of pre-designed spectral envelopes, or by frequency-warping the envelopes,
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to name a few possibilities.
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<ADDRESS>
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Miller Puckette
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2006-12-30
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