Anthony Mattox Interaction Design + Digital Art

I’ve been playing around with Pure Data for a while now, along with a few other pieces of audio software. The results of this can be heard in the audio for my recent flash game, Pulsus.

Pure Data is a great application which allows you to control every detail building audio applications from very basic components. Unfortunately, much of the program is very poorly documented, so I’d thought I’d share a few things I’ve managed to figure out.

Here’s a quick walkthrough to create a very basic midi controlled, polyphonic synthesizer in pure data. It doesn’t sound like much, but it’ll get things working.

Generating a Tone

This basic synthesizer will consist of two patches, one patch to generate a tone, and another which will read a midi signal and control a few instances of the tone generator.

This is the oscillator patch which generates a tone from a midi message. The tone is created on the left side and the envelope (the change in volume over the course of a note) on the right.

The tone patch has an inlet at the top, which accepts a message containing a pitch/velocity pair, and an outlet at the bottom which sends out the audio signal generated. The [unpack] object splits the pitch and velocity. The pitch is sent to a series of objects on the left side which generate the tone. The velocity is sent to the right where the envelope will be generated.

The pitch, in the form of a midi note number, is converted to a frequency in Hertz which is then sent to an oscillator, generating an sine wave.

Two messages will be sent for each midi note, the first when the note begins and the second when the note ends. The ‘note off’ message will be the same but the velocity will be 0. The velocity, how hard the note was hit, is usually used to set the volume of the note. The velocity is sent to a [select] object. If it is zero, the message is sent to release the note, otherwise to attack. Each of these send a message to a [vline~] object which sends a signal for the amplitude which either ramps up or down.

Finally, the tone is multiplied by the envelope and sent out the parent patch. When the note is hit, a message is also sent to the oscillator which resets the phase.

MIDI and Polyphony

Now that we have our oscillator we need to create a patch which will control a few instances of it. The patch will receive midi data from a hardware controller or from another piece of software. The midi settings will need to be configured for PD to receive the data.

The PD patch which takes midi input and handles the voice allocation, controlling a number of oscillators.

At the top of our patch we have a [notein] object which reads midi from channel 1. [notein] will output pitch/velocity pairs. The [poly] object handles the allocation of different voices. When pitch/velocity pairs are sent to it it sends on the data with the addition of a voice number.

In order to play multiple notes at once, the messages will be routed to a number of different oscillators, each a different ‘voice’. When [poly] receives a note, it will give the number of the first available voice. When it gets a ‘note off’ message (velocity of 0) it makes that voice available again so that that oscillator can play another note when it needs to.

Now we can route the processed messages to a number of instances of the ‘tone’ patch. We do this by packing the messages into lists and using the [route] object which sorts them based on the first element in the message. Awesome. Finally we just add all the signals from the oscillators together and send them to the audio output. It first also multiplies the signal by a amplitude set by the slider so that we can adjust the volume of the synth.

Ok, So What Now?

Now that we have a simple little synth we should be able to get some sounds. A midi signal can come from a physical midi controller or from software. You can use a program, like Ableton Live, to create a composition and then send out midi. You can also create other pure data patches to generate midi. A patch could create generative melodies, act as a sequencer, or create midi from another input such as the keyboard.

The synth at this point just plays a sine wave for each note. It’s a start, but not thrilling. To create different timbres, the [osc~] object can be replaced with something to play different waveforms and we could add modulators or a more complex envelope generator. On top of that we can run the audio signal through filters either in pure data or in other software. A bit of reverb will make it sound a little less flat.

If you have any suggestions post them in the comments. I’m certainly no expert, and I’d love to hear some other methods.

Recently I performed with a musical group, creating generative imagery as a backdrop. I decided try using quartz composer again, primarily because it renders graphics so quickly and smoothly, but also because it’s fairly easy to put something together. Unfortunately I ran into a significant roadblock. Quartz composer has no way to control elements while they are running. Although you can completely restructure and modify your script while it is running, there are no interface elements to control simple changes in values. Obviously, this makes QC so much less valuable as a performance tool.

Having already set up most of my script in Quartz Composer I decided to find a way to control it some other way. Pure Data, an application similar to Quartz but for the production of sound, allows you to easily build interfaces. The data is then sent out of pure data as midi controller values, sent through a virtual midi channel, and grabbed by Quartz Composer. From there the values can be used for whatever is necessary.

Figuring out the details took a little while and some help from my friend Michael Scoot-Nelson, but the setup is actually quite simple. In Pure Data, just set up as many controller as you need in the form of sliders or other UI elements.

In this case, I have a number of vertical sliders labelled for a particular application. Above each controller I have an object which receives a bang [r b]. In Pure Data the value from each slider is only sent when the slider is changed. Sending a bang to the value ‘b’ pushes all the values down and sends them out to the midi channel. The Sliders should be set to a range from 0-127.

The values from the sliders are plugged into the first inlet of control out objects (ctlout). The ctlout objects also need a controller number and a channel number. These are used to get the value on the other end. The channels can all be set to one, and each slider should have it’s own controller number.

In Quartz composer, a ‘Midi Controller Receiver’ object will grab the values sent from Pure Data. Viewing it’s settings in the inspector, the number of inputs can be changed. Each input is listed with an integer, these are the controller numbers which must be matched to the ones set in pure data. The channels being received can also be changed.

There is one final step to getting everything hooked up. In Pure Data, under Preferences > Midi Settings must be set to an Midi Channel that Quartz Composer is listening for. On a Mac it’s easy to set up this channel with the IAC Driver in Audio Midi Setup.

I also created this little guy here. One element I was controlling in Quartz Composer (a particle generator) could only be set on or off and I needed it to vary smoothly. This creates what is essentially a pulse width modulation simulated analog output. Pressing the ‘on’ and ‘off’ buttons do just that, and pressing the pwm toggle lets you use the slider to set it more or less on.

Metro’s turn the output to on every tenth of a second. It is then turned off (ten times a second) after a short delay (less than a tenth of a second). If the slider is at the bottom it turns back off almost immediately, so for the greater part of a second it is off. If the slider is at the top, it takes nearly a tenth of a second to turn back off, so it is mostly on. In the middle it will be on half the time. The result, at least for my particle generator, was the appearance of it’s rate changing while really it was just turning on and off very quickly. I’m not sure where else this would be useful, but here it is.

It might be a little hard to read, but if it would be useful to anyone I can upload the pd file somewhere.

For a while I’ve been interested in exploring sound as a new medium. Pure Data is a sound program which I’ve been particularly interested in. The program is something like a visual programming language, with a similar interface to quartz composer. Objects which represent chunks of code are placed on the canvas. These objects have inputs and outputs through which they send and receive data in the form of numbers, strings, and audio signals. Some graphic interface elements call also be added to control applications.

Starting to work with Pure Data is a little intimidating. Objects added to the stage are blank and you have to type into them what object the should be. Until you understand what the basic objects are and how they interact trying to get anything working isn’t easy. For at least a good while I’ve been opening up Pure Data every few months only to put it away again after beating my head against it for a while.

For my recent flash game, Pulsus, I decided to create the sound using Pure Data to force myself to learn the program. I managed to cobble together a basic understanding and build a few synthesizers and sound generators.

I used this first patch to create most of the sound effects in the game. For a number of oscillators the pitch and envelope can be changed. The pitches can create harmonics, harmonies, or dissonant chords. The envelope, the volume over the course of the sound, creates pulsing tones, short beats, and any other type of tone. I also added an amplitude modulator and a global envelope to add some more control.

In my next experiment I created a simple mono synthesizer. Key inputs, from my computer keyboard, are mapped into midi notes. When a key is pressed the frequency slides to that note if another note is still playing. Key presses trigger the envelope generator which reads data from an array (top right). The synth also has frequency and amplitude modulators and reads the waveform from a table to include harmonics.

Next I build a polyphonic synthesizer which has a separate oscillator for every note in the scale.

Here are the pure data files for these patches in case they might be useful to anyone, but again they are not super efficient, organized, or annotated.

• tone generator
• monosynth
• polysynth
• polysynth tone (an abstract for the polysynth)

These are my first moderately successful explorations with Pure Data. Some things, I realize, are not done as efficiently as possible, but I’m working things out in the next iteration. My next frustration is to find a way to control the instruments I build. I need a midi sequencer with which I can construct songs that could then send the midi info to Pure Data. I tried using a garage band plugin to output midi info from garage band but It came out pretty garbled in PD, I could be doing something wrong though. Any thoughts on how I should go about this?