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HREF="node47.html">Audio signals as control</A>
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<H1><A ID="SECTION00780000000000000000">
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Operations on control streams</A>
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</H1>
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<P>
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So far we've discussed how to convert between control streams and audio
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streams. In addition to this possibility, there are four types of
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operations
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you can perform on control streams to get other control streams. These control
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stream operations have no corresponding operations on audio signals. Their
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existence explains in large part why it is useful to introduce a whole
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control structure in parallel with that of audio signals.
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<P>
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The first type consists of
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<A ID="3671"></A>
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<I>delay</I>
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operations, which offset the time values associated with a control stream.
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In real-time systems the delays can't be negative in value. A control stream
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may be delayed by a constant amount, or alternatively, you can delay each event
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separately by different amounts.
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<P>
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Two different types of delay are used in practice:
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<A ID="3673"></A>
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<I>simple</I> and
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<A ID="3675"></A>
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<I>compound</I>. Examples of each are shown in Figure <A HREF="#fig03.10">3.10</A>. A simple
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delay acting on a control stream schedules each event, as it comes in,
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for a time in the future. However, if another event arrives at the input
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before the first event is output, the first event is forgotten in favor of the
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second. In a compound delay, each event at the input produces an output, even
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if other inputs arrive before the output appears.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig03.10"></A><A ID="3680"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 3.10:</STRONG>
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Delay as an operation on a control stream: (a) block
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diagram; (b) effect of a simple delay on a control stream; (c) effect
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of a compound delay.</CAPTION>
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<TR><TD><IMG
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WIDTH="338" HEIGHT="384" BORDER="0"
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SRC="img335.png"
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ALT="\begin{figure}\psfig{file=figs/fig03.10.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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A second operation on control steams is
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<A ID="3683"></A><I>merging</I>:
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taking two control streams and combining
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all the events into a new one. Figure <A HREF="#fig03.11">3.11</A> (part a) shows how this
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and the remaining operations are represented in block diagrams.
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<P>
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Part (b) of the figure shows the effect of merging two streams. Streams may
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contain more than one event at the same time. If two streams to be merged
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contain events at the same time, the merged stream contains them both, in a
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well-defined order.
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<P>
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A third type of operation on control streams is
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<A ID="3686"></A><I>pruning</I>.
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Pruning a control stream means looking at the associated data and letting only
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certain elements through. Part (c) shows an example, in
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which events (which each have an associated number) are passed through only if
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the number is positive.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig03.11"></A><A ID="3690"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 3.11:</STRONG>
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Operations on control streams (besides delay): (a) block diagrams;
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(b) merging; (c) pruning; (d) resynchronizing.</CAPTION>
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<TR><TD><IMG
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WIDTH="344" HEIGHT="533" BORDER="0"
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SRC="img336.png"
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ALT="\begin{figure}\psfig{file=figs/fig03.11.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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Finally, there is the concept of <A ID="3693"></A>
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<I>resynchronizing</I> one control stream to another, as shown in part (d).
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Here one control stream (the source) contributes values which are
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put onto the time sequence of a second one (the sync). The value given the
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output is always the most recent one from the source stream. Note that any
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event from the source may appear more than once (as suggested in the figure),
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or, on the other hand, it might not appear at all.
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<P>
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Again, we have to consider what happens when the two streams each contain an
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event at the same time. Should the sync even be considered as happening before
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the source (so that the output gets the value of the previous source event)?
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Or should the source event be considered as being first so that its value goes
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to the output at the same time? How this should be disambiguated is a design
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question, to which various software environments take various approaches. (In
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Pd it is controlled explicitly by the user.)
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