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<H1><A ID="SECTION00850000000000000000"></A>
<A ID="sect4.voicealloc"></A>
<BR>
Voice allocation
</H1>
<P>
It is frequently desirable to automate the selection of voices to associate
with individual
<A ID="4677"></A><I>tasks</I>
(such as notes to play). For example, a musician playing
at a keyboard can't practically choose which voice should go with each note
played. To automate voice selection we need a voice allocation algorithm, to
be used as shown in Figure <A HREF="#fig04.09">4.9</A>.
<P>
Armed with a suitable voice allocation algorithm, the control source need not
concern itself with the detail of which voice is taking care of which task;
algorithmic note generators and sequencers frequently rely on this. On the
other hand, musical writing for ensembles frequently specifies explicitly
which instrument plays which note, so that the notes will connect to each other
end-to-end in a desirable way.
<P>
<DIV ALIGN="CENTER"><A ID="fig04.09"></A><A ID="4682"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.9:</STRONG>
Polyphonic voice allocation.</CAPTION>
<TR><TD><IMG
WIDTH="166" HEIGHT="388" BORDER="0"
SRC="img372.png"
ALT="\begin{figure}\psfig{file=figs/fig04.09.ps}\end{figure}"></TD></TR>
</TABLE>
</DIV>
<P>
One simple voice allocation algorithm works as shown in Figure <A HREF="#fig04.10">4.10</A>. Here we
suppose that the voice bank has only two voices, and we try to allocate voices
for the tasks <IMG
WIDTH="11" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img4.png"
ALT="$a$">, <IMG
WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img21.png"
ALT="$b$">, <IMG
WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img293.png"
ALT="$c$">, and <IMG
WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img28.png"
ALT="$d$">. Things go smoothly until task <IMG
WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img28.png"
ALT="$d$">
comes along, but then we see no free voices (they are taken up by <IMG
WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img21.png"
ALT="$b$"> and <IMG
WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img293.png"
ALT="$c$">).
We could now elect either to drop task <IMG
WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img28.png"
ALT="$d$">, or else to steal the voice of
either task <IMG
WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img21.png"
ALT="$b$"> or <IMG
WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img293.png"
ALT="$c$">. In practice the best choice is usually to steal one.
In this particular example, we chose to steal the voice of the oldest task,
<IMG
WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img21.png"
ALT="$b$">.
<P>
<DIV ALIGN="CENTER"><A ID="fig04.10"></A><A ID="4688"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.10:</STRONG>
A polyphonic voice allocation algorithm, showing voice stealing.</CAPTION>
<TR><TD><IMG
WIDTH="371" HEIGHT="218" BORDER="0"
SRC="img373.png"
ALT="\begin{figure}\psfig{file=figs/fig04.10.ps}\end{figure}"></TD></TR>
</TABLE>
</DIV>
<P>
If we happen to know the length of the tasks <IMG
WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img21.png"
ALT="$b$"> and <IMG
WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img293.png"
ALT="$c$"> at the
outset of task <IMG
WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img28.png"
ALT="$d$">, we may be able to make a better choice of which voice
to steal. In this example it might have been better to steal from <IMG
WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img293.png"
ALT="$c$">, so
that <IMG
WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img28.png"
ALT="$d$"> and <IMG
WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img21.png"
ALT="$b$"> would be playing together at the end and not <IMG
WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img28.png"
ALT="$d$"> alone.
In some situations this information will be available when the choice must be
made, and in some (live keyboard input, for example) it will not.
<P>
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<ADDRESS>
Miller Puckette
2006-12-30
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