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diff --git a/desiredata/doc/1.manual/x2.htm b/desiredata/doc/1.manual/x2.htm deleted file mode 100644 index dd33b149..00000000 --- a/desiredata/doc/1.manual/x2.htm +++ /dev/null @@ -1,1276 +0,0 @@ -<!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"> - </HEAD> - - -<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> - |