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<TITLE>Controlling reverberators</TITLE>
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HREF="node111.html">Artificial reverberation</A>
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HREF="node111.html">Artificial reverberation</A>
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<B> <A NAME="tex2html2223"
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HREF="node4.html">Contents</A></B>
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<H2><A NAME="SECTION001161000000000000000">
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Controlling reverberators</A>
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</H2>
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<P>
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Artificial reverberation is used almost universally in recording or sound
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reinforcement to sweeten the overall sound. However, and more interestingly,
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reverberation may be used as a sound source in its own right. The special case
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of infinite reverberation is useful for grabbing live sounds
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and extending them in time.
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<P>
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To make this work in practice it is necessary to open the input of the
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reverberator only for a short period of time, during which the input sound is
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not varying too rapidly. If an infinite reverberator's input is left open for
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too long, the sound will collect and quickly become an indecipherable mass. To
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``infinitely reverberate" a note of a live instrument, it is best to wait until
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after the attack portion of the note and then allow perhaps 1/2 second of the
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note's steady state to enter the reverberator. It is possible to build chords
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from a monophonic instrument by repeatedly opening the input at different
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moments of stable pitch.
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig07.16"></A><A NAME="8114"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 7.16:</STRONG>
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Controlling a reverberator to capture sounds selectively: (a)
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the network; (b) examples of how to control the input gain and feedback to
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capture two sounds at times <IMG
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WIDTH="16" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
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SRC="img37.png"
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ALT="$t_1$"> and <IMG
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WIDTH="16" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
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SRC="img38.png"
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ALT="$t_2$">, and to hold them until a later
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time <IMG
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WIDTH="16" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
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SRC="img39.png"
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ALT="$t_3$">.</CAPTION>
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<TR><TD><IMG
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WIDTH="449" HEIGHT="484" BORDER="0"
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SRC="img780.png"
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ALT="\begin{figure}\psfig{file=figs/fig07.16.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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Figure <A HREF="#fig07.16">7.16</A> shows how this can be done in practice. The two most
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important controls are the reverberator's input and feedback
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gains. To capture a sound, we set the feedback gain to one (infinite
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reverberation time) and momentarily open the input at time <IMG
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WIDTH="16" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
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SRC="img37.png"
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ALT="$t_1$">. To add other
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sounds to an already held one, we simply reopen the input gain at the
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appropriate moments (at time <IMG
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WIDTH="16" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
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SRC="img38.png"
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ALT="$t_2$"> in the figure, for example). Finally, we
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can erase the recirculating sound, thus both fading the output and emptying the
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reverberator, by setting the feedback gain to a value less than one (as at time
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<IMG
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WIDTH="16" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
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SRC="img39.png"
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ALT="$t_3$">). The further we reduce the feedback gain, the faster the output will
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decay.
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<B> Next:</B> <A NAME="tex2html2227"
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HREF="node113.html">Variable and fractional shifts</A>
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<B> Up:</B> <A NAME="tex2html2221"
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<B> <A NAME="tex2html2223"
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HREF="node4.html">Contents</A></B>
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