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222 lines
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222 lines
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<H1><A ID="SECTION00860000000000000000"></A>
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<A ID="sect4.tags"></A>
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<BR>
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Voice tags
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</H1>
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<P>
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Suppose now that we're using a voice bank to play notes, as in the example
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above, but suppose the notes <IMG
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WIDTH="11" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img4.png"
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ALT="$a$">, <IMG
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WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img21.png"
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ALT="$b$">, <IMG
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WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img293.png"
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ALT="$c$">, and <IMG
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WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img28.png"
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ALT="$d$"> all have the same
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pitch, and furthermore that all their other parameters are identical. How
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can we design a control stream so that, when any one note is turned off,
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we know which one it is?
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<P>
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This question doesn't come up if the control source is a clavier keyboard
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because it's impossible to play more than one simultaneous note on a single key.
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But it could easily arise algorithmically, or simply as a result of merging
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two keyboard streams together. Moreover, turning notes off is only the
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simplest example of a more general problem, which is how, once having set
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a task off in a voice bank, we can get back to the same voice to guide
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its evolution as a function of real-time inputs or any other unpredictable
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factor.
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<P>
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To deal with situations like this we can add one or more
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<A ID="4693"></A><I>tags</I> to the message starting a note (or, in general, a task).
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A tag is any collection of data with which we can later identify the task,
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which we can then use to search for the voice that is allocated for it.
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<P>
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Taking again the example of Figure <A HREF="node65.html#fig04.10">4.10</A>, here is one way we might
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write those four tasks as a control stream:
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<P>
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<PRE>
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start-time end-time pitch ...
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1 3 60 ...
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2 8 62
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4 6 64
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5 8 65
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</PRE>
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<P>
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In this representation we have no need of tags because each message (each line
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of text) contains all the information we need in order to specify the entire
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task. (Here we have assumed that the tasks <IMG
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WIDTH="11" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img4.png"
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ALT="$a$">, ..., <IMG
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WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img28.png"
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ALT="$d$"> are in
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fact musical notes with pitches 60, 62, 64, and 65.)
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In effect we're representing each task as a single event in a control stream
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(Section <A HREF="node43.html#sect3.controlstreams">3.3</A>).
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<P>
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On the other hand, if we suppose now that we do not know in advance the length
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of each note, a better representation would be this one:
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<P>
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<PRE>
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time tag action parameters
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1 a start 60 ...
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2 b start 62 ...
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3 a end
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4 c start 64 ...
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5 d start 65 ...
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6 c end
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8 b end
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8 d end
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</PRE>
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<P>
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Here each note has been split into two separate events to start and end it.
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The labels <IMG
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WIDTH="11" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img4.png"
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ALT="$a$">, ..., <IMG
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WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img28.png"
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ALT="$d$"> are used as tags;
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we know which start goes with which end since their tags are the same. Note
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that the tag is not necessarily related at all to the voice that will be used
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to play each note.
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<P>
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The MIDI standard does not supply tags; in normal use, the pitch of a note
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serves also as its tag (so tags are constantly being re-used.) If two
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notes having the same pitch must be addressed separately (to slide their
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pitches in different directions for example), the MIDI channel may be used (in
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addition to the note) as a tag.
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<P>
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In real-time music software it is often necessary to pass back and forth
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between the event-per-task representation and the tagged one above,
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since the first representation is better suited to storage and graphical
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editing, while the second is often better suited to real-time
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operations.
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<P>
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<B> Next:</B> <A ID="tex2html1530"
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HREF="node67.html">Encapsulation in Pd</A>
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<B> Up:</B> <A ID="tex2html1524"
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HREF="node58.html">Automation and voice management</A>
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<B> Previous:</B> <A ID="tex2html1518"
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HREF="node65.html">Voice allocation</A>
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<B> <A ID="tex2html1526"
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HREF="node4.html">Contents</A></B>
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HREF="node201.html">Index</A></B>
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<ADDRESS>
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Miller Puckette
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2006-12-30
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