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165 lines
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<H2><A ID="SECTION0011105000000000000000">
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Order of execution in non-recirculating delay lines</A>
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</H2>
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<P>
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In non-recirculating delay networks, it should be possible to place the
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operation of writing into the delay line earlier in the sequence than that
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of reading it. There should thus be no lower limit on the length of the
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delay line (except whatever is imposed by the interpolation scheme; see
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Section <A HREF="node113.html#sect7-fractional">7.7</A>). In languages such as Csound, the sequence
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of unit generator computation is (mostly) explicit, so this is easy to
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specify. In graphical patching environments, however, the order is
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implicit and another approach must be taken to ensuring that, for example,
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a <TT>delwrite~</TT> object is computed before the corresponding
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<TT>delread~</TT> object. One way of accomplishing this is shown in example
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G05.execution.order.pd (Figure <A HREF="#fig07.28">7.28</A>).
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<P>
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<DIV ALIGN="CENTER"><A ID="fig07.28"></A><A ID="8410"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 7.28:</STRONG>
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Using subpatches to ensure that delay lines are written before they
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are read in non-recirculating networks:
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(a) the <TT>delwrite~</TT> and <TT>vd~</TT> objects might be executed in either
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the "right" or the "wrong" order; (b) the <TT>delwrite~</TT> object is inside the
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<TT>pd delay-writer</TT>
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subpatch and the <TT>vd~</TT> object is inside the <TT>pd delay-reader</TT> one.
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Because of the audio connection between the two subpatches, the order of
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execution of the read/write pair is forced to be the correct one.</CAPTION>
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<TR><TD><IMG
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ALT="\begin{figure}\psfig{file=figs/fig07.28.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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In part (a) of the figure, the connections in the patch do not determine which
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order the two delay operations appear in the sorted sequence of tilde object
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computation; the <TT>delwrite~</TT> object could be computed either before or
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after the <TT>vd~</TT> object. If we wish to make sure the writing operation
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happens before the reading operation, we can proceed as in part (b) of the
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figure and put the two operations in subpatches, connecting the two via
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audio signals so that the first subpatch must be computed before the
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second one. (Audio computation in subpatches is done atomically, in the sense
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that the entire subpatch contents are considered as the audio computation for
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the subpatch as a whole. So everything in the one subpatch happens before
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anything in the second one.)
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<P>
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In this example, the "right"
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and the "wrong" way to make the comb filter have audibly different results.
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For delays less than 64 samples, the right hand side of the patch (using
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subpatches) gives the correct result, but the left hand side can't produce
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delays below the 64 sample block size.
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HREF="node120.html">Order of execution and</A>
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<ADDRESS>
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Miller Puckette
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2006-12-30
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