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172 lines
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172 lines
6.3 KiB
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<H2><A ID="SECTION00593000000000000000">
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Smoothed amplitude control with an envelope generator</A>
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</H2>
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<A ID="sect1.ex.3"></A>
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<P>
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As Figure <A HREF="node12.html#fig01.06">1.6</A> shows, one way to make smooth amplitude changes in a
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signal without clicks is to multiply it by the output of an envelope generator
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as shown in block diagram form in Figure <A HREF="node12.html#fig01.07">1.7</A>. This may be
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implemented in Pd using the <TT>line~</TT> object:
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<P>
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<BR><!-- MATH
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$\fbox{ $ \mathrm{line}\sim $}$
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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="img152.png"
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ALT="\fbox{ $ \mathrm{line}\sim $}">:
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<A ID="1404"></A>envelope generator.
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The output is a signal which ramps linearly from one value to another over
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time, as determined by the messages received. The inlets take messages to
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specify target values (left inlet) and time delays (right inlet). Because
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of a general rule of Pd messages, a pair of numbers sent to the left inlet
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suffices to specify a target value and a time together. The time is in
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milliseconds (taking into account the sample rate), and the target value is
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unitless, or in other words, its output range should conform to whatever
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input it may be connected to.
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<P>
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Example A03.line.pd demonstrates the use of a <TT>line~</TT> object to control the
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amplitude of a sinusoid. The active part is shown in Figure <A HREF="node20.html#fig01.12">1.12</A>
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(part b).
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The six message boxes are all connected to the <TT>line~</TT> object, and are
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activated by clicking on them; the top one, for instance, specifies that the
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<TT>line~</TT> ramp (starting at wherever its output was before receiving the
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message) to the value 0.1 over two seconds. After the two seconds elapse,
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unless other messages have arrived in the meantime, the output remains
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steady at 0.1. Messages may arrive before the two seconds elapse, in which
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case the <TT>line~</TT> object abandons its old trajectory and takes up a
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new one.
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<P>
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Two messages to <TT>line~</TT> might arrive at the same time or so close together
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in time that no DSP computation takes place between the two; in this case, the
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earlier message has no effect, since <TT>line~</TT> won't have changed its output
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yet to follow the first message, and its current output, unchanged, is then
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used as a starting point for the second segment. An exception to this rule
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is that, if <TT>line~</TT> gets a time value of zero, the output value is
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immediately set to the new value and further segments will start from the
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new value; thus, by sending two pairs, the first with a time value of
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zero and the second with a nonzero time value, one can independently specify
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the beginning and end values of a segment in <TT>line~</TT>'s output.
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<P>
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The treatment of <TT>line~</TT>'s right inlet is unusual among Pd objects in that
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it forgets old values; a message with a single number such as "0.1" is
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always equivalent to the pair, "0.1 0". Almost any other object will retain
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the previous value for the right inlet, instead of resetting it to zero.
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<P>
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Example A04.line2.pd shows the <TT>line~</TT> object's output graphically. Using
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the various message boxes, you can recreate the effects shown in Figure
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<A HREF="node12.html#fig01.06">1.6</A>.
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HREF="node20.html">Amplitude control in decibels</A>
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<ADDRESS>
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Miller Puckette
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2006-12-30
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