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176 lines
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<H2><A ID="SECTION0011103000000000000000">
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Variable delay line</A>
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</H2>
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<P>
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The next example, G03.delay.variable.pd (Figure <A HREF="#fig07.25">7.25</A>), is another
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recirculating comb filter, this time using a variable-length delay line. One
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new object is introduced here:
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<P>
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<DIV ALIGN="CENTER"><A ID="fig07.25"></A><A ID="8215"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 7.25:</STRONG>
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The flanger: an interpolating, variable delay line.</CAPTION>
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<TR><TD><IMG
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WIDTH="527" HEIGHT="425" BORDER="0"
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ALT="\begin{figure}\psfig{file=figs/fig07.25.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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$\fbox{ \texttt{vd\~}}$
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-->
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<IMG
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WIDTH="44" HEIGHT="39" ALIGN="MIDDLE" BORDER="0"
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SRC="img825.png"
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ALT="\fbox{ \texttt{vd\~}}">:
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<A ID="8426"></A>Read from a delay line, with a time-varying delay time. As with the
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<TT>delread~</TT> object, this reads from a delay line whose name is specified
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as a creation argument. Instead of using a second argument and/or
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control messages to specify the delay time, for the <TT>vd~</TT> object the delay in milliseconds is specified by an incoming audio signal.
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The delay line is read using four-point (cubic) interpolation; the minimum
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achievable delay is one sample.
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<P>
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Here the objects on the left side, from the top down to the
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<TT>clip -0.2 0.2</TT> object,
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form a waveshaping network; the index is set by the
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"timbre" control, and the waveshaping output varies between a near sinusoid
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and a bright, buzzy sound. The output is added to the output of the
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<TT>vd~</TT> object. The sum is then high pass filtered (the <TT>hip~</TT> object
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at lower left), multiplied by a
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feedback gain, clipped, and written into the delay line at bottom right. There
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is a control at right to set the feedback gain; here, in contrast with the
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previous example, it is possible to specify a gain greater than one in order
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to get unstable feedback. For this reason the second <TT>clip~</TT> object is
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inserted within the delay loop (just above the <TT>delwrite~</TT> object) so
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that the signal cannot exceed 1 in absolute value.
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<P>
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The length of the delay is controlled by the signal input to the
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<TT>vd~</TT> object. An oscillator with variable frequency and gain, in the
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center of the figure, provides the delay time. The oscillator is added to
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one to make it nonnegative before multiplying it by the "cycle depth" control,
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which effectively sets the range of delay times. The minimum possible
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delay time of 1.46 milliseconds is added so that the true range of delay times
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is between the minimum and the same plus twice the "depth". The reason for this
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minimum delay time is taken up in the discussion of the next example.
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<P>
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Comb filters with variable delay times are sometimes called <I>flangers</I>.
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As the delay time changes the peaks in the frequency response move up and down
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in frequency, so that the timbre of the output changes constantly in a
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characteristic way.
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Miller Puckette
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2006-12-30
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