Building Drums in PD

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Autor:	Frank Barknecht
Contact:	fbar@footils.org

Abstract

This quicktoot describes how PD can be used to build a simple drum +synthesizer with graphical controls and a step sequencer.

Contents

Small is beautiful
The Creator
Bring The Noise
Let It Swing
Going faster

Small is beautiful

Pure Data - or short PD - is a multimedia software environment written +by Miller Puckette and others that can be used to make art in several +media like music, sound or graphics. In this quicktoot we will use the +sound generating capabilities of PD to build a module for drum sounds. +Our drum synthesizer will use a synthesis algorithm that's very +similar to the early analog or digital drumsynths of the eighties. We +will not use pre-recorded samples but instead employ simple sound +generators. You will be surprised, how effective this is and how much +variety is possible with just two or three sound sources. On the other +hand our design leaves room for improvment. By the end of this article +you will be able to add these on your own. Our drummer includes a +preset database: When you have found a sound, that you like, you can +save it and recall it again later.

I will not go into much detail on how to install or how to use PD: You +should read the documentation that comes with PD for that. I will of +course explain the more advanced features of PD used and everything +that is needed to follow this tutorial.

The Creator

Creating something in PD is very much like the genesis of a world as described +in, well, the Bible's first book "Genesis". In the beginning you have nothing, +just a large white area. You, the Creator, can create objects by a simple +button press "Ctrl-1", which actually means "Let there be an object". And there +was an object. But your first object is a small empty rectangle and it can do +nothing, it has no way of communicating, it isn't alive yet. To create a living +object you have to give it a name, you have to call it by typing the name onto +the object, for example "noise~". And there was noise. With time you can +populate the area with more objects, you can let them talk to each other, you +can create more worlds, and you will create by giving all of them names. We +maybe come to [moses] later. Our drumsynth will have the name "angriff", +which is german for "attack". So our goal is, that someone can create a +drumsynth by typing the name "angriff" on a new object.

This is achieved in PD by a so called "abstraction". An abstraction is +simply a saved PD patch. So our very first and basically unusable +version of angriff is this: A nearly empty patch with a noise~ object +inside, saved under the name "angriff-01.pd".

angriff-01.pd, an unusable first version.

Even this can be used as an object. Just create a new, empty patch +(with "Ctrl-n"), put an object in it and call it "angriff-01", i.e. the +name of our first drum patch without the .pd-suffix.

Using the unusable angriff-01.pd in another patch.

PD searches for something that matches the object "angriff-01" in +several places: in the list of builtin objects like "noise~", in +external libraries or in its path for abstractions. If angriff-01.pd +is in the same directory as the using patch or in a path known to PD +through the startup option "-path /a/pd-path/", it gets found. If you +click on angriff-01, you can open it to see, what's inside. +Abstractions itself can use other abstractions so it's a good idea to +put often used functionality into abstractions. As our final drumsynth +should have a sound generating part and a GUI to control this, we will +seperate both parts from each other by use of abstractions. For the +synthesis of sound we will build and use an abstraction called "drumcenter" +inside of the main "angriff" patch. The main patch includes the control +elements for the parameters provided by "drumcenter". This way our +synthesis model and the view to the user are separated.

Bring The Noise

How do drums drum? Although human dummers can spend hours tuning their +drumset, drums are generally non-pitched, percussive sounds. So for +our first usable version of a drumsound we take some noise as a sound +source and put an amplitude envelope around the noise. A simple, +builtin envelope generator in PD is line~. drumcenter-01.pd show the +noise, the envelope and both multiplied together.

Enveloped noise, drumcenter-01.pd

If you listen to this patch, it already sounds a bit like a snare, +doesn't it? To make this sound a bit more variable we can use filtered +noise, for example with a lowpass or highpass filter. PD comes with +both, so lets use them. Her's the noise filtered with the lop~ filter +at a cutoff frequency of 400 Hertz:

Lowpass filtered and enveloped noise, drumcenter-02.pd

Now our patch is beginning to get a bit crammed and chaotic, so let's +replace some chords with "send" and "receive" pairs, abbreviated "s" and "r". +If we give those pairs names starting with $0, they get names starting +with unique numbers instead of the $0 while performing. This way, we +can later use several instances of our patches without conflicts +between their own send and receive variables. We can also hide stuff +we don't need to see in so called subpatches. Subpatches are areas +inside a patch that are hidden in an object box starting with [pd]. +They are like curtains hiding things in a room, but the hidden objects +are a full part of the surrounding patch. The special objects [inlet], +[outlet], [inlet~] and [outlet~] create points of connections between +the surroundig patch and the subpatch. They work with abstractions as well.

Cleaning up in the house, drumcenter-03.pd

Let It Swing

I lied in the previous chapter: Drums are indeed pitched, although +just a little bit pitched. If a drummer kicks a bassdrum, the drum +begins to oscillate slightly at a low frequency. But this oscillation +is soon damped to an even lower frequency and then disapears. To +simulate this behaviour, we use PD's [osc~] object with an added +frequency envelope as show in drumcenter-04.pd:

Percussive oscillator, drumcenter-04.pd

The right inlet of [osc~] is used here to set the start phase of the +oscillator. If we wouldn't set it, it would be different everytime we +start the envelope, because an object like the [osc~] is always on and +goes through its cycle even if we don't listen to it. The main inlet +sets the frequency, that's going from a starting value to a lower +frequency in a short time.

Going faster

So far we used [line~] for all envelopes and a controlling message +like [1, 0 50(. This tells [line~] to first go immediatly to 1, then +start going to 0 for 50 milliseconds. For our oscillator frequency +this is fine, but instantly going to an amplitude of 1 produces +clicks, as you might have heard. So we need to specify an attack time, +in that the amplitude raises from 0. The first guess would be a +message like [1 10, 0 50(, but that simply doesn't work. So this has to +be solved with two messages to the [line~] object, for example [1 10( +to go in 10 msecs to 1 and then the release: [0 50(. Between them a +[del] is placed, that delays for as long as the first line segment's +duration is. But this also has a problem, as shown in the next figure: +a [del] always delays in blocks of 64 samples, and that is a time of +around 1.45 msecs at a sampling rate of 44100 Hertz. A bit too long +for the short attack times used in drum synths.

Two line~ segments started with delay, fastline.pd

This problem isn't trivial at all and it appears everywhere, one wants +to contol messages in PD quicker than the signal blocksize allows, for +example in granular synthesis patches. A solution to this are " +Time-Tagged Triggers" (T3) as proposed by Gerhard Eckel and Manuel +Rocha Iturbide for Max/FTS. In PD T3-objects are made available as +externals in IEMLIB. Time-Tagged Triggers are a replacment for the +standard "bang" message that is normally used in PD or Max. Eckel +writes: "In Max/FTS, a T3 is nothing else than a message containing one +floating-point number which specifies the delay in ms after which, +counting from the current tick, the trigger should go off." We will now +use the T3 break point envelope generator [t3_bpe] to generate faster +envelopes.