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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
+
+<HTML>
+ <HEAD>
+ <TITLE>Pd Documentation 2</TITLE>
+ <meta http-equiv="Content-Type" content="text/html">
+ <link rel="stylesheet" type="text/css" href="pdmanual.css" media="screen">
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+<BODY>
+
+<H2>Pd Documentation chapter 2: theory of operation</H2>
+
+<P>
+<A href="index.htm#s2"> back to table of contents</A>
+<BR><BR>
+</P>
+
+<P>
+
+<P> The purpose of this chapter is to describe Pd's design and how it is
+supposed to work. Practical details about how to obtain, install, and run Pd
+are described in the next chapter. To learn digital audio processing basics
+such as how to generate time-varying sounds that don't click or fold over, try
+the on-line book,
+<A HREF="http://www.crca.ucsd.edu/~msp/techniques.htm"
+<I> Theory and Techniques of Electronic Music </I>.
+
+<H3> <A name=s1> 2.1 overview </A> </H3>
+
+<P>Pd is a real-time graphical programming environment for audio and graphical
+processing. It resembles the Max/MSP system but is much simpler and more
+portable; also Pd has two features not (yet) showing up in Max/MSP: first,
+via Mark Dank's GEM package, Pd can be used for simultaneous computer
+animation and computer audio. Second, an experimental facility is provided
+for defining and accessing data structures.
+
+<H3> <A name=s1.1> 2.1.1. the main window, canvases, and printout </A> </H3>
+
+<P>When Pd is running, you'll see a main "Pd" window, and possibly one or more
+"canvases" or "patches". The main Pd window looks like this:
+
+<CENTER><P>
+ <IMG src="fig1.1.png" ALT="pd window">
+</P></CENTER>
+
+<P> There are peak level and clip indicators for audio input and output; these
+report peak levels over all input and all output channels. Note that DC
+shows up as an input level; many cards have DC levels which show up in the
+50s. To see an RMS audio level, select "test audio and MIDI" from the Media
+menu. The main window display is intended only to help you avoid clipping
+on input and output. You can turn the peak meters on and off using the
+control at lower left.
+
+<P> At lower right is a control to turn audio processing on and off
+globally. Turning audio off stops the computation and relinquishes any audio
+devices Pd is using. The "Media" menu is also provided, with accelerators
+"Control-." to turn audio computation off and "Control-/" to turn it on. When
+audio is on, Pd is computing audio samples in real time according to whatever
+patches you have open (whether they are visible or not).
+
+<P> The DIO (Digital I/O) error indicator flashes if there is a synchronization
+error for audio input or output. (But note that on some platforms Pd doesn't
+find out about them. If you never see red, you're probably not seeing the
+truth.)
+Click the "DIO errors" button to see a list of recent errors.
+This indicator should turn red whenever the
+computation runs late (so that the DAC FIFOs fill and/or the ADC FIFOs empty)
+or if audio input and output are not running at the same rate. See
+<a href="x3.htm#s2"> audio and MIDI support </A>.
+
+<P> The bottom part of the Pd window is an area for printout from objects in
+patches, and/or for messages from Pd itself.
+
+<P> Pd documents are called "patches" or "canvases."
+Each open document has one main window and any number of
+sub-windows. The sub-windows can be opened and closed but are always running
+whether you can see them or not. Here is a simple Pd patch:
+
+<CENTER><P>
+ <IMG src="fig1.2.jpg" ALT="hello world patch">
+</P></CENTER>
+
+<P>There are four <I> text boxes </I> in this patch: a number box (showing zero),
+an object box showing "print," and two comments. The number box and the object
+box are connected, the number box's output to the print box's input. Boxes may
+have zero or more inputs and/or outputs, with the inputs on top and the outputs
+on bottom.
+
+<P>
+Pd's printout appears on the main ``Pd" window,
+unless you redirect it elsewhere.
+
+<H3> <A name="s1.2"> 2.1.2. object boxes </A> </H3>
+<P> Pd patches can have four types of boxes: <I> object, message, GUI, </I>
+and <I> comment </I>.
+
+<P> You make <I> objects </I> by typing text into object boxes. The text is
+divided into <I> atoms </I> separated by white space. The first atom specifies
+what type of object Pd will make, and the other atoms, called <I> creation
+arguments </I>, tell Pd how to initialize the object. If you type for example,
+
+<CENTER><P>
+ <IMG src="fig1.3.jpg" ALT="object">
+</P></CENTER>
+
+<P>the "+" specifies the <I> class </I> of the object.
+In this case the object will be the kind that carries out addition,
+and the "13" initializes the amount to add.
+
+<P> Atoms are either numbers or <I>
+symbols </I> like "+". Anything that is not a valid number os considered a
+symbol. Valid numbers may or may not have a decimal point (for instance, 12,
+15.6, -.456), or may be
+written in exponential notation (such as "4.5e6", which means "4.5 multiplied
+by 10 six times, i.e., 4500000). Negative exponentials divide by 10 (so
+that 1.23e-5 comes to 0.0000123).
+
+<P> Non-valid numbers which are read as symbols
+include things like "+5" and "0..6" as well as words and names such as "Zack"
+or "cat". The symbols "gore", "Gore", and "GORE" are all distinct.
+
+<P> The text you type into an object box determines how
+many and what kinds of inlets and outlets the object will have. Some
+classes (like "+" always have a fixed arrangement of inlets and outlets,
+and in the case of other classes, the inlets and outlets will depend on the
+creation arguments.
+
+<P>Here for example is a simple MIDI synthesizer:
+
+<CENTER><P>
+ <IMG src="fig1.4.png" ALT="simple MIDI synthesizer">
+</P></CENTER>
+
+<P>This patch mixes <I> control </I> objects (notein, stripnote, and ftom) with
+<I> tilde </I> objects osc~, *~, and dac~. The control objects carry out their
+function sporadically, as a result of one or more type of <I> event </I>. In
+this case, incoming MIDI note messages set off the control computation. The
+result of the computation is, when the note happens to be a "note on" (and not
+a "note off", to compute the frequency in cycles per second and pass it on to
+the oscillator ("osc~").
+
+<P> The second half of the patch, the osc~, *~, and dac~ objects, compute audio
+samples, in the same way as an analog synthesizer works. The osc~ object is
+acting as the interface between the two regimes, in that it takes control
+messages to set its frequency but talks to "*~" using an audio signal. Audio
+signals aren't sporadic; they are continuous streams of numbers. As a result
+tilde objects act under very different rules from control objects. The audio
+portion of the patch is always running, whether MIDI messages arrive or not. On
+the other hand, the function of control computations is to insert calculations
+between the audio computation which may change audio computation parameters
+such as the frequency of an oscillator.
+
+<P> The connections in the patch (the lines between the boxes) are also of two
+types: control and signal. The type of connection depends on the outlet it
+comes from. Signal connections are represented by thicker lines than control
+connections; in the patch above, the two bottom conections are signal and the
+others are control. In general, a control connection may be made to a signal
+inlet; if numbers are sent over it they are automatially converted to
+signals. Signal connections may not be made to control inlets; some sort
+of explicit conversion must be specified.
+
+<H3> <A name="s1.3"> 2.1.3. message and GUI boxes </A> </H3>
+
+<P>The border of a box tells you how its text is interpreted and how the box
+functions. Object boxes (as in the previous example) use the text to create
+objects when you load a patch or type text onto a new one. If you retype the
+text in an object box, the old one is discarded and a new one is created, using
+the new creation arguments. The contents of an object box describe a message
+which is sent to Pd to create the object.
+
+<P> <I> Message </I> boxes interpret the text as a message to send whenever
+the box is activated (by an incoming message or with the mouse.) The message
+may be sent many times while the patch is running (as opposed to object boxes
+whose message is used once to create the object). Instead of going straight
+to Pd, the message box's message (or messages) go either to the box's outlet
+or to other specified receiving objects. In the example
+below, the message box, when clicked, sends the message "21" to an object
+box which adds 13 to it.
+
+<CENTER><P>
+ <IMG src="fig1.5.jpg" ALT="[message( --> [object] -> [number]">
+</P></CENTER>
+
+<P> The third box shown is a <I> GUI </I> ("graphical user interface") box. GUI
+boxes come in many forms including number boxes (as in this example), toggles,
+sliders, and so on. Whereas the appearance of an object or message box is
+static 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.
+(There are also shift- and alt-click actions; see <A href="x2.htm#s2.7">
+getting help </A> to find out how to look this up).
+
+<P> You can also create a "symbol" box which is like a number box but deals
+in symbols like "cat." You can type your own strings in (followed by "enter")
+or use it to display strings which arrive as messages to its inlet.
+
+<H3> <A name="s1.4"> 2.1.4. patches and files </A> </H3>
+
+<P>When you save a patch to a file, Pd doesn't save the entire state of all the
+objects in the patch, but only what you see: the objects' creation arguments
+and their interconnections. Certain data-storage objects have functions for
+reading and writing other files to save and restore their internal state.
+
+<P>Pd finds files using a <I> path </I> which can be specified as part of Pd's
+startup arguments. The path specifies one or more directories, separated by
+colons (semicolons if you're using windows.) Most objects which can read files
+search for them along the search path, but when Pd writes files they go to
+the directory where the patch was found.
+
+<H3> <A name=s2> 2.2. editing Pd patches </A> </H3>
+
+<H3> <A name=s2.1> 2.2.1. edit and run mode </A> </H3>
+
+<P> A patch can be in edit or run mode; this really only affects how mouse
+clicks affect the patch. In edit mode, clicking and dragging selects and
+moves boxes or makes and cuts connections; in run mode clicking on boxes sends
+them messages which they react to in different ways. In run mode, number and
+message boxes can be used as controls. Normally, when you are in a performance
+you will stay in run mode; to change the patch you go to edit mode.
+
+<H3> <A name=s2.2> 2.2.2. creating boxes </A> </H3>
+
+<P> You can create boxes (objects, messages, GUIs, and comments) using the
+"put" menu. Note the handy accelerators. Object and message boxes are empty
+at first; drag them where you want them and type in the text. The GUI
+objects (which come in several flavors) require no typing; just create and
+place them.
+
+<P> You will often find it more convenient to select a box and "duplicate" it
+(in the Edit menu) than to use the "Put" menu. If you select and duplicate
+several items, any connections between them will be duplicated as well.
+
+<H3> <A name=s2.3> 2.2.3. the selection </A> </H3>
+
+<P>Boxes in a Pd window may be selected by clicking on them. To select more
+than one object you may use shift-click or click on a blank portion of
+the window and drag the cursor to select all objects within a rectangle.
+
+<P>Clicking on an unselected object, message, or comment box makes the text
+active, i.e., ready to be text edited. (If you select using the rectangle
+method, the text isn't activated.) Once you've activated a text box, you
+may type into it (replacing the selected text) or use the mouse to change the
+selection.
+
+<P> You may also select a single connection (patch cord) by clicking on it.
+You can't have connections and boxes selected simultaneously.
+
+<H3> <A name=s2.4> 2.2.4. deleting, cutting, and pasting </A> </H3>
+
+<P>If you select a box, a connection, or several boxes, and if you haven't made
+any text active, you can "delete" the selection by hitting the backspace or
+delete key. You can also "cut" "copy" and "paste" using menu items. Notice
+that pasting puts the new object(s) right down on top of the old ones.
+
+<P>The "duplicate" menu item performs a copy and paste with a small offset so you
+can see the new boxes. You can drag them together to a new place on the screen.
+
+<P>You can cut and paste between windows within Pd but cut/paste isn't
+integrated with the OS in any way. Cut/copy/paste for activated text in boxes
+isn't implemented yet, although in Linux and Irix at least you can "X-paste"
+into and out of "text" dialogs (created with the "edit text" menu item.)
+
+<H3> <A name=s2.5> 2.2.5. changing the text </A> </H3>
+
+<P> To change a text item, you can select it and then edit the text. If you
+only click once, the entire text is selected and your typing will replace
+everything. Click again and drag to select a portion of the text to retype.
+
+<P> If there's
+more than a small amount of text (in a comment, for example) you might want to
+select the text and choose "text editor" from the Edit menu, which opens a text
+editing window with a copy of the text in it. Hitting "send" in that window is
+exactly equivalent to retyping the text into Pd; you can send it to more than
+one box in sequence if you want.
+
+<P> If you click a box and move the mouse without releasing the button this
+displaces the entire box. If you wish to displace a box which is already
+selected, first de-select the box by clicking outside it; otherwise you will be
+selecting text instead of moving the box.
+
+<P> <I> The updated text only becomes part of the patch when you de-select the
+object. </I> Changing the text in an "object" box deletes the old
+object and creates a new one; the internal state of the old one is lost.
+
+<H3> <A name=s2.6> 2.2.6. connecting and disconnecting boxes </A> </H3>
+
+<P>To make a connection between two boxes, click on any outlet of the first
+one, drag toward an inlet of the second one, and release. You can
+release the mouse button anywhere within the target object and the connection
+will be made to the nearest inlet.
+
+<P>Connections are broken by selecting them and using "cut" or the backspace
+or delete key.
+
+<H3> <A name=s2.7> 2.2.7. popup menu for properties, open, and help </A> </H3>
+
+<P> All the "clicking" mentioned above is done with the left mouse button.
+The right button, instead, gives a popup menu offering "properties," "open,"
+and "help".
+(For Macintosh users who may only have one button on their mouse,
+double-clicking is mapped to right-click.)
+
+<P> Selecting "help" on an object gets
+a Pd patch that demonstrates how to use it. "Help" for the canvas as a whole
+(right-clicking outside any object) gives a list of all built-in objects.
+
+<P> The "open" menu item is only enabled if you right-click on a subpatch
+(see below) and causes Pd to open it. Ordinary subpatches may also be opened
+by clicking on them, but for "graph-on-parent" ones, this is the only way to
+do it.
+
+<P> The "properties" dialog allows you to change certain settings of GUI
+objects, or of the patch itself (by clicking outside any box.)
+
+<H3> <A name=s2.7> 2.2.8. miscellaneous </A> </H3>
+
+<P> Control-q "quits" Pd, but asks you to comfirm the quit. To quit without
+having to confirm, use command-shift-Q.
+
+<H3> <A name="s3"> 2.3. messages </A> </H3>
+
+<P> In Pd, objects intercommunicate by sending messages and/or audio signals.
+Pd messages are sporadic, like MIDI messages or music N "Note cards."
+
+<H3> <A name="s3.1"> 2.3.1. anatomy of a message </A> </H3>
+
+<P>Messages contain a selector followed by
+any number of arguments. The selector is a symbol, which appears in the patch
+as a non-numeric string with no white space, semicolons, or commas. The
+arguments may be symbols or numbers. Numbers in Pd are kept in 32-bit floating
+point, so that they can represent integers exactly between -8388608 and
+8388608. (In Max, there are separate data types for integers and floating
+point numbers; Pd uses only float.)
+
+<P> When a message is passed to something (which is often an inlet of a box
+but could be anything that can receive a message), the selector of the message
+is checked against the receiver. If the receiver recognizes messages of that
+selector, it carries out some corresponding action. For instance, here is a
+"float" object:
+
+<CENTER><P>
+ <IMG src="fig3.1.jpg" ALT="float object">
+</P></CENTER>
+
+<P> The two rectangles at the top are usually both called "inlets" but
+the one at the left directs incoming messages to the "float" object itself,
+whereas the one at the right directs messages to an auxiliary "inlet"
+object. The float object proper (represented by the left-hand inlet) accepts
+messages with selector "float" and "bang". The right-hand inlet takes only
+the message selector "float". These two selectors, along with "symbol" and
+"list", are usually used to denote an object's main action, whatever it may be,
+so that objects can be interconnected with maximum flexibility.
+
+<P> It is possible to type messages which start with a number,
+which cannot be used as a selector. A single number is always given the
+"float" selector automatically, and a message with a number followed by other
+arguments is given the selector "list".
+
+<H3> <A name="s3.2"> 2.3.2. depth first message passing </A> </H3>
+
+<P> In Pd whenever a message is initiated, the receiver may then send out
+further messages in turn, and the receivers of those messages can send yet
+others. So each message sets off a tree of consequent messages. This tree is
+executed in depth first fashion. For instance in the patch below:
+
+<CENTER><P>
+ <IMG src="fig3.2.jpg" ALT="depth first message passing">
+</P></CENTER>
+
+<P> the order of arrival of messages is either A-B-C-D or A-C-D-B. The "C"
+message is not done until the "D" one is also, and the "A" is not done until
+all four are. It is indeterminate which of "B" or "C" is done first; this
+depends on what order you made the connections in (in Max, it's automatically
+sorted right to left).
+
+<P> Message-passing can give rise to infinite loops of the sort shown here:
+
+<CENTER><P>
+ <IMG src="fig3.3.jpg" ALT="infinite message passing loop">
+</P></CENTER>
+
+<P> Here the left-hand "+" can't finish processing until the right-hand one has
+been sent the result "2", which can't finish processing that until the
+left-hand one has been sent "3", and so on. Pd will print an error message
+reporting a "stack overflow" if this happens.
+
+<P> However, it is legal to make a loop if there is a "delay" object somewhere
+in it. When the "delay" receives a message it schedules a message for the
+future (even if the time delay is 0) and is then "finished;" Pd's internal
+scheduler will wake the delay back up later.
+
+<H3> <A name="s3.3">
+2.3.3. hot and cold inlets and right to left outlet order </A> </H3>
+
+<P> With few exceptions (notably "timer"), objects treat their leftmost
+inlet as "hot" in the sense that messages to left inlets can result in output
+messages. So the following is a legal (and reasonable) loop construct:
+
+<CENTER><P>
+ <IMG src="fig3.4.jpg" ALT="hot and cold inlets">
+</P></CENTER>
+
+<P>Here the "f" is an abbreviation for "float". Note that the "+ 1" output is
+connected to the right-hand inlet of "f". This "cold" inlet merely stores the
+value for the next time the "f" is sent the "bang" message.
+
+<P>It is frequently desirable to send messages to two or more inlets of an object
+to specify its action. For instance, you can use "+" to add two numbers; but
+to do it correctly you must make sure the right hand inlet gets its value
+first. Otherwise, when the left hand side value comes in, "+" will carry out
+the addition (since the left hand inlet is the "hot" one) and will add this
+value to whatever was previously sitting in the right hand inlet.
+
+<P> Problems can arise when a single outlet is connected (either directly or
+through arbitrarily long chains of message passing) to different inlets of a
+single object. In this case it is indeterminate which order the two inlets will
+receive their messages. Suppose for example you wish to use "+" to double a
+number. The following is incorrect:
+
+<CENTER><P>
+ <IMG src="fig3.5.jpg" ALT="incorrect inlet connection">
+</P></CENTER>
+
+<P> Here, I connected the left inlet before connecting the right hand one (although
+this is not evident in the appearance of the patch.) The "+" thus adds the
+new input (at left) to the previous input (at right).
+
+<P> The "trigger" object, abbreviated "t", can be used to split out connections
+from a single outlet in a determinate order. By convention, all objects in Pd,
+when sending messages out more than one outlet, do so from right to left. If
+you connect these to inlets of a second object without crossing wires, the
+second object will get its leftmost inlet last, which is usually what you
+want. Here is how to use "trigger" to disambiguate the previous example:
+
+<CENTER><P>
+ <IMG src="fig3.6.jpg" ALT="trigger to disambiguate">
+</P></CENTER>
+
+<P> "Cold" (non-leftmost) inlets are almost universally used to store single
+values (either numbers or symbols.) With the exception of "line" and "line~",
+these values are "sticky," i.e., once you set the value it is good until the
+next time you set it. (The "line" exception is for sanity's sake.)
+
+<P> One more question sometimes comes up in execution order, which is
+the order in which two messages are sent to a single "cold" inlet. In this
+situation, since the messages are merged, the last value to be received is
+the value that is used in the computation.
+
+<H3> <A name="s3.4"> 2.3.4. message boxes </A> </H3>
+
+Message boxes are text boxes in which you type a message. When the message
+box is activated, either by clicking on it or sending something to its inlet,
+the message or messages are sent, either to the message box's outlet or
+elsewhere as specified.
+
+<CENTER><P>
+ <IMG src="fig3.7.jpg" ALT="message boxes">
+</P></CENTER>
+
+<P>The first of the message boxes above contains the single number 1.5; this
+message has an implicit selector of "float." The second is a list with three
+numbers in it, and in the third, the selector is "my" and the two arguments are
+the number 5 and the symbol "toes."
+
+<P> Multiple messages may be separated by commas as shown:
+
+<CENTER><P>
+ <IMG src="fig3.8.jpg" ALT="multiple messages in one box">
+</P></CENTER>
+
+<P>Here the three messages are the numbers 1, 2, and 3, and they are sent in
+sequence (with no intervening time between them, as with the "trigger" object,
+and having depth-first consequences so that whatever chain of actions depending
+on "1" takes place before anything depending on "2" and so on.)
+
+<P> Semicolons may also separate messages. A message following a semicolon must
+specify a symbol giving a destination (in other words, semicolons are like
+commas except that they clear the "current destination"
+so that the next message specifies a new one). The "current destination" is
+at first the message box's own outlet. In the example below, the leading
+semicolon immediately redirects messages from the outlet to an object named
+"fred" (which is here a receive object), and likewise the next message is sent
+to "sue."
+
+
+<CENTER><P>
+ <IMG src="fig3.9.jpg" ALT="semicolons to send messages">
+</P></CENTER>
+
+<P>Certain other objects (Pd windows, for example, and arrays) have Pd names and
+can be sent messages this way. Also, the special object "pd" is defined to
+which you may send messages to start and stop DSP.
+
+<P> You can put variables in message boxes as shown below:
+
+<CENTER><P>
+ <IMG src="fig3.10.jpg" ALT="variables in message boxes">
+</P></CENTER>
+
+<P>Here, "$1", etc., refer to the arguments of the arriving message (and aren't
+defined if you send a "bang" message or if you click on the message box to
+activate it.) Dollar sign variables are either numbers or symbols depending
+on the incoming message; if symbols, you may even use them to specify variable
+message selectors or destinations.
+
+<H3> <A name="s4"> 2.4. audio signals </A> </H3>
+
+<P>
+Using Pd you can build audio patches which can synthesize musical sounds,
+analyze incoming sounds, process incoming sounds to produce transformed
+audio outputs, or integrate audio processing with other media. This section
+describes how Pd treats audio signals.
+
+<H3> <A name="s4.1"> 2.4.1. sample rate and format </A> </H3>
+
+<P>
+Pd's audio signals are internally kept as 32-bit floating point numbers, so
+you have all the dynamic range you could want. However, depending on your
+hardware, audio I/O is usually limited to 16 or 24 bits. Inputs all appear
+between the values of -1 and 1; and output values will be clipped to that range.
+Pd assumes a sample rate of 44100 unless you override this (
+in Pd's command line or in the "audio setup" dialog).
+
+<P>
+Pd can read or write samples to files either in 16-bit or 24-bit fixed point
+or in 32-bit floating point, in WAV, AIFF, or AU format, via the soundfiler,
+readsf, and writesf objects.
+
+<H3> <A name="s4.2"> 2.4.2. tilde objects and audio connections </A> </H3>
+
+<P>Audio computations in Pd are carried out by "tilde objects" such as "osc~"
+whose names conventionally end in a tilde character to warn you what they
+are. Tilde objects can intercommunicate via audio connections. When audio
+computation is turned on, or when you change the audio network while audio is
+on, Pd sorts all the tilde objects into a linear order for running; then this
+linear list is run down in blocks of 64 samples each; at 44100 Hz. this means
+the audio network runs every 1.45 milliseconds.
+
+<P> Inlets or outlets are configured in Pd either for messages or audio; it's
+an error to connect an audio outlet to a non-audio inlet or vice versa; usually
+these errors are detected at "sort time" when audio is started or the network
+changed with audio running. An object's leftmost inlet may accept both audio
+and messages; any other inlet is either one or the other.
+
+<P>
+The audio network, that is, the tilde objects and their interconnections,
+must be acyclic. If there are loops, you will see the error message at "sort
+time." When errors are reported at sort time there is no easy way to
+find the source of the error. You can build algorithms with feedback using
+nonlocal signal connections.
+
+<P>
+Your subpatches can have audio inlets and outlets via the inlet~ and outlet~
+objects.
+
+<H3> <A name=s4.3> 2.4.3. converting audio to and from messages </A> </H3>
+
+<P> If you want to use a control value as a signal, you can use the sig~ object
+to convert it. The +~, -~, *~, /~, osc~, and phasor~ objects can be configured
+to take control or signal inputs.
+
+<P> The other direction, signal to control, requires that you specify at what
+moments you want the signal sampled. This is handled by the snapshot~ object,
+but you can also sample a signal with tabwrite~ and then get access it via
+tabread or tabread4 (note the missing tildes!). There are also analysis
+objects, the simplest of which is "env~", the envelope follower.
+
+<H3> <A name=s4.4> 2.4.4. switching and blocking </A> </H3>
+
+<P>You can use the switch~ or block~ objects to turn portions of your audio
+computation on and off and to control the block size of computation. There
+may be only one switch~ or block~ object in any window; it acts on the entire
+window and all of its subwindows, which may still have their own nested
+switch~/block~ objects. Switch~ and block~ take a block size and an overlap
+factor as arguments; so for instance, "block~ 1024 4" specifies 1024 sample
+blocks, overlapped by a factor of 4 relative to the parent window. Switch~
+carries a small computational overhead in addition to whatever overhead is
+associated with changing the block size.
+
+<P> Larger block sizes than 64 should result in small increases in run-time
+efficiency. Also, the fft~ and related objects operate on blocks so that
+setting the block size also sets the number of FFT channels. You may wish
+to use block sizes smaller than 64 to gain finer resolutions of message/audio
+interaction, or to reduce "block delay" in feedback algorithms. At the
+(untested) extreme, setting the block size to one allows you to write your
+own recursive filters.
+
+<P> You can use switch~ to budget your DSP computations; for instance you might
+want to be able to switch between two synthesis algorithms. To do this, put
+each algorithm in its own subpatch (which can have sub-sub patches in turn, for
+a voice bank for instance), and switch each one off as you switch the other one
+on. Beware of clicks; if you have a line~ controlling output level, give it
+time to ramp to zero before you switch it off or it will be stuck at a nonzero
+value for the next time it comes back on.
+
+<P> When a subpatch is switched off its audio outputs generate zeros; this
+costs a fairly small overhead; a cheaper way to get outputs is to use throw~
+inside the switched module and catch~ outside it.
+
+<H3> <A name=s4.5> 2.4.5. nonlocal signal connections </A> </H3>
+
+<P>You may wish to pass signals non-locally, either to get from one window to another, or
+to feed a signal back to your algorithm's input. This can be done using
+throw~/catch~, send~/receive~, or delwrite~/delread~ pairs. Throw~ and catch~
+implement a summing bus; throw~ adds into the bus and catch~ reads out the
+accumulated signal and zeros the bus for the next time around. There can be
+many throw~ objects associated with a single catch~, but a throw~ can't talk to
+more than one catch~. You can reset the destination of a throw~ if you want to.
+
+<P> Send~ just saves a signal which may then be receive~d any number of times; but
+a receive~ can only pick up one send~ at a time (but you can switch between
+send~s if you want.)
+
+<P> Don't try to throw~ and catch~ or send~ and receive~ between windows with
+different block sizes. The only re-blocking mechanisms which are well tested
+are inlet~ and outlet~.
+
+<P> When you send a signal to a point that is earlier in the sorted list of tilde
+objects, the signal doesn't get there until the next cycle of DSP computation,
+one block later; so your signal will be delayed by one block (1.45 msec by
+default.) Delread~ and delwrite~ have this same restriction, but here the 1.45
+msec figure gives the minimum attainable delay. For nonrecursive algorithms, a
+simple flanger for example, you might wish to ensure that your delread~ is
+sorted after your delwrite~. The only way to ensure this is to create the
+delread~ after you created the delwrite~; if things get out of whack, just
+delete and re-create the delread~.
+
+<H3> <A name=s5> 2.5. scheduling </A> </H3>
+
+<P>Pd uses 64-bit floating point numbers to represent time, providing sample
+accuracy and essentially never overflowing. Time appears to the user
+in milliseconds.
+
+<H3> <A name=s5.1> 2.5.1. audio and messages </A> </H3>
+
+<P>Audio and message processing are interleaved in Pd. Audio processing is
+scheduled every 64 samples at Pd's sample rate; at 44100 Hz. this gives a
+period of 1.45 milliseconds. You may turn DSP computation on and off by
+sending the "pd" object the messages "dsp 1" and "dsp 0."
+
+<P> In the intervals between, delays might time out or external conditions
+might arise (incoming MIDI, mouse clicks, or whatnot). These may cause a
+cascade of depth-first message passing; each such message cascade is completely
+run out before the next message or DSP tick is computed. Messages are never
+passed to objects during a DSP tick; the ticks are atomic and parameter changes
+sent to different objects in any given message cascade take effect
+simultaneously.
+
+<P> In the middle of a message cascade you may schedule another one at a delay
+of zero. This delayed cascade happens after the present cascade has finished,
+but at the same logical time.
+
+<H3> <A name=s5.2> 2.5.2. computation load </A> </H3>
+
+<P> The Pd scheduler maintains a (user-specified) lead on its computations;
+that is, it tries to keep ahead of real time by a small amount in order to be
+able to absorb unpredictable, momentary increases in computation time. This
+is specified using the "audiobuffer" or "frags" command line flags (see <a
+href="x3.htm" name=s3>getting Pd to run </A>).
+
+<P> If Pd gets late with respect to real time, gaps (either occasional or
+frequent) will appear in both the input and output audio streams. On the
+other hand, disk streaming objects will work correctly, so that you may use
+Pd as a batch program with soundfile input and/or output. The "-nogui"
+and "-send" startup flags are provided to aid in doing this.
+
+<P> Pd's "realtime" computations compete for CPU time with its own GUI, which
+runs as a separate process. A flow control mechanism will be provided someday
+to prevent this from causing trouble, but it is in any case wise to avoid
+having too much drawing going on while Pd is trying to make sound. If a
+sub-window is closed, Pd suspends sending the GUI update messages for it;
+but not so for miniaturized windows as of version 0.32. You should really
+close them when you aren't using them.
+
+<H3> <A name=s5.3> 2.5.3. determinism </A> </H3>
+
+<P>All message cascades that are scheduled (via "delay" and
+its relatives) to happen before a given audio tick will happen as scheduled
+regardless of whether Pd as a whole is running on time; in other words,
+calculation is never reordered for any real-time considerations. This is done
+in order to make Pd's operation deterministic.
+
+<P> If a message cascade is started by an external event, a time tag is given
+it. These time tags are guaranteed to be consistent with the times at which
+timeouts are scheduled and DSP ticks are computed; i.e., time never decreases.
+(However, either Pd or a hardware driver may lie about the physical time an
+input arrives; this depends on the operating system.) "Timer" objects which
+measure time intervals measure them in terms of the logical time stamps of the
+message cascades, so that timing a "delay" object always gives exactly the
+theoretical value. (There is, however, a "realtime" object that measures real
+time, with nondeterministic results.)
+
+<P> If two message cascades are scheduled for the same logical time, they are
+carried out in the order they were scheduled.
+
+<H3> <A name=s6> 2.6. semantics </A> </H3>
+
+This section describes how objects in Pd are created, how they store data and
+how object and other boxes pass messages among themselves.
+
+<H3> <A name=s6.1> 2.6.1. creation of objects </A> </H3>
+
+The text in a box has a different function depending on whether it is a message,
+atom (number/symbol), or object box. In message boxes the text specifies the
+message or messages it will send as output. In atom boxes the text changes
+at run time to show the state of the box, which is either a number or a symbol.
+
+<P> In an object box, as in a message box, the text specifies a message; but
+here the message is to be passed to Pd itself, once, and the
+message's effect is to create the object in question. When you open a file,
+all the objects created are created using their text as "creation messages."
+If you type a new message into an object box (or change it), the old object is
+destroyed and the message is used to create the new one.
+
+<P> The selector of the message (the first word in the message) is a selector
+which Pd interprets to mean which type of object to create. Any message
+arguments (called "creation arguments") are used to parameterize the object
+being created. Thus in "makenote 64 250" the selector "makenote" determines
+the class of object to create and the creation arguments 64 and 250 become the
+initial velocity and duration.
+
+<H3> <A name=s6.2> 2.6.2. persistence of data </A> </H3>
+
+Among the design principles of Pd is that patches should be printable, in the
+sense that the appearance of a patch should fully determine its functionality.
+For this reason, if messages received by an object change its action, since the
+changes aren't reflected in the object's appearance, they are not saved as part
+of the file which specifies the patch and will be forgotten when the patch is
+reloaded. In the same way, if you delete and then recreate an object the
+original object's state is not retained but is instead reinitialized (possibly
+as specified by creation arguments.)
+
+<P> An exception is made for subpatches whose "state" is the configuration of
+the subpatch; as a special case, this configuration is restored when the
+patch is read from a file. Also, if you rename the subpatch, for instance
+typing "pd jane" instead of "pd spot," the contents of the patch are preserved
+and only the text in the object box and the window title of the subpatch are
+changed.
+
+<P> It is probably bad style to specify creation arguments ala "makenote 64 250"
+if you are going to override them later; this is confusing to anyone who tries
+to understand the patch.
+
+<H3> <A name=s6.3> 2.6.3. message passing </A> </H3>
+
+Messages in Pd consist of a selector (a symbol) and zero or more arguments
+(which may be symbols or numbers). To pass a message to an object, Pd first
+checks the selector against the class of the object. Message boxes all are
+of one class and they all take the same incoming messages and dispense them
+according to their state, that is, the text typed into the box. The same
+holds for atom boxes (number or symbol) except that their state may change
+(it consists of the number or symbol showing).
+
+<P> Object boxes may have many different classes. The class is usually
+determined by the selector of the creation message, i.e., the first atom of the
+creation message which is usually a symbol.
+
+<P> Each class comes with a fixed collection of messages it may be sent. For
+example, the "float" or "f" object takes "bang" and "float." These messages
+are sent to "float" objects (objects whose class is float) via the leftmost,
+hot inlet. (The right inlet is a separate, auxiliary object.) Objects of
+class "float" respond to the message "bang" by outputting their current value,
+that is, by sending a "float" message to their outlet. They respond to "float"
+messages by setting their value and then outputting it.
+
+<P> Each other class (like "float") in Pd has its own protocol for responding
+to messages it is sent, and may take "float" and "bang" messages, or others
+in addition or instead of them.
+
+<H3> <A name=s6.4> 2.6.4. inlets and lists </A> </H3>
+
+The leftmost connection point at the top of most objects represents the object
+itself. Any other dark rectangle is a separate object called an "inlet"
+although in Pd there are 4 individual inlet classes. The class of the inlet
+determines which messages it will take: symbol, float, or other; and the inlet
+forwards the message either to the object proper or to some proxy, usually
+one that the object creates for the occasion.
+
+<P> Unless they arrange otherwise by defining a "list" method, objects respond
+to the "list" message by distributing the arguments of the message to their
+inlets, except for the first argument which is passed as a "float" or
+"symbol" message to the object proper.
+
+<H3> <A name=s6.5> 2.6.5. dollar signs </A> </H3>
+
+In message or object boxes, message arguments starting with a dollar sign
+and a number (like "$1" or "$3-bazoo") are variables which are substituted
+with values supplied as part of the environment the message is passed in.
+In the case of message boxes, the environment consists of the arguments of
+the "list" message (possibly extrapolated from "bang," "float,"
+or other) that the message box is responding to. Thus, if a message box gets
+"23 skidoo" and if it contains the text, "$2 until $1," out comes the message,
+"skidoo until 23."
+
+<P> Object boxes contain text which forms a message to be sent to Pd to create
+and initialize the object. Here, $1, etc., are taken from the context in which
+the patch was loaded. When the patch is a new document or opened from a file
+the "$" variables are undefined. But if the patch is an abstraction (see the
+next section) they are
+taken from the abstractions' creation arguments.
+
+<P> Constructions such as "$1-x" are expanded by string concatenation. This
+is the mechanism for making local variables. In particular, $0 in an abstraction
+is a counter which is guaranteed to be unique to that abstraction, so sends and
+receives with names like "$0-bear" can be used as local send/receive pairs.
+
+<P> Note that the expansion of variables such as $0 and $1 only works at the
+beginning of the symbol; so, for instance, "rats-$1" will not be expanded.
+Occasionally you may want to have double or triple substitutions; this can
+be done one stage at a time by nesting abstractions (with each subpatch
+adding its own $-variable to the beginning of a symbol and passing that on
+as argument to a further abstraction.)
+
+<P> For example, if you want to get dog-food, dog-ears, and cat-food, for
+example, have an abstraction "a1" that invokes an abstraction "a2" twice, as
+"a2 $1-food" and "a2 $1-ears", and then in a third patch call a1 twice, as
+"a1 cat" and "a1 dog". Inside the four "a2" copioes, $1 will evaluate to
+"dog-food", "cat-food", "dog-ears", and "cat-ears".
+
+<H3> <A name="s7"> 2.7. subpatches </A> </H3>
+
+Pd offers two mechanisms for making subpatches, called "one-off subpatches"
+and "abstractions." In either case the subpatch appears as an object box
+in a patch. If you type "pd" or "pd my-name" into an object box, this creates
+a one-off subpatch. For instance, in this fragment:
+
+<CENTER><P> <IMG src="fig7.1.jpg" ALT="subpatch"> </P></CENTER>
+
+the box in the middle, if clicked on, opens the sub-patch shown here:
+
+<CENTER><P> <IMG src="fig7.2.jpg" ALT="open subpatch window"> </P></CENTER>
+
+<P> The contents of the subpatch are saved as part of the parent patch, in
+one file. If you make several copies of a subpatch you may change them
+individually.
+
+<P> The objects, "inlet,", "inlet~," "outlet," and "outlet~,", when put in a
+subpatch, create inlets and outlets for the object box containing the subpatch.
+This works equally for one-off subpatches and abstractions. The inlet~ and
+outlet~ versions create inlets and outlets for audio signals. You can't mix
+messages and audio in a subpatch inlet or outlet; they must be one or the other
+exclusively. Inlets and outlets appear on the invoking box in the same left-to-right
+order as they appear in the subpatch.
+
+<H3> <A name="s7.1"> 2.7.1. abstractions </A> </H3>
+
+<P> To make an abstraction, save a patch with a name such as "abstraction1.pd"
+and then invoke it as "abstraction1" in an object box:
+
+<CENTER><P> <IMG src="fig7.3.jpg" ALT="abstraction"> </P></CENTER>
+
+<P> Here we're invoking a separate file, "abstraction1.pd", which holds the
+patch shown here (the border is the same as for the subpatch above):
+
+<CENTER><P> <IMG src="fig7.4.jpg" ALT="abstraction example"> </P></CENTER>
+
+You may create many instances of "abstraction1" or invoke it from several
+different patches; and changing the contents of "abstraction1" will affect all
+invocations of it as they are created. An analogy from the "c" programming
+language is that one-off subpatches are like bracketed blocks of code and
+abstractions are like subroutines.
+
+<P> Abstractions are instantiated by typing the name of a patch (minus the ".pd"
+extension) into an object box. You may also type arguments; for instance if
+you have a file "my-abstraction.pd" you may type "my-abstraction 5" to set the
+variable $1 to 5. This is defined only for object boxes (not for messages) in
+the abstraction. (For message boxes, "$1", etc, have a different meaning as
+described above.) If you want to send a message with a $1 in the sense of a
+creation argument of an abstraction, you must generate it with an object box
+such as "float $1", "symbol $1", or perhaps "pack $1 $2", which may then be
+sent to a message box.
+
+<P> The corresponding feature in Max (both Opcode and Ircam) was the "#1"
+construct. In a Max abstraction, "#1", etc., are replaced by the creation
+argument. This has the disadvantage that you can't edit the abstraction as
+instantiated in the patch since the "#" variables are substituted. In Pd the
+"$" variables in object boxes are spelled literally as "$" variables so that
+it's meaningful to edit them from within their calling patch. On the Pd side,
+however, there is the disadvantage that it's confusing to have "$" expanded at
+a different time in an object box than in a message box. In an object box, the
+"$" argument is expanded at creation time, and in a message box, at message
+time.
+
+<H3> <A name="s7.2"> 2.7.2. Graph-on-parent subpatches </A> </H3>
+
+If you open the "properties" dialog for a subpatch or an abstraction, you can
+check the "graph on parent" box to have the controls of the subpatch/abstraction
+appear on the parent. For instance, here is an invocation of "abstraction2":
+
+<CENTER><P> <IMG src="fig7.5.jpg" ALT="graph-on-parent abstraction"> </P></CENTER>
+
+where the patch "abstraction2.pd" contains:
+
+<CENTER><P> <IMG src="fig7.6.jpg" ALT="inside graph-on-parent abstraction"> </P></CENTER>
+
+Here, the number box in the abstraction shows up on the box that invoked
+the abstraction. The "graph on parent" flag is set in the abstraction
+(and is saved as part of the abstraction); to set it, open the "properties"
+dialog for the "abstraction2" canvas by right-clicking on any white space
+in the patch.
+
+<P> To open the subpatch, right click on the object and select "open". (On
+Macintoshes without a 2-button mouse, you can double-click in edit mode
+instead.) It doesn't work just to click on the object in run mode since clicks
+are sent to visible controls and/or arrays.
+
+<P> When the sub-patch is closed, all controls in it appear on the object
+instead; so the number box in the sub-patch in the example above is the same
+one as you see in the box. Only controls are made visible in this way
+
+<H3> <A name=s8> 2.8. numeric arrays </A> </H3>
+
+Linear arrays of numbers recur throughout the computer musician's bag of tricks,
+beginning with the wavetable oscillator. The wavetable oscillator later was
+reinvented as the looping sampler. Also, table lookup is used for nonlinear
+distortion of audio signals. In the domain of control, arrays of numbers
+can specify control mappings, probability densities, voicing data, and much
+more.
+
+<P> Arrays in Pd should be allocated (and possible read in from a file) before
+beginning to make sound, since memory allocation and disk operations may take
+long enough to cause audio buffer overruns or underruns. Pd provides two ways
+to define new arrays, as "graphs" and "tables". In either case the array
+has a pre-defined name and size (i.e., number of points). Elements of the
+array are stored as floating-point numbers, 4 bytes apiece
+
+<P> If you use an array to store a one-second sound at 44.1 kHz you will need
+176 kilobytes, or a one-minute sound, 10.6 megabytes. To store a sound with
+two or more channels, use a separate array for each channel.
+
+<P> Arrays are also useful as transfer functions, for example for nonlinear
+distortion of an audio signal, or to map a control onto a synthesis parameter.
+In situations like this one typically uses much shorter arrays, of no more
+than a few hundred elements. They are also useful for storing measured
+spectra derived from the fft~ objects, and probably for many other uses.
+
+<P> Arrays usually appear within subpatches created to house them, whether
+in "graph on parent" form (so that you see them within a rectangle drawn on
+the containing patch), or as a regular subpatch (which you see as a text box.)
+In the "graph on parent" form, an array appears as shown:
+
+<CENTER><P> <IMG src="fig8.1.jpg" ALT="array"> </P></CENTER>
+
+<P> Arrays are indexed from 0 to N-1 where N is the number of points in the
+array. You can read an array value using the tabread object:
+
+<CENTER><P> <IMG src="fig8.2.jpg" ALT="array indexing"> </P></CENTER>
+
+Here we see that the third point of the array (index 2) has the value 0.4.
+To write into the array you can use the tabwrite object:
+
+<CENTER><P> <IMG src="fig8.3.jpg" ALT="setting an value in an array"> </P></CENTER>
+
+In this example, sending the message sets the third element to 0.5. (You
+may also send the two numbers to the two inlets separately.)
+
+<P> The two previous examples showed control operations to read and write from
+and to arrays. These may also be done using audio signals. For example,
+the patch below creates a 440 Hz. tone with "array1" as a waveform:
+
+<CENTER><P> <IMG src="fig8.4.jpg" ALT="setting an array with a waveform"> </P></CENTER>
+
+Here phasor~'s outputs a sawtooth wave, repeating 440 times per second, whose
+output range is from 0 to 1. The multiplier and adder adjust the range from
+1 to 11, and then the values are used as indices for tabread4~, which is a
+4-point interpolating table lookup module. (Much more detail is available in
+the audio example patches in the "pure documentation" series.)
+
+<P> To create a new array, select "array" from the "put" menu. Up will come
+a dialog window to set initial properties of the array. By default, a
+new graph is created to hold the array, but it may also be housed in the
+most recently created graph instead. Other properties may be specified there
+and/or changed later using the "properties" dialog.
+
+<P> If you select "properties" on an array in a graph, you two dialogs, one
+for the array and one for the graph. The array dialog looks like this:
+
+<CENTER><P> <IMG src="fig8.5.jpg" ALT="array properties window"> </P></CENTER>
+
+You may use this to change the name and size, in addition to another property,
+"save contents". If "save contents" is selected, the array's values are stored
+in the containing patch; otherwise they're initialized to zero each time the
+patch is reloaded. If you intend to use arrays to store sounds, you will
+probably not wish to store them in the patch but as separate soundfiles. This
+will be more efficient, and you may also then use a sound editor to modify them
+outside Pd.
+
+<P> If you check "delete me" and then "OK", the array will be deleted. This is
+an odd interface for deleting an object, and is only provided because Pd
+lacks a mechanism for selecting arrays (so that "cut" could serve).
+
+<P> The graph dialog (which also pops up) is shown here:
+
+<CENTER><P> <IMG src="fig8.6.jpg" ALT="graph properties"> </P></CENTER>
+
+<P> The X bounds initially range from 0 to the number of points in the table
+minus one (this is a good choice for arrays, although graphs holding other
+kinds of objects might require other X bounds.) The Y bounds should be
+chosen to reflect the natural range of the table, so that stored sounds
+would naturally range from -1 to 1, but a sequence of frequency values might
+range from 0 to 20,000. Finally, you choose the screen size of the graph,
+width and height, in screen pixels.
+
+<P> Many other operations are defined for arrays; see the related patches
+in the tutorial (starting at 2.control/15.array.pd) for more possibilities.
+
+<H3> <A name=s9> 2.9. Data structures </A> </H3>
+(Note: this section is adapted from an article submitted to ICMC 2002.)
+
+<P> The original idea in developing Pd was to make a real-time computer music
+performance environment like Max, but somehow to include also a facility for
+making computer music scores with user-specifiable graphical representations.
+This idea has important precedents in Eric Lindemann's Animal and Bill Buxton's
+SSSP. An even earlier class of precedents lies in the rich variety of paper
+scores for electronic music before it became practical to offer a
+computer-based score editor. In this context, scores by Stockhausen (<I>
+Kontakte</I> and <I> Studie II</I>) and Yuasa (<I>Toward the Midnight Sun</I>)
+come most prominently to mind, but also Xenakis's <I>Mycenae-alpha</I>, which,
+although it was realized using a computer, was scored on paper and only
+afterward laboriously transcribed into the computer.
+
+<P> Pd is designed to to offer an extremely unstructured environment for
+describing data structures and their graphical appearance. The underlying
+idea is to allow the user to display any kind of data he or she wants to,
+associating it in any way with the display. To accomplish this Pd introduces
+a graphical data structure, somewhat like a data structure out of the C
+programming language, but with a facility for attaching shapes and colors to
+the data, so that the user can visualize and/or edit it. The data itself can
+be edited from scratch or can be imported from files, generated
+algorithmically, or derived from analyses of incoming sounds or other data
+streams.
+
+Here is one simple
+example of a very short musical sketch realized using Pd:
+
+<CENTER><P> <IMG src="fig9.1.jpg" ALT="graphical score"> </P></CENTER>
+
+The example, which only lasts a few seconds, is a polyphonic collection of
+time-varying noise bands. The graphical ``score" consists of six objects, each
+having a small grab point at left, a black shape to show dynamic, and a colored
+shape to show changing frequency and bandwidth. The horizontal axis represents
+time and the vertical axis, frequency (although, as explained later, this
+behavior isn't built into pd). The dynamic and frequency shapes aren't
+constrained to be connected or even to be proximate, but since they pertain to
+the same sound their horizontal positions line up. In this example the last
+(furthest-right) object is percussive (as seen by the black shape) and has a
+fixed frequency and bandwidth, whereas the large, articulated shape in the
+center has a complicated trajectory in both frequency and dynamic. The color
+of the frequency trace determines the voice number used to realize it.
+
+<P> Each object is thus composed of a combination of scalar values (color;
+aggregate position in X and Y coordinates) and array values (time/value
+pairs for the black traces and time/frequency/bandwidth triples for the
+colored ones.) This is all specified by the user using Pd's ``template"
+mechanism.
+
+<P> Here is the template associated with the graphical objects
+shown above:
+
+<CENTER><P> <IMG src="fig9.2.jpg" ALT="template for graphical score"> </P></CENTER>
+
+Templates consist of a data structure definition (the "struct" object) and
+zero or more drawing instructions ("filledpolygon" and "plot"). The "struct"
+object gives the template the name, "template-toplevel." The data structure
+is defined to contain three floating point numbers named "x", "y", and
+"voiceno," and two arrays, one named "pitch" whose elements belong to another
+template named "template-pitch," and similarly for the array "amp."
+
+<P> In general, data structures are built from four data types: scalar floats
+and symbols, arrays (whose elements share another, specified template) and
+lists (whose elements may have a variety of templates). The contents of a Pd
+window themselves form a list. Pd's correlate of Max's "table" object is
+implemented as a top-level array whose elements are scalars containing a single
+floating-point number.
+
+<P> Data structures in Pd may nest arbitrarily deeply using the array and list
+types. For example, a collection of sinusoidal tracks from an analysis engine
+could be implemented as an array of arrays of (pitch, amplitude)
+pairs; this appears as example 12 in Pd's FFT object online tutorial.
+
+<P> After the "struct" object in the template shown above, the remaining
+three objects are <I> drawing instructions </I> , first for a rectangle
+("filledpolygon"), and then for two arrays. The various graphical
+attributes that are specified for drawing instructions may be numerical
+constants or data structure field names; in the latter case the value varies
+depending on the data. For instance, the second creation argument to
+"plot" is the color. The first "plot" plots the "amp" field and the
+color is given as 0, or black. The second one plots "pitch" using the color
+"voiceno". In this way the color of the second trace is attached to the
+"voiceno" slot in the data structure, so that color will vary according to its
+"voiceno" slot.
+
+<H3> <A name="s9.1"> 2.9.1. Traversal </A> </H3>
+
+<P> Pd objects are provided to traverse lists and arrays, and to address
+elements of data structures for getting and setting. Here is a patch showing
+how these facilities could be used, for example, to sequence the graphical
+score shown above:
+
+<CENTER><P> <IMG src="fig9.3.jpg" ALT="traversal example patch"> </P></CENTER>
+
+<P> Pd has no built-in sequencer, nor even any notion that "x" values should be
+used as a time axis. (However, a "sort" function is provided, which reorders
+a list from left to right, on the assumption that users might often want to use Pd
+data collections as x-ordered sequences.) Recording sequences of events into
+lists, and/or playing the lists back as sequences, are functionalities that the
+user is expected to supply on top of Pd's offerings, which, it is hoped, would
+allow those functionalities within a much larger range of possibilities, to
+include random re-orderings of events, score following, self-modifying scores,
+reactive improvisation, and perhaps much more.
+
+<P> Traversal of data is made possible by adding a new type of atom, "pointer",
+to the two previously defined types that make up messages, to wit, numbers and
+symbols. Unlike numbers and symbols, pointers have no printed form and thus
+can't be uttered in message boxes. Traversal objects such as "pointer" and
+"get" (among several others) can generate or use pointers. The pointer data
+type is also integrated into pipe-fitting objects such as "pack",
+"unpack",
+and "route".
+
+<P> In the patch shown above, the topmost "pointer" object holds a pointer to
+the next object to "play" (by sending it to one of the "voice"
+abstractions at bottom.) The pointer object takes a "traverse" message to
+set it to the head of the list (named "pd-data"), and "next" messages to
+move to (and output) the next datum in the list (i.e., the next in the list of
+six objects in the score). Another "pointer" object is also used, further
+down, as a storage cell for pointers just as "float" is for numbers.
+
+<P> The center of any sequencer is always the "delay" object, which must be
+fed the time difference between each event (including the non-event of hitting
+"start") and the next. As we extract each of the six objects in the score, we
+must wait the delay for playing that object, and then send its pointer to one
+of the "voice" abstractions to play it. However, we have to inspect the
+object itself to know the delay before playing it. So, in the loop, we peel off
+the first remaining object to play and inspect the time difference between it
+and the previous one, using this value to set the delay, but also storing the
+pointer in the lower "pointer" and "pack" objects.
+
+<P> The time difference needed to set the delay object is obtained using the
+"get template-toplevel x" object. (This is converted to incremental time
+("-"), corrected for tempo, and fed to the delay.) Pd provides
+the "get" and "set"
+objects for reading and writing values from data structures.
+The two "get" objects shown here obtain the "x" and "voiceno" fields
+of the current object. The template name (template-toplevel) is supplied
+to the "get" objects so that they can look up the offset of the necessary
+field(s) in advance, for greater run-time efficiency.
+
+<P> Once the delay has expired, the object's pointer is recalled (the lower
+"pointer" object), and the voice number is recalled. This is packed with
+the pointer itself and routed, so that the pointer goes to the appropriate
+voice. The voice number is shown as the color of the frequency trace in
+"999" units (first digit red, second green, third blue) and the "route" is
+arbitrarily set up to select among the six primary and secondary colors plus
+black.
+
+<P> The details of extracting the pitch and dynamic breakpoints from the arrays
+defined in the template are managed in the "voice" abstraction.
+The "voice"
+abstraction receives a
+pointer to a given object and manages the sequencing of the arrays; so it
+contains two sequencers itself. The nesting of the overall structure of
+the sequencer patch mirrors the nesting of the original data structures.
+Finally, the voice abstraction puts its audio output on a summing bus.
+
+<P> More general patches can easily be constructed which access heterogeneous lists
+of objects (having different templates). In this way, an arbitrarily rich
+personal "score language" can be developed and sequenced.
+
+<H3> <A name=s9.2> 2.9.2. Accessing and changing data </A> </H3>
+
+<P> In general, accessing or changing data is done via "pointers" to
+"scalars". Numbers and symbols within scalars are accessed using the
+"get" object and changed, in the same way, using "set". Since lists
+and arrays are composed of scalars, every actual number or symbol in a data
+heap will be a number or symbol element of some scalar. To access them, it
+suffices to have objects to chase down elements of lists and arrays (given
+either a global name or a pointer to the containing scalar).
+
+<P> Lists are traversed in the way shown above; to get to a sublist of a scalar,
+the "get" object will provide a pointer, in the same way as it provides
+"float" or "symbol" elements of scalars. For arrays, an
+"element" object is provided which, given a scalar, a field name and
+a number, chases down the numbered, scalar, element of the named array field.
+
+<P> To alter "float" or "symbol" elements of scalars is straightforward
+using the "set" object, but arrays and lists can't be set by assignment;
+there is no suitable data type available within messages. Lists could
+possibly be "settable" by passing pointers to other lists, but permitting this
+would have required either automatically doing deep copies of data structures
+to carry out the assignments, or else implementing a garbage collecting memory
+management system, either of which would be difficult to realize within
+real-time computation time constraints. Instead, all the data hanging from a
+scalar is considered as belonging to that scalar, and is left in memory until
+the scalar is deleted; the data may be changed atom by atom, but primitives
+are not provided which would imply unpredictable execution times.
+
+<P> The "getsize" and "setsize" objects are provided to access or change
+the number of elements in the array. For lists, an "append" object
+appends a new scalar for a given template to a list, after the element pointed
+to. (To insert a scalar at the beginning of a list, the pointer can be set to
+the "head" of the list, a formal location before the first list item.)
+Deletion is less flexible; the only operation is to delete an entire list.
+(There's no reason not to provide finer-grain deletion mechanisms except that
+it's not clear how to protect against stale pointers efficiently, except by
+voiding the entire collection of pointers into a list.)
+
+<H3> <A name=s9.3> 2.9.3. Editing </A> </H3>
+
+<P> The graphical score shown above can be edited by dragging breakpoints, or
+by adding and deleting them, using mouse clicks. Also, entire objects or
+collections of them may be copied, pasted, and dragged around the screen.
+Alternatively, there is an editable (or computer generate-able or parse-able)
+text representation for the data, which may be seen or changed in a dialog
+window or read and written to external text files.
+
+<P> Since the graphical presentation of data objects is determined by drawing
+instructions, the drawing instructions are interpreted backwards to alter data
+as a result of mouse operations. If a given graphical dimension is controlled
+by a variable, that variable is then controlled by dragging along that
+dimension; if the dimension is constant, it can't be altered by dragging.
+
+<P> Tricky situations can arise when the user changes the contents of templates.
+A change in drawing instructions can be accommodated by simply tracking
+down and redrawing all data objects using the template. However, changing
+the "struct" object itself make for less straightforward situations. The
+user might wish to reorder fields, delete them, add new ones, or rename them.
+When a "struct" object changes, Pd automatically conforms the data from the old
+structure to the new one. Fields with the same name as previously are maintained
+(reordering them as necessary); and if a field disappears but another of the
+same type appears, the new one(s) are taken to be renamings of the old one(s)
+in order of appearance. New fields which cannot be matched in this way with
+previously existing ones are assumed to be new and are initialized.
+
+<P> It can happen that two "struct" objects compete to define the same data
+structure, or that the user reads in data from a file which expects a different
+version of the structure, or alternatively, that the "struct" object for
+existing data objects disappears. For this reason, Pd maintains a private
+representation of the last active version of a "struct" until all
+similarly named "structs," as well as all data using that "struct", have
+disappeared. If the user introduces a new version of the "struct" and only
+later deletes the "current" one, the data is only conformed to the new version
+once the old one is deleted. In this way we avoid getting into situations
+where data is left hanging without its structure definition, or where data ends
+up belonging to two or more structures of the same name. The worst that can
+happen is that data may lose their drawing instructions, in which case Pd
+supplies a simple default shape.
+
+<H3> <A name=s9.4> 2.9.4. Limitations </A> </H3>
+
+<P> When examples get more complicated and/or dense than the one shown here, it
+becomes difficult to see and select specific features of a data collection;
+more work is needed to facilitate this.
+There should be some facility for turning drawing instructions on and off, or
+perhaps for switching between versions of a template, depending on the user's
+desired view. There should also be a callback facility in the template for
+when an object is edited with the mouse, so that the user can bind actions to
+mouse clicks.
+
+<P> More generally, the collection of traversal objects that Pd provides is
+adequate to support a variety of modes of data collection and use, such as
+analysis and sequencing. But the patches required to traverse the data
+collections are not always simple. It would be desirable to find a more
+straightforward mechanism than that provided by the "pointer", "get"
+and "set" objects.
+
+<P> The "data" facility, although part of the original plan for Pd, has only
+recently been implemented in its current form, and as (hopefully) the user base
+grows there will surely be occasions for many further extensions of the data
+handling primitives and the graphical presentation and editing functions.
+
+</BODY>
+</HTML>
+