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<H2><A NAME="SECTION001241000000000000000"></A>
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<A NAME="sect08.subtractive"></A>
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<BR>
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Subtractive synthesis
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</H2>
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<P>
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<A NAME="10517"></A><I>Subtractive synthesis</I> is the technique of using filters to shape the
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spectral envelope of a sound, forming another sound, usually preserving
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qualities of the original sound such as pitch, roughness, noisiness, or
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graniness. The spectral envelope of the resulting sound is the product of the
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spectral envelope of the original sound with the frequency response of the
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filter. Figure <A HREF="#fig08.24">8.24</A> shows a possible configuration of source,
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filter, and result.
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig08.24"></A><A NAME="10522"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 8.24:</STRONG>
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Subtractive synthesis: (a) spectrum of input sound; (b) filter frequency
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response; (c) spectrum of output sound.</CAPTION>
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<TR><TD><IMG
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WIDTH="409" HEIGHT="387" BORDER="0"
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SRC="img986.png"
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ALT="\begin{figure}\psfig{file=figs/fig08.24.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 filter may be constant or time-varying. Already in wide use by the mid
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1950s, subtractive synthesis boomed with the introduction of the
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voltage-controlled filter (VCF), which became widely available in the mid 1960s
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with the appearance of modular synthesizers. A typical VCF has two inputs: one
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for the sound to filter, and one to vary the center or cutoff frequency of the
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filter.
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<P>
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A popular use of a VCF is to control the center frequency of a resonant filter
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from the same ADSR generator that controls the amplitude; a possible block
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diagram is shown in Figure <A HREF="#fig08.25">8.25</A>. In this configuration,
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the louder portion of a note (loudness roughly controlled by the multiplier
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at the bottom) may also be made to sound brighter, using the filter, than
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the quieter parts; this can mimic the spectral evolution of strings or
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brass instruments over the life of a note.
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig08.25"></A><A NAME="10528"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 8.25:</STRONG>
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ADSR-controlled subtractive synthesis.</CAPTION>
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<TR><TD><IMG
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WIDTH="223" HEIGHT="318" BORDER="0"
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ALT="\begin{figure}\psfig{file=figs/fig08.25.ps}\end{figure}"></TD></TR>
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</TABLE>
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