Quick Introduction to Pd

Pd documents are called patches. They correspond roughly to the boxes in the abstract block diagrams shown earlier in this chapter, but in detail they are quite different, because Pd is an implementation environment, not a specification language.

A Pd patch, such as the one shown in Figure 1.10, consists of a collection of boxes connected in a network. The border of a box tells you how its text is interpreted and how the box functions. In part (a) of the figure we see three types of boxes. From top to bottom they are:

• a message box. Message boxes, with a flag-shaped border, interpret the text as a message to send whenever the box is activated (by an incoming message or with a pointing device). The message in this case consists simply of the number ``21".

• an object box. Object boxes have a rectangular border; they interpret the text to create objects when you load a patch. Object boxes may hold hundreds of different classes of objects--including oscillators, envelope generators, and other signal processing modules to be introduced later--depending on the text inside. In this example, the box holds an adder. In most Pd patches, the majority of boxes are of type ``object". The first word typed into an object box specifies its class, which in this case is just ``+". Any additional (blank-space-separated) words appearing in the box are called creation arguments, which specify the initial state of the object when it is created.

• a number box. Number boxes are a particular type of GUI box. Others include push buttons and toggle switches; these will come up later in the examples. The number box has a punched-card-shaped border, with a nick out of its top right corner. Whereas the appearance of an object or message box is fixed when a patch is running, a number box's contents (the text) changes to reflect the current value held by the box. You can also use a number box as a control by clicking and dragging up and down, or by typing values in it.

In Figure 1.10 (part a) the message box, when clicked, sends the message ``21" to an object box which adds 13 to it. The lines connecting the boxes carry data from one box to the next; outputs of boxes are on the bottom and inputs on top.

Figure 1.10: (a) three types of boxes in Pd (message, object, and GUI); (b) a simple patch to output a sinusoid.

Figure 1.10 (part b) shows a Pd patch which makes a sinusoid with controllable frequency and amplitude. The connecting patch lines are of two types here; the thin ones are for carrying sporadic messages, and the thicker ones (connecting the oscillator, the multiplier, and the output dac~ object) carry digital audio signals. Since Pd is a real-time program, the audio signals flow in a continuous stream. On the other hand, the sporadic messages appear at specific but possibly unpredictable instants in time.

Whether a connection carries messages or signals depends on the box the connection comes from; so, for instance, the + object outputs messages, but the *~ object outputs a signal. The inputs of a given object may or may not accept signals (but they always accept messages, even if only to convert them to signals). As a convention, object boxes with signal inputs or outputs are all named with a trailing tilde (``~") as in ``*~" and ``osc~".