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159 lines
5.4 KiB
159 lines
5.4 KiB
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<H2><A ID="SECTION001252000000000000000">
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Prefabricated time-varying band-pass filter</A>
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</H2>
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<P>
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Time-varying band-pass filtering, as often used in classical
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subtractive synthesis (Section <A HREF="node152.html#sect08.subtractive">8.4.1</A>), can be done
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using the <TT>vcf~</TT> object, introduced here:
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<P>
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<BR><!-- MATH
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$\fbox{ \texttt{vcf\~}}$
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-->
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<IMG
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WIDTH="53" HEIGHT="39" ALIGN="MIDDLE" BORDER="0"
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SRC="img1022.png"
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ALT="\fbox{ \texttt{vcf\~}}">:
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<A ID="10846"></A>a "voltage controlled" band-pass filter,
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similar to <TT>bp~</TT>, but with a signal inlet to control center frequency.
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Both <TT>bp~</TT> and <TT>vcf~</TT> are one-pole resonant filters as
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developed in Section <A HREF="node143.html#sect8.twopolebandpass">8.3.4</A>; <TT>bp~</TT> outputs only
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the real part of the resulting signal, while <TT>vcf~</TT> outputs the
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real and imaginary parts separately.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig08.29"></A><A ID="10612"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 8.29:</STRONG>
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The <TT>vcf~</TT> band-pass filter, with its center frequency
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controlled by an audio signal (as compared to
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<TT>bp~</TT> which takes only control messages to set its center frequency. </CAPTION>
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<TR><TD><IMG
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ALT="\begin{figure}\psfig{file=figs/fig08.29.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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Example H04.filter.sweep.pd (Figure <A HREF="#fig08.29">8.29</A>) demonstrates using the <TT>vcf~</TT> object for a simple and characteristic subtractive synthesis task. A
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<TT>phasor~</TT> object (at top) creates a sawtooth wave to filter. (This is
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not especially good practice as we are not controlling the possibility of
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foldover; a better sawtooth generator for this purpose will be developed in
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Chapter 10.) The second <TT>phasor~</TT> object (labeled "LFO for sweep")
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controls the time-varying center frequency. After adjusting to set the depth
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and a base center frequency (given in MIDI units), the result is converted into
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Hertz (using the <TT>tabread4~</TT> object) and passed to <TT>vcf~</TT> to set
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its center frequency. Another example of using a <TT>vcf~</TT> object for
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subtractive synthesis is demonstrated in example H05.filter.floyd.pd.
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<ADDRESS>
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Miller Puckette
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2006-12-30
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