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186 lines
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186 lines
5.8 KiB
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<H2><A ID="SECTION001251000000000000000">
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Prefabricated low-, high-, and band-pass filters</A>
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</H2>
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<P>
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Patches H01.low-pass.pd, H02.high-pass.pd, and H03.band-pass.pd (Figure <A HREF="#fig08.28">8.28</A>) show Pd's
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built-in filters, which implement filter designs described in Sections
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<A HREF="node140.html#sect08.low-pass">8.3.1</A>, <A HREF="node141.html#sect08.highpass">8.3.2</A> and <A HREF="node143.html#sect8.twopolebandpass">8.3.4</A>.
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Two of the patches also use a noise generator we have not introduced before.
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We will need four new Pd objects:
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<P>
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<BR><!-- MATH
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$\fbox{\texttt{lop\~}}$
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-->
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<IMG
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WIDTH="47" HEIGHT="39" ALIGN="MIDDLE" BORDER="0"
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SRC="img1014.png"
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ALT="\fbox{\texttt{lop\~}}">:
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<A ID="10842"></A>one-pole low-pass filter. The left inlet takes a signal to be filtered, and
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the right inlet takes control messages to set the
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cutoff frequency of the filter. The filter is normalized so that the
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gain is one at frequency 0.
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<P>
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<BR><!-- MATH
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$\fbox{ \texttt{hip\~}}$
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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="img1016.png"
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ALT="\fbox{ \texttt{hip\~}}">:
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<A ID="10843"></A>one-pole, one-zero high-pass filter, with the same inputs and outputs as
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<TT>lop~</TT>, normalized to have a gain of one at the Nyquist frequency.
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<P>
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<BR><!-- MATH
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$\fbox{ \texttt{bp\~}}$
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-->
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<IMG
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WIDTH="44" HEIGHT="39" ALIGN="MIDDLE" BORDER="0"
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SRC="img1017.png"
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ALT="\fbox{ \texttt{bp\~}}">:
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<A ID="10844"></A>resonant filter. The middle inlet takes control messages to set the center
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frequency, and the right inlet to set "q".
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<P>
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<BR><!-- MATH
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$\fbox{ \texttt{noise\~}}$
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-->
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<IMG
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WIDTH="69" HEIGHT="39" ALIGN="MIDDLE" BORDER="0"
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SRC="img1019.png"
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ALT="\fbox{ \texttt{noise\~}}">:
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<A ID="10845"></A>white noise generator. Each sample is an independent
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pseudo-random number, uniformly distributed from -1 to 1.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig08.28"></A><A ID="10600"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 8.28:</STRONG>
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Using prefabricated filters in Pd: (a) a low-pass filter, with
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white noise as a test input; (b) using a high-pass filter to remove a
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signal component of frequency 0.</CAPTION>
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<TR><TD><IMG
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WIDTH="491" HEIGHT="190" BORDER="0"
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SRC="img1021.png"
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ALT="\begin{figure}\psfig{file=figs/fig08.28.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 first three example patches demonstrate these three filters (see Figure
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<A HREF="#fig08.28">8.28</A>). The <TT>lop~</TT> and <TT>bp~</TT> objects are demonstrated
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with noise as input; <TT>hip~</TT> as shown is used to remove the DC (zero
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frequency) component of a signal.
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<ADDRESS>
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Miller Puckette
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2006-12-30
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