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<TITLE>Quick Introduction to Pd</TITLE>
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<B> Next:</B> <A NAME="tex2html780"
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HREF="node15.html">About the Software Examples</A>
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HREF="node15.html">About the Software Examples</A>
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<B> <A NAME="tex2html776"
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<H2><A NAME="SECTION00581000000000000000">
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Quick Introduction to Pd</A>
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</H2>
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<P>
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Pd documents are called
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<A NAME="1211"></A><I>patches</I>. They correspond roughly to the
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boxes in the abstract block diagrams shown earlier in this chapter, but in
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detail they
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are quite different, because Pd is an implementation
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environment, not a specification language.
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<P>
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A Pd patch, such as the ones shown in
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Figure <A HREF="#fig01.10">1.10</A>, consists of a collection of
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<A NAME="1214"></A><I>boxes</I>
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connected in a network.
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The border of a box tells you how its text is interpreted and how the box
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functions. In part (a) of the figure we see three types of boxes. From
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top to bottom they are:
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<UL>
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<LI>a
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<A NAME="1217"></A><A NAME="1218"></A><I>message box</I>.
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Message boxes, with a flag-shaped border, interpret the text as a message to
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send whenever the box is
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activated (by an incoming message or with a pointing device). The message in this
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case consists simply of the number ``21".
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<P>
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</LI>
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<LI>an
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<A NAME="1220"></A><A NAME="1221"></A><I>object box</I>.
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Object boxes have a rectangular border; they interpret the text to create
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objects
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when you load a patch. Object boxes may hold hundreds of different
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classes of objects--including oscillators, envelope generators, and other
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signal processing modules to be introduced later--depending on the text
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inside. In this example, the box holds an adder. In most Pd patches, the
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majority of boxes are of type ``object". The first word typed into an object
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box specifies its
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<A NAME="1223"></A><I>class</I>,
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which in this case is just ``+". Any additional (blank-space-separated) words
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appearing in the box are called
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<A NAME="1225"></A><A NAME="1226"></A><I>creation arguments</I>,
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which specify the initial state of the object when it is created.
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<P>
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</LI>
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<LI>a
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<A NAME="1228"></A><A NAME="1229"></A><I>number box</I>.
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Number boxes are a particular type of
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<A NAME="1231"></A><A NAME="1232"></A><I>GUI box</I>. Others include push buttons and
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toggle switches; these will come up later in the examples.
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The number box
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has a punched-card-shaped border, with a nick out of its top right corner.
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Whereas the appearance
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of an object or message box is fixed when a patch is running, a number box's
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contents (the text) changes to reflect the current value held by the box. You
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can also use a number box as a control by clicking and dragging up and down, or
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by typing values in it.
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<P>
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</LI>
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</UL>
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In Figure
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<A HREF="#fig01.10">1.10</A> (part a) the message box, when clicked, sends the message ``21" to an
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object box which adds 13 to it. The lines connecting the boxes carry data
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from one box to the next; outputs of boxes are on the bottom and inputs on top.
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig01.10"></A><A NAME="1238"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 1.10:</STRONG>
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(a) three types of boxes in Pd (message, object, and GUI); (b) a simple patch to output a
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sinusoid.</CAPTION>
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<TR><TD><IMG
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WIDTH="500" HEIGHT="206" BORDER="0"
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SRC="img144.png"
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ALT="\begin{figure}\psfig{file=figs/fig01.10.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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Figure <A HREF="#fig01.10">1.10</A> (part b) shows a Pd patch which
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makes a sinusoid with controllable frequency and amplitude. The connecting
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patch lines are of two types here; the thin ones are for carrying sporadic
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<A NAME="1242"></A><I>messages</I>,
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and the thicker ones (connecting the oscillator, the multiplier, and the output
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<TT>dac~</TT> object) carry digital audio signals. Since Pd is a real-time
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program, the audio signals flow in a continuous stream. On the other hand, the
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sporadic messages appear at specific but possibly unpredictable instants in
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time.
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<P>
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Whether a connection carries messages or signals depends on the box
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the connection comes from; so, for instance, the <TT>+</TT>
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object outputs messages, but the <TT>*~</TT> object
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outputs a signal. The inputs of a given object may or may not accept
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signals (but they always accept messages, even if only to convert them to
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signals). As a convention, object boxes with signal inputs or outputs
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are all named with a trailing tilde (``<TT>~</TT>") as in ``<TT>*~</TT>"
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and ``<TT>osc~</TT>".
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