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.
