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177 lines
6.3 KiB
177 lines
6.3 KiB
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HREF="node61.html">Continuous and discontinuous control</A>
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<H2><A ID="SECTION00831000000000000000"></A>
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<A ID="sect4.muting"></A>
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<BR>
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Muting
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</H2>
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<P>
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The
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<A ID="4634"></A><I>muting</I>
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technique is to apply an envelope to the output amplitude, which is quickly
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ramped to zero before the parameter change and then restored afterward. It
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may or may not be the case that the discontinuous changes will result in a signal
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that rises smoothly from zero afterward. In Figure <A HREF="#fig04.05">4.5</A> (part a),
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we take the example of an amplitude envelope (the output signal of an ADSR
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generator), and assume that the discontinuous change is to start a new note at
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amplitude zero.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig04.05"></A><A ID="4639"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.5:</STRONG>
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Muting technique for hiding discontinuous changes: (a) the
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envelope (upper graph) is set discontinuously to zero and
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the muting signal (lower graph) ramps down in advance to prepare for the change, and
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then is restored (discontinuously) to its previous value; (b) the envelope
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changes discontinuously between two nonzero values; the muting signal must
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both ramp down beforehand and ramp back up afterward.</CAPTION>
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<TR><TD><IMG
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WIDTH="327" HEIGHT="334" BORDER="0"
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SRC="img368.png"
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ALT="\begin{figure}\psfig{file=figs/fig04.05.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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To change the ADSR generator's output discontinuously we
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<A ID="4642"></A><I>reset</I> it. This is a different operation from triggering it; the
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result is to make it jump to a new value, after which we may either simply
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leave it there or trigger it anew. Figure <A HREF="#fig04.05">4.5</A> (part a)
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shows the effect of resetting and retriggering an ADSR generator.
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<P>
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Below the ADSR generator output we see the appropriate muting signal, which ramps to zero
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to prepare for the discontinuity. The amount of time we allow for muting
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should be small (so as to disrupt the previous sound as little as possible)
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but not so small as to cause audible artifacts in the output.
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A working example of this type of muting was already shown on Page
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<A HREF="node53.html#sect3.noloopsampler"><IMG ALIGN="BOTTOM" BORDER="1" ALT="[*]"
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SRC="crossref.png"></A>; there we allowed 5 msec for ramping down.
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The muting signal is multiplied by the output of the process to be de-clicked.
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<P>
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Figure <A HREF="#fig04.05">4.5</A> (part b) shows the situation in which we suppose the
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discontinuous change is between two possibly nonzero values. Here the muting
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signal must not only ramp down as before (in advance of the discontinuity) but
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must also ramp back up afterward. The ramp-down time need not equal the
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ramp-up time; these must be chosen, as always, by listening to the output
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sound.
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<P>
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In general, muting presents the difficulty that you must start the muting
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operation in advance of making the desired control change. In real-time
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settings, this often requires that we intentionally delay the control change.
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This is another reason for keeping the muting time as low as possible.
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(Moreover, it's a bad idea to try to minimize delay by conditionally
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omitting the ramp-down period when it isn't needed; a constant delay
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is much better than one that varies, even if it is smaller on average.)
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HREF="node61.html">Continuous and discontinuous control</A>
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HREF="node61.html">Continuous and discontinuous control</A>
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<ADDRESS>
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Miller Puckette
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2006-12-30
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