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original version by: Nikos Drakos, CBLU, University of Leeds
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<H2><A ID="SECTION00664000000000000000">
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Looping samplers</A>
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</H2>
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<A ID="sect2.example.loop"></A>
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<P>
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In most situations, you'll want a more automated way than moving the mouse to
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specify wavetable read locations; for instance, you might want to be able to
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play a sample at a steady transposition; you might have several samples playing
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back at once (or other things requiring attention), or you might want to switch
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quickly between samples or go to prearranged locations. In the next few
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examples we'll develop an automated looping sample reader, which, although only
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one of many possible approaches, is a powerful and often-used one.
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<P>
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Patches B05.sampler.loop.pd and B06.sampler.loop.smooth.pd show how to do this: the former in the simplest
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possible way and the latter (pictured in Figure <A HREF="#fig02.15">2.15</A>, part a)
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incorporating a second waveshape to envelope the sound as described in
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Section <A HREF="node29.html#sect2.enveloping">2.3</A>. One new object class
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is introduced here:
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<P>
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<DIV ALIGN="CENTER"><A ID="fig02.15"></A><A ID="2443"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 2.15:</STRONG>
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(a) a looping sampler with a synchronized envelope (B06.sampler.loop.smooth.pd); (b) the
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same, but with a control for read location (B08.sampler.nodoppler.pd).</CAPTION>
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<TR><TD><IMG
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WIDTH="546" HEIGHT="547" BORDER="0"
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SRC="img281.png"
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ALT="\begin{figure}\psfig{file=figs/fig02.15.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{ $\mathrm{cos}\sim$\ }$
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-->
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<IMG
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WIDTH="63" HEIGHT="33" ALIGN="MIDDLE" BORDER="0"
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SRC="img282.png"
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ALT="\fbox{ $\mathrm{cos}\sim$\ }">:
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<A ID="2559"></A>compute the cosine of <IMG
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WIDTH="21" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img16.png"
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ALT="$2\pi $"> times the input signal (so that 0 to 1 makes a whole
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cycle). Unlike the table reading classes in Pd, <TT>cos~</TT> handles
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wraparound so that there is no range limitation on its input.
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<P>
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In Figure <A HREF="#fig02.15">2.15</A> (part a), a <TT>phasor~</TT> object supplies both
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indices into the wavetable (at right) and phases for a half-cosine-shaped
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envelope function at left. These two are multiplied, and the product is
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high-pass filtered and output. Reading the wavetable is straightforward; the
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phasor is multiplied by a "chunk size" parameter, added to 1, and used as an
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index to <TT>tabread4~</TT>The chunk size parameter is multiplied by
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441 to convert it from hundredths of a second to samples. This corresponds
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exactly to the block diagram shown in Figure <A HREF="node28.html#fig02.05">2.5</A>, with a segment
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location of 1. (The segment location can't be 0 because 1 is the minimum index
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for which <TT>tabread4~</TT> works.)
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<P>
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The left-hand signal path in the example corresponds to the enveloping
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wavetable lookup technique shown in Figure <A HREF="node29.html#fig02.07">2.7</A>. Here the sawtooth
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wave is adjusted to the range (-1/4, 1/4) (by subtracting and multiplying by
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0.5), and then sent to <TT>cos~</TT>. This reads the cosine
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function in the range (<IMG
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WIDTH="41" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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SRC="img283.png"
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ALT="$-\pi/2$">, <IMG
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WIDTH="29" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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SRC="img5.png"
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ALT="$\pi /2$">), thus giving only the positive half
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of the waveform.
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<P>
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Part (b) of Figure <A HREF="#fig02.15">2.15</A> introduces a third parameter, the "read
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point", which specifies where in the sample the loop is to start. (In part (a)
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we always started at the beginning.) The necessary change is simple enough:
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add the "read point" control value, in samples,
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to the wavetable index and proceed as before. To avoid discontinuities
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in the index we smooth the read point value using
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<TT>pack</TT> and <TT>line~</TT> objects, just as we did in
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the first sampler example (Figure <A HREF="node35.html#fig02.14">2.14</A>).
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<P>
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This raises an important, though subtle, issue. The Momentary
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Transposition Formula (Page <A HREF="node28.html#sect2.momentaryformula"><IMG ALIGN="BOTTOM" BORDER="1" ALT="[*]"
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SRC="crossref.png"></A>) predicts that, as
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long as the chunk size and read point aren't changing in time, the
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transposition is just the frequency times the chunk size (as always, using
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appropriate units; Hertz and seconds, for example, so that the product is
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dimensionless). However, varying the chunk size and read point in time will
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affect the momentary transposition, often in very noticeable ways, as can be
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heard in Example B07.sampler.scratch.pd. Example B08.sampler.nodoppler.pd (the one shown in the figure) shows
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one possible way of controlling this effect, while introducing a new object
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class:
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<P>
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<BR><!-- MATH
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$\fbox{ $\mathrm{samphold}\sim$\ }$
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-->
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<IMG
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WIDTH="108" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
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SRC="img284.png"
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ALT="\fbox{ $\mathrm{samphold}\sim$\ }">:
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<A ID="2560"></A>a sample and hold unit. (This will be familiar to analog synthesizer users,
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but with a digital twist; for more details see Section <A HREF="node47.html#sect3.analog">3.7</A>.)
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This stores a single sample of the left-hand-side
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input and outputs it repeatedly, until caused by the right-hand-side input (also
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a digital audio signal, called the <EM>trigger</EM>) to overwrite the stored
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sample with a new one--again
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from the left-hand-side input. The unit acquires a new sample whenever
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the trigger's numerical value falls from one sample to the next. This is
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designed to be easy to pair with <TT>phasor~</TT> objects, to facilitate
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triggering on phase wraparounds.
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<P>
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Example B08.sampler.nodoppler.pd uses two <TT>samphold~</TT> objects to update the values
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of the chunk size and read point, exactly when the <TT>phasor~</TT> wraps around, at which moments the cosine envelope is at zero so the effect
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of the instantaneous changes can't be heard. In this situation we can apply
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the simpler Transposition Formula for Looping Wavetables to relate frequency,
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chunk size, and transposition. This is demonstrated in Example B09.sampler.transpose.pd (not shown).
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<P>
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HREF="node32.html">Examples</A>
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HREF="node35.html">Using a wavetable as</A>
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HREF="node4.html">Contents</A></B>
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<ADDRESS>
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Miller Puckette
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2006-12-30
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