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HREF="node40.html">Audio and control computations</A>
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<B> Previous:</B> <A ID="tex2html1262"
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HREF="node48.html">Operations on control streams</A>
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<H1><A ID="SECTION00790000000000000000">
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Control operations in Pd</A>
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</H1>
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<P>
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So far we have used Pd mostly for processing audio signals, although as early
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as Figure <A HREF="node16.html#fig01.10">1.10</A> we have had to make the distinction between Pd's
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notion of audio signals and of control streams: thin connections carry
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control streams and thick ones carry audio. Control streams in Pd appear
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as sequences of
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<A ID="3697"></A>
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<I>messages</I>. The messages may contain data (most often, one or more
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numbers), or not. A
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<A ID="3699"></A>
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<I>numeric control stream</I>
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(Section <A HREF="node43.html#sect3.controlstreams">3.3</A>) appears as a (thin) connection that
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carries numbers as messages.
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<P>
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Messages not containing data make up <I>time sequences</I>. So that you can
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see messages with no data, in Pd they are given the (arbitrary) symbol "bang".
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<P>
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<DIV ALIGN="CENTER"><A ID="fig03.12"></A><A ID="3705"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 3.12:</STRONG>
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The four control operations in Pd: (a) delay; (b) merging;
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(c) pruning; (d) resynchronizing.</CAPTION>
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<TR><TD><IMG
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WIDTH="332" HEIGHT="224" BORDER="0"
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SRC="img337.png"
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ALT="\begin{figure}\psfig{file=figs/fig03.12.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 four types of control operations described in the previous section can
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be expressed in Pd as shown in Figure <A HREF="#fig03.12">3.12</A>.
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<A ID="3709"></A>
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<I>Delays</I>
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are accomplished
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using two explicit delay objects:
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<P>
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<BR><!-- MATH
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$\fbox{ $\mathrm{del}$\ }$
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-->
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<IMG
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WIDTH="45" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
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SRC="img338.png"
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ALT="\fbox{ $\mathrm{del}$\ }">,
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<BR><!-- MATH
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$\fbox{ $\mathrm{delay}$\ }$
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-->
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<IMG
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WIDTH="61" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
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SRC="img339.png"
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ALT="\fbox{ $\mathrm{delay}$\ }">:
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<A ID="3843"></A>simple delay. You can specify the delay time in a creation argument or via
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the right inlet. A "bang" in the left inlet sets the delay, which then outputs
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"bang" after the specified delay in milliseconds. The delay is <I>simple</I>
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in the sense that sending a bang to an already set delay resets it to a new
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output time, canceling the previously scheduled one.
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<P>
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<BR><!-- MATH
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$\fbox{ $\mathrm{pipe}$\ }$
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-->
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<IMG
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WIDTH="54" HEIGHT="40" ALIGN="MIDDLE" BORDER="0"
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SRC="img340.png"
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ALT="\fbox{ $\mathrm{pipe}$\ }">:
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<A ID="3844"></A>compound delay. Messages coming in the left inlet appear on the output
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after the specified delay, which is set by the first creation argument. If
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there are more creation arguments, they specify one or more inlets for numeric
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or symbolic data the messages will contain. Any number of messages may be
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stored by <TT>pipe</TT> simultaneously, and messages may be reordered as
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they are output depending on the various delay times given for them.
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<P>
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<A ID="3721"></A>
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<I>Merging</I>
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of control streams in Pd is accomplished not by explicit objects
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but by Pd's connection mechanism itself. This is shown in part (b) of the
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figure with number boxes as an example. In general, whenever more than one
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connection is made to a control inlet, the control streams are merged.
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<P>
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Pd offers several objects for
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<A ID="3723"></A>
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<I>pruning</I> control streams, of which two are shown in part (c) of the
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figure:
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<P>
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<BR><!-- MATH
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$\fbox{ $\mathrm{moses}$\ }$
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-->
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<IMG
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WIDTH="65" HEIGHT="33" ALIGN="MIDDLE" BORDER="0"
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SRC="img341.png"
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ALT="\fbox{ $\mathrm{moses}$\ }">:
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<A ID="3845"></A>prune for numeric range. Numeric messages coming in the left inlet
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appear on the left output if they are smaller than a threshold value (set
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by a creation argument or by the right inlet), and out the right inlet
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otherwise.
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<P>
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<BR><!-- MATH
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$\fbox{ $\mathrm{select}$\ }$
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-->
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<IMG
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WIDTH="63" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
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SRC="img342.png"
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ALT="\fbox{ $\mathrm{select}$\ }">,
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<BR><!-- MATH
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$\fbox{ $\mathrm{sel}$\ }$
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-->
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<IMG
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WIDTH="42" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
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SRC="img343.png"
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ALT="\fbox{ $\mathrm{sel}$\ }">:
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<A ID="3846"></A>prune for specific numbers. Numeric messages coming in the left inlet produce
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a "bang" on the output only if they match a test value exactly. The test
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value is set either by creation argument or from the right inlet.
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<P>
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Finally, Pd takes care of
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<A ID="3734"></A>
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<I>resynchronizing</I>
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control streams implicitly in its connection mechanism, as illustrated by part
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(d) of the figure. Most objects with more than one inlet synchronize all other
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inlets to the leftmost one. So the <TT>float</TT> object shown in the figure
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resynchronizes its right-hand-side inlet (which takes numbers) to its
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left-hand-side one. Sending a "bang" to the left inlet outputs the most
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recent number <TT>float</TT> has received beforehand.
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