Pure Data is the brains behind the whole thing. It is the audio programming language which makes it possible to actually process the incoming sound in an infinitely more flexible way than any other effect pedal.
Pd’s official website describes it as a “real-time graphical programming environment for audio, video, and graphical processing. It is the third major branch of the family of patcher programming languages known as Max (Max/FTS, ISPW Max, Max/MSP, jMax, etc.) originally developed by Miller Puckette and company at IRCAM. The core of Pd is written and maintained by Miller Puckette and includes the work of many developers, making the whole package very much a community effort.”
It is difficult to explain in a few words what one can do with Pure Data. It is not an audio workstation for recording and editing like Ardour or Qbase, nor is it a digital effect rack, or a sequencing application. It does nothing at all until you’ve written a program (a “patch”).
Pd is a programming environment. This means that once you’ve installed it, well, you can’t hear anything at all. Much like a soldering iron and a bag full of electronic parts isn’t a fuzz stompbox, Pd isn’t an audio application. You basically have to do all the work.
This can be frustrating at first (I remember the first time I installed it… It took me about 48 hours to make a sound). I wouldn’t be lying if I told you that taming Pd requires a fair amount of work. But it’s like learning to read and write : it takes months, even years, but you can end up writing novels.
That being said, Pd has one major advandage over other audio programming environments (like CSound or SuperCollider) : the language is graphical. A Pd patch isn’t a text document with lines and lines of code, it is a network of graphical objects linked together. This feature makes it pretty easy to understand how a simple patch works (SEE introduction to programming with PD).
Pd also benefits from a dedicated pool of users/developpers, who have helped develop Pd over the years and added countless new features (like GEM for 3D graphics).
Pure Data can be downloaded in the Vanilla version (the one published by its creator Miller Puckette) and in the Extended version (including all the extra features developped by the community). I personnaly recommend using Pd-extended.
Below are a few useful links to get started.