Example A01.sinewave.pd, shown in Figure 1.11, contains essentially the simplest possible patch that makes a sound, with only three object boxes. (There are also comments, and two message boxes to turn Pd's ``DSP" (audio) processing on and off.) The three object boxes are:
: sinusoidal oscillator. The left hand side input and the output are digital audio signals. The input is taken to be a (possibly time-varying) frequency in Hertz. The output is a sinusoid at the specified frequency. If nothing is connected to the frequency inlet, the creation argument (440 in this example) is used as the frequency. The output has peak amplitude one. You may set an initial phase by sending messages (not audio signals) to the right inlet. The left (frequency) inlet may also be sent messages to set the frequency, since any inlet that takes an audio signal may also be sent messages which are automatically converted to the desired audio signal.
: multiplier. This exists in two forms. If a creation argument is specified (as in this example; it's 0.05), this box multiplies a digital audio signal (in the left inlet) by the number; messages to the right inlet can update the number as well. If no argument is given, this box multiplies two incoming digital audio signals together.
: audio output device. Depending on your hardware, this might not actually be a Digital/Analog Converter as the name suggests; but in general, it allows you to send any audio signal to your computer's audio output(s). If there are no creation arguments, the default behavior is to output to channels one and two of the audio hardware; you may specify alternative channel numbers (one or many) using the creation arguments. Pd itself may be configured to use two or more output channels, or may not have the audio output device open at all; consult the Pd documentation for details.
The two message boxes show a peculiarity in the way messages are parsed in message boxes. Earlier in Figure 1.10 (part a), the message consisted only of the number 21. When clicked, that box sent the message ``21" to its outlet and hence to any objects connected to it. In this current example, the text of the message boxes starts with a semicolon. This is a terminator between messages (so the first message is empty), after which the next word is taken as the name of the recipient of the following message. Thus the message here is ``dsp 1" (or ``dsp 0") and the message is to be sent, not to any connected objects--there aren't any anyway--but rather, to the object named ``pd". This particular object is provided invisibly by the Pd program and you can send it various messages to control Pd's global state, in this case turning audio processing on (``1") and off (``0").
Many more details about the control aspects of Pd, such as the above, are explained in a different series of example patches (the ``control examples") in the Pd release, but they will only be touched on here as necessary to demonstrate the audio signal processing techniques that are the subject of this book.