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<H2><A NAME="SECTION0011104000000000000000">
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Order of execution and lower limits on delay times</A>
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</H2>
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<P>
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When using delays (as well as other state-sharing tilde objects in Pd), the
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order in which the writing and and reading operations are
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done can affect the outcome of the computation. Although the tilde objects
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in a patch may have a complicated topology of audio connections, in reality
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Pd executes them all in a sequential order, one after the other, to compute
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each block of audio output. This linear order is guaranteed to be compatible
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with the audio interconnections, in the sense that no tilde object's computation
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is done until all its inputs, for that same block, have been computed.
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<P>
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Figure <A HREF="#fig07.26">7.26</A> shows two examples of tilde object topologies and their
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translation into a sequence of computation. In part (a) there are four
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tilde objects, and because of the connections, the object <TT>a~</TT> must
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produce its output before either of <TT>b~</TT> or <TT>c~</TT> can run;
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and
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both of those in turn are used in the computation of <TT>d~</TT>. So the
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possible orderings of these four objects are ``a-b-c-d" and
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``a-c-b-d". These
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two orderings will have exactly the same result unless the computation of
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<TT>b~</TT> and <TT>c~</TT> somehow affect each other's output (as
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delay operations might, for example).
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig07.26"></A><A NAME="8233"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 7.26:</STRONG>
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Order of execution of tilde objects in Pd: (a), an acyclic network.
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The objects may be executed in either the order ``a-b-c-d" or ``a-c-b-d". In
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part (b), there is a cycle, and there is thus no compatible linear ordering of
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the objects because each one would need to be run before the other.</CAPTION>
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<TR><TD><IMG
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WIDTH="273" HEIGHT="155" BORDER="0"
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SRC="img827.png"
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ALT="\begin{figure}\psfig{file=figs/fig07.26.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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Part (b) of the figure shows a cycle of tilde objects. This network cannot
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be sorted into a compatible sequential order, since each of <TT>a~</TT> and
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<TT>b~</TT> requires the other's output to be computed first. In general,
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a sequential ordering of the tilde objects is possible if and only if there
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are no cycles anywhere in the network of tilde objects and their audio
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signal interconnections. Pd reports an error when such a cycle appears.
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(Note that the situation for <I>control</I> interconnections between objects
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is more complicated and flexible; see the Pd documentation for details.)
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<P>
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To see the effect of the order of computation on a
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<TT>delwrite~</TT>/<TT>delread~</TT> pair, we can write explicitly the input
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and output signals in the two possible orders, with the minimum possible
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delay. If the write operation comes first, at a block starting at sample
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number <IMG
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WIDTH="18" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img3.png"
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ALT="$N$">, the operation can be written as:
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<BR><P></P>
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<DIV ALIGN="CENTER">
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<!-- MATH
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\begin{displaymath}
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x[N], \ldots, x[N+B-1] \longrightarrow \fbox{\texttt{delwrite\~}}
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\end{displaymath}
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-->
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<IMG
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WIDTH="279" HEIGHT="29" BORDER="0"
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SRC="img828.png"
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ALT="\begin{displaymath}
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x[N], \ldots, x[N+B-1] \longrightarrow \fbox{\texttt{delwrite\~}}
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\end{displaymath}">
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</DIV>
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<BR CLEAR="ALL">
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<P></P>
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where <IMG
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WIDTH="16" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img311.png"
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ALT="$B$"> is the block size (as in Section <A HREF="node42.html#sect3.control">3.2</A>).
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Having put those particular samples into the delay line, a following
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<TT>delread~</TT> is able to read the same values out:
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<BR><P></P>
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<DIV ALIGN="CENTER">
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<!-- MATH
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\begin{displaymath}
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\fbox{\texttt{delread\~}} \longrightarrow x[N], \ldots, x[N+B-1]
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\end{displaymath}
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-->
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<IMG
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WIDTH="270" HEIGHT="29" BORDER="0"
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SRC="img829.png"
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ALT="\begin{displaymath}
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\fbox{\texttt{delread\~}} \longrightarrow x[N], \ldots, x[N+B-1]
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\end{displaymath}">
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</DIV>
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<BR CLEAR="ALL">
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<P></P>
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<P>
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On the other hand, suppose the <TT>delread~</TT> object comes before the
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<TT>delwrite~</TT>. Then the samples <!-- MATH
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$x[N], \ldots, x[N+B-1]$
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-->
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<IMG
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WIDTH="163" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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SRC="img830.png"
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ALT="$x[N], \ldots, x[N+B-1]$"> have not
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yet been stored in the delay line, so the most recent samples that
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may be read belong to the previous block:
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<BR><P></P>
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<DIV ALIGN="CENTER">
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<!-- MATH
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\begin{displaymath}
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\fbox{\texttt{delread\~}} \longrightarrow x[N-B], \ldots, x[N-1]
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\end{displaymath}
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-->
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<IMG
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WIDTH="270" HEIGHT="29" BORDER="0"
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SRC="img831.png"
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ALT="\begin{displaymath}
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\fbox{\texttt{delread\~}} \longrightarrow x[N-B], \ldots, x[N-1]
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\end{displaymath}">
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</DIV>
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<BR CLEAR="ALL">
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<P></P>
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<BR><P></P>
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<DIV ALIGN="CENTER">
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<!-- MATH
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\begin{displaymath}
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x[N], \ldots, x[N+B-1] \longrightarrow \fbox{\texttt{delwrite\~}}
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\end{displaymath}
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-->
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<IMG
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WIDTH="279" HEIGHT="29" BORDER="0"
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SRC="img828.png"
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ALT="\begin{displaymath}
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x[N], \ldots, x[N+B-1] \longrightarrow \fbox{\texttt{delwrite\~}}
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\end{displaymath}">
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</DIV>
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<BR CLEAR="ALL">
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<P></P>
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Here the minimum delay we can possibly obtain is the block size <IMG
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WIDTH="16" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img311.png"
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ALT="$B$">. So the
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minimum delay is either <IMG
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WIDTH="11" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img179.png"
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ALT="$0$"> or <IMG
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WIDTH="16" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img311.png"
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ALT="$B$">, depending on the order in which the
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<TT>delread~</TT> and <TT>delwrite~</TT>objects are sorted into a sequence of
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execution.
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<P>
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Looking back at the patches of Figures <A HREF="node118.html#fig07.24">7.24</A> and <A HREF="node119.html#fig07.25">7.25</A>, which
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both feature recirculating delays, the <TT>delread~</TT> or <TT>vd~</TT> object must be placed
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earlier in the sequence than the <TT>delwrite~</TT> object. This is true of any
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design in which a delay's output is fed back into its input. The minimum
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possible delay is <IMG
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WIDTH="16" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img311.png"
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ALT="$B$"> samples. For a (typical) sample rate of 44100 Hertz and
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block size of 64 samples, this comes to 1.45 milliseconds. This might not
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sound at first like a very important restriction. But if you are trying to tune
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a recirculating comb filter to a specific pitch, the highest you can get only
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comes to about 690 Hertz. To get shorter recirculating delays you must increase
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the sample rate or decrease the block size.
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<P>
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Example G04.control.blocksize.pd (Figure <A HREF="#fig07.27">7.27</A>) shows how the block size can be
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controlled in Pd using
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a new object:
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig07.27"></A><A NAME="8407"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 7.27:</STRONG>
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A patch using block size control to lower the loop delay below
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the normal 64 samples: (a) the main patch; (b) the ``delay-writer" subpatch
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with a <TT>block~</TT> object and a recirculating delay network.</CAPTION>
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<TR><TD><IMG
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WIDTH="558" HEIGHT="252" BORDER="0"
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SRC="img832.png"
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ALT="\begin{figure}\psfig{file=figs/fig07.27.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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<BR><!-- MATH
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$\fbox{ \texttt{block\~}}$
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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="img833.png"
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ALT="\fbox{ \texttt{block\~}}">,
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<BR><!-- MATH
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$\fbox{ \texttt{switch\~}}$
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-->
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<IMG
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WIDTH="78" HEIGHT="39" ALIGN="MIDDLE" BORDER="0"
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SRC="img834.png"
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ALT="\fbox{ \texttt{switch\~}}">:
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<A NAME="8427"></A><A NAME="8428"></A>Set the local block size of the patch window the object sits in. Block sizes
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are normally powers of two. The <TT>switch~</TT> object, in addition, can be used to
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turn audio computation within the window on and off, using control messages.
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Additional creation arguments can set the local sample rate and specify
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overlapping computations (demonstrated in Chapter <A HREF="node163.html#chapter-fft">9</A>).
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<P>
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In part (a) of the figure (the main patch), a rectangular pulse is sent
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to the <TT>pd delay-writer</TT> subpatch, whose output is then returned to the
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main patch. Part (b) shows the contents of the subpatch,
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which sends the pulses into
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a recirculating delay. The <TT>block~</TT> object specifies that, in this
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subpatch, signal computation uses a block size (<IMG
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WIDTH="16" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img311.png"
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ALT="$B$">) of only one. So the
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minimum achievable delay is one sample instead of the default 64.
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<P>
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Putting a pulse (or other excitation signal) into a recirculating comb filter
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to make a pitch is sometimes called
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<A NAME="8260"></A><I>Karplus-Strong synthesis</I>,
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having been described in a paper by them [<A
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HREF="node202.html#r-karplus83">KS83</A>], although the idea
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seems to be older. It shows up for example in Paul Lansky's 1979 piece,
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<I>Six Fantasies on a Poem by Thomas Campion</I>.
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<P>
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