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<H2><A ID="SECTION00885000000000000000">
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Polyphonic synthesis: sampler</A>
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</H2>
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<A ID="4831"></A>
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<P>
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We move now to an example using dynamic voice allocation as described in
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Section <A HREF="node65.html#sect4.voicealloc">4.5</A>. In the additive synthesis examples shown
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previously, each voice is used for a fixed purpose. In the
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present example, we allocate voices from a bank as needed to play
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notes in a control stream.
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<P>
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Example D11.sampler.poly.pd (Figure <A HREF="#fig04.20">4.20</A>) shows the polyphonic sampler, which
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uses the abstraction <TT>sampvoice</TT> (whose interior
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is shown in Figure <A HREF="#fig04.21">4.21</A>).
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The techniques for altering the pitch and other parameters in a one-shot
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sampler are shown in Example D10.sampler.notes.pd (not shown) which in turn is derived from
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the original one-shot sampler from the previous chapter (C05.sampler.oneshot.pd, shown
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in Figure <A HREF="node53.html#fig03.14">3.14</A>).
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<P>
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<DIV ALIGN="CENTER"><A ID="fig04.20"></A><A ID="4839"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.20:</STRONG>
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A polyphonic sampler demonstrating voice allocation and use
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of tags.</CAPTION>
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<TR><TD><IMG
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WIDTH="459" HEIGHT="598" BORDER="0"
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SRC="img393.png"
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ALT="\begin{figure}\psfig{file=figs/fig04.20.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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<DIV ALIGN="CENTER"><A ID="fig04.21"></A><A ID="4895"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.21:</STRONG>
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The <TT>sampvoice</TT> abstraction used in the polyphonic sampler
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of Figure <A HREF="#fig04.20">4.20</A>.</CAPTION>
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<TR><TD><IMG
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WIDTH="562" HEIGHT="630" BORDER="0"
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SRC="img394.png"
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ALT="\begin{figure}\psfig{file=figs/fig04.21.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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The <TT>sampvoice</TT> objects in Figure <A HREF="#fig04.20">4.20</A> are arranged in a
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different kind of summing bus from the ones before; here, each one adds its
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own output to the signal on its inlet, and puts the sum on its outlet. At the
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bottom of the eight objects, the outlet therefore holds the sum of all eight.
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This has the advantage of being more explicit than the <TT>throw~</TT> /
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<TT>catch~</TT> busses, and is preferable when visual clutter is not a
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problem.
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<P>
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The main job of the patch, though, is to distribute the "note" messages to
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the <TT>sampvoice</TT> objects. To do this we must introduce some new Pd
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objects:
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<P>
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<BR><!-- MATH
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$\fbox{ $\mathrm{mod}$}$
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-->
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<IMG
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WIDTH="50" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
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SRC="img395.png"
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ALT="\fbox{ $\mathrm{mod}$}">:
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<A ID="4910"></A><A ID="4911"></A>Integer modulus. For instance, 17 mod 10 gives 7, and -2 mod 10 gives 8.
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There is also an integer division object named <TT>div</TT> ; dividing 17 by
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10 via <TT>div</TT> gives 1, and -2 by 10 gives -1.
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<P>
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<BR><!-- MATH
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$\fbox{ $\mathrm{poly}$}$
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-->
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<IMG
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WIDTH="49" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
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SRC="img397.png"
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ALT="\fbox{ $\mathrm{poly}$}">:
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<A ID="4912"></A>Polyphonic voice allocator. Creation arguments give the number of
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voices in the bank and a flag (1 if voice stealing is needed, 0 if not).
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The inlets are a numeric tag at left and a flag at right indicating whether
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to start or stop a voice with the given tag (nonzero numbers meaning "start"
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and zero, "stop"). The outputs are, at left, the voice number, the tag
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again at center, and the start/stop flag at right. In MIDI applications, the
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tag can be pitch and the start/stop flag can be the note's velocity.
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<P>
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<BR><!-- MATH
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$\fbox{ \texttt{makenote}}$
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-->
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<IMG
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WIDTH="86" HEIGHT="39" ALIGN="MIDDLE" BORDER="0"
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SRC="img398.png"
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ALT="\fbox{ \texttt{makenote}}">:
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<A ID="4913"></A>Supply delayed note-off messages to match note-on messages. The inlets are
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a tag and start/stop flag ("pitch" and "velocity" in MIDI usage) and the
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desired duration in milliseconds. The tag/flag pair are repeated to
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the two outlets as they are received; then, after the delay, the tag is
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repeated with flag zero to stop the note after the desired duration.
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<P>
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The "note" messages contain fields for pitch, amplitude, duration,
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sample number, start location in the sample, rise time, and decay time. For
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instance, the message,
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<PRE>
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60 90 1000 2 500 10 20
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</PRE>
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(if received by the <TT>r note</TT> object)
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means to play a note at pitch 60 (MIDI units), amplitude 90 dB, one second
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long, from the wavetable named "sample2", starting at a point 500 msec
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into the wavetable, with rise and decay times of 10 and 20 msec.
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<P>
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After unpacking the message into its seven components, the patch
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creates a tag for the note. To do this, first the <TT>t b f</TT> object
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outputs a bang after the last of the seven parameters appear separately. The
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combination of the <TT>+</TT>, <TT>f</TT>, and <TT>mod</TT> objects acts
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as a counter that repeats after a million steps, essentially generating
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a unique number corresponding to the note.
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<P>
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The next step is to use the <TT>poly</TT> object to determine which voice to play
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which note. The <TT>poly</TT> object expects separate messages to start
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and stop tasks (i.e., notes). So the tag and duration are first fed to the
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<TT>makenote</TT> object, whose outputs include a flag ("velocity") at
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right and the tag again at left. For each tag <TT>makenote</TT> receives, two pairs
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of numbers are output, one to start the note, and another, after a delay
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equal to the note duration, to stop it.
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<P>
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Having treated <TT>poly</TT> to this separated input, we now have to strip
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the messages corresponding to the ends of notes, since we really only need
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combined "note" messages with
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duration fields. The <TT>stripnote</TT> object does this job. Finally, the
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voice number we have calculated is prepended to the seven parameters we
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started with (the <TT>pack</TT> object), so that the output of the
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<TT>pack</TT> object looks like this:
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<PRE>
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4 60 90 1000 2 500 10 20
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</PRE>
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where the "4" is the voice number output by the <TT>poly</TT> object.
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The voice number is used to route the message
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to the desired voice using the <TT>route</TT> object. The appropriate
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<TT>sampvoice</TT> object then gets the original list starting with
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"60".
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<P>
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Inside the <TT>sampvoice</TT> object (Figure <A HREF="#fig04.21">4.21</A>), the message
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is used to control the <TT>tabread4~</TT> and surrounding <TT>line~</TT> and <TT>vline~</TT> objects. The control takes place with a delay of
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5 msec as in the earlier sampler example. Here, however, we must
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store the seven parameters of the note (earlier there were no
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parameters). This is done using the six <TT>f</TT> objects, plus the
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right inlet of the rightmost <TT>delay</TT> object. These values are used after
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the delay of 5 msec. This is done in tandem with the muting
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mechanism described on Page <A HREF="node62.html#sect4.muting"><IMG ALIGN="BOTTOM" BORDER="1" ALT="[*]"
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SRC="crossref.png"></A>, using another
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<TT>vline~</TT> object.
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<P>
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When the 5 msec have elapsed, the <TT>vline~</TT> object in charge of
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generating the wavetable index gets its marching orders (and,
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simultaneously, the wavetable number is set for the <TT>tabread4~</TT> object and the amplitude envelope generator starts its attack.)
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The wavetable index must be set discontinuously to the starting index, then
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ramped to an ending index over an appropriate time duration to obtain the
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needed transposition. The starting index in samples is just 44.1 times the
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starting location in milliseconds, plus one to allow for four-point table
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interpolation. This becomes the third number in a packed
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list generated by the <TT>pack</TT> object at the center of the voice patch.
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<P>
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We arbitrarily decide that the ramp will last ten thousand seconds (this is the
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"1e+07" appearing in the message box sent to the wavetable index generator),
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hoping that this is at least as long as any note we will play. The ending index
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is the starting index plus the number of samples to ramp through. At a
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transposition factor of one, we should move by 441,000,000 samples during those
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10,000,000 milliseconds, or proportionally more or less depending on the
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transposition factor. This transposition factor is computed by the <TT>mtof</TT> object, dividing by 261.62 (the frequency corresponding to MIDI note 60) so
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that a specified "pitch" of 60 results in a transposition factor of one.
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<P>
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These and other parameters are combined in one message
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via the <TT>pack</TT> object so that the following message boxes can
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generate the needed control messages. The only novelty is
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the <TT>makefilename</TT> object, which converts numbers such as "2" to
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symbols such as "sample2" so that the <TT>tabread4~</TT> object's
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wavetable may be set.
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<P>
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At the bottom of the voice patch we see how a summing bus is implemented inside
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a subpatch; an <TT>inlet~</TT> object picks up the sum of all the preceding
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voices, the output of the current voice is added in, and the result is
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sent on to the next voice via the <TT>outlet~</TT> object.
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<ADDRESS>
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Miller Puckette
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2006-12-30
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