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<H3>Streams of Events and Iteration</H3>
Often it is necessary to repeat some simple event iteratively in order to
form a stream of similar events. One example of this might be repeatedly
striking an object at short (random) intervals in order to create a
dense granular texture. This section describes a common technique
for implementing such streams of events <sup><A HREF="UserManual_156.html#footnote-2">2</A></sup>.
<P>In order to describe an iterated event we will use the technique of
nested control structures described in the previous section, but in
a particular way, which allows an
event to reschedule itself once its time is up. The following example
script schedules a series of events to occur at one second intervals.
Each individual event is trivial in nature, simply printing a message
to the shell window showing the time at which it occurs (performance-time,
not real-time).
<P><PRE>
Audio rate: 44100;
Param eventStart=0.0, eventDur=0.01, interval=1.0;
Init:
...
Score 10 secs:
At eventStart for eventDur:
At start:
Print "Time=", Time, newline;
...
At end:
eventStart += interval;
...
...
...
</PRE>
<P>The first thing to note about this script is that it contains a
hierarchy of nested control structures. The outermost <CODE>Score</CODE>
control structure contains a single <CODE>At..for</CODE> structure, which in
turn contains two further <CODE>At</CODE> structures. The rest of the script
is quite straightforward to understand. The parameters <CODE>eventStart</CODE>
and <CODE>eventDur</CODE> are used to define the start time and duration of
each event and the parameter <CODE>interval</CODE> is used to define the
interval between successive events. The key element is the use of the
<CODE>At end:</CODE> control structure. Every time the event occurs
the body of the <CODE>At end:</CODE> structure is executed just once at the
very end of the event, and when it is a new start time is calculated
for the next event.
<P>The script produces the following output:
<P><PRE>
Sample rate=44100 KHz
Score duration=10 seconds
Time=0
Time=1
Time=2
Time=3
Time=4
Time=5
Time=6
Time=7
Time=8
Time=9
Time=10
</PRE>
<P>Of course the time interval between events does not have to be fixed.
The value by which the <CODE>eventStart</CODE> parameter is incremented
can be derived from an arbitrary mathematical expression (see
section <A HREF="UserManual_79.html">*</A> for details of expression syntax).
Since expressions can include numerical values derived from physical
attributes read off the various instruments and devices, this technique
opens the way for quite complex self-evolving events to be described.
This is one of <B>Tao</B>'s strengths: any physical attribute, such as the
velocity of a point on an instrument or the current height of a hammer
device can potentially be used as input to an algorithm which is playing
the very same instruments and devices.
<P>
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<tr><td colspan="3" class="addr"><!-- bottom panel --><ADDRESS><FONT SIZE=-1>©1999,2000 Mark Pearson
<A HREF="mailto:m.pearson@ukonline.co.uk">m.pearson@ukonline.co.uk</A> April 30, 2000</ADDRESS><BR></td></tr><!-- end bottom panel --></table></BODY></HTML>
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