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Synth Design

The sounds you hear in Sonic Pi are all produced by the SuperCollider synthesis engine. These synths and effects have to be defined beforehand in a special kind of binary file called a synthdef. The built in synthdefs are loaded up on boot so they are ready to trigger.

Design Methods

Synthdefs are ultimately compiled from source code into binary files, but there are two main ways you can do this. You can either use the Clojure based library called Overtone, or directly use SuperCollider's own language instead.

You can find the source code for the synthdefs that currently ship with Sonic Pi in the folder

etc/synthdefs/designs/

Most of these were originally designed in Overtone. Please note however, that for new synth designs that are intended to be distributed with Sonic Pi,the preference is that they are created with SuperCollider, as it is much easier to set up, create and maintain synths this way. As such, the instructions for using Overtone still work, but if you are considering submitting a new synth design for distribution with Sonic Pi, we strongly recommend that you create your synth with SuperCollider directly.

(Note also that detailed information on Overtone and SuperCollider syntax is beyond the scope of this document. However, the information contained below should be a reasonable guide to getting set up and running, as well as giving a brief outline of both the source code and process required to create and use your own synths).

Synth design constraints
Using Overtone to design and compile your synths
Using SuperCollider to design and compile your synths

Synth design constraints

If you want your synth to work with Sonic Pi's automatic stereo sound infrastructure you need to ensure your synth outputs a stereo signal to an audio bus with an index specified by a synth arg named out_bus.

Additionally, your synth must self-terminate at some point - Sonic Pi will not tidy up zombied synths.

Using Overtone to design and compile your synths

Editing the synthdefs

You'll need the following to be able to compile the synths yourself:

  • The latest version of Overtone (git clone git@github.com:overtone/overtone.git)
  • Leiningen
  • A text editor setup to work with Clojure

After cloning the Overtone repo, cd into the folder and edit the project.clj file to add the Sonic Pi synthdefs folder to the locations it looks in when requiring Clojure files.

  :native-path "native"
  :min-lein-version "2.0.0"
  ;; this is the line to add
  :source-paths  ["src"  "/Users/foo/sonic-pi/etc/synthdefs/designs"]
  ;; make sure the path points to your installation of Sonic Pi
Starting a REPL

cd into the Overtone folder and run

$ lein repl

Make a note of the port number that nREPL starts on:

nREPL server started on port 49223 ...
Connecting the editor to the REPL and booting Overtone

If you're using vim and vim-fireplace:

  • open a new terminal window
  • cd back into the Sonic Pi folder
  • Open etc/synthdefs/designs/overtone/sonic-pi/src/sonic_pi/core.clj
  • type :Connect then hit enter
  • when prompted, enter the nREPL port number from earlier
  • Visual select the namespace definition and type : followed by Eval then enter

This should start booting Overtone which will take a little while. If there are no errors, your vim is successfully hooked up to Overtone!

If you're using emacs:

  • open a new terminal window
  • open emacs and go to the project.clj for Overtone
  • M-x cider-jack-in to connect to the running nREPL
  • Navigate back into the Sonic Pi folder and open etc/synthdefs/designs/overtone/sonic-pi/src/sonic_pi/core.clj
  • Evaluate the namespace as you normally would for Clojure in emacs

The synth design file

Taking the example of a basic synth, let's have a look at what the bits are doing:

(ns sonic-pi.synths.studio
  (:use [overtone.live])
  (:require [sonic-pi.synths.core :as core]))

(do
  (without-namespace-in-synthdef
    (defsynth sonic-pi-recorder
         [out-buf 0 in_bus 0]
         (disk-out out-buf (in in_bus 2))))


  (uncomment
    (core/save-synthdef sonic-pi-recorder)))

This is the definition used to define the record functionality in Sonic Pi.

The without-namespace-in-synthdef from sonic-pi.synths.core needs to wrap the defsynth for the synth to follow the correct naming convention.

The core/save-synthdef manages the workflow for saving the compiled synthdef file into the correct folder and also the graphviz design which is used for documentation.

By evaluating the whole form this should cause all the files to be saved to the correct places.

Using SuperCollider to design and compile your synths

You will need to have Supercollider installed on your computer. Simply define your synth with SuperCollider's built in language, and use the writeDefFile() command to store the compiled synthdef into a directory of your choice. You can then dynamically load your synthdefs with the load_synthdefs fn.

The synth design file

Below is a small example of a synth design:

    SynthDef(\\piTest,
             {|freq = 200, amp = 1, out_bus = 0 |
               Out.ar(out_bus,
                      SinOsc.ar([freq,freq],0,0.5)* Line.kr(1, 0, 5, amp, doneAction: 2))}
    ).writeDefFile(\"/Users/sam/Desktop/my-synths\");

This is a simple synth that causes two Sine wave oscillators to sound on two separate channels for a short time.

Let's examine it at a high level:

SynthDef is the SuperCollider function that creates a Synth Definition.

Here, we pass SynthDef two parameters: \\piTest, (the name we want to give to the synth, preceded by \\), and a function enclosed in curly brackets {...}, that describes the components of the synth that will generate the sound. (This needs to adhere to the Sonic Pi synth design constraints as mentioned above).

Next, writeDefFile() is used by SuperCollider to create the compiled binary file from the source definition, and we pass it the location of a folder where the compiled file is then stored.

To actually trigger the creation of the compiled file, once we have entered the above synthdef source code into a new file in SuperCollider, we can select the menu item Language > Evaluate File. The 'Post Window' (SuperCollider's log window) will show -> a SynthDef, indicating that the code was successfully executed and the result was a new synthdef file, as desired. The new file will be waiting in the location we set with writeDefFile().

Making the synth available in Sonic Pi

There are two choices for making your synth available in Sonic Pi.

'Loose' integration into Sonic Pi

Integrating a synth into Sonic Pi 'loosely' will allow you to use it but will not make features such as autocompletion of synth opts available.

To enable a synth in this manner, firstly, make sure that the setting 'Enable external synths and FX' is turned on in the Preferences pane under Audio > Synths and FX.

Next, in your Sonic Pi code, call load_synthdefs with the path to the folder containing the synthdefs you've compiled and they'll be immediately available to Sonic Pi via the synth fn.

Lastly, for synths like these that are not tightly integrated into Sonic Pi, You call them by name with a string value - eg: synth 'piTest'.

'Tight' integration into Sonic Pi

Integrating your synth into Sonic Pi in a tighter manner will allow you to use it without having to enable external synths and FX or explicitly load the synth path into memory beforehand. It will also enable features such as synth opt autocompletion.

However, there's a little more involved when doing it this way. You need to add the appropriate metadata to app/server/ruby/lib/sonicpi/synths/synthinfo.rb and re-compile the app.

Lastly, as with built-in synths, you would call the synth by name with a symbol. Eg: synth :piTest.

We look forward to hearing about your synth and FX creations - have fun!

Gated synths

As described above, standard Sonic Pi synths always self-terminate. That is, they have an ADSR envelope with a finite duration, so they finish, and then they are automatically cleaned up from memory. This is a design decicision which aims at preventing users from starting synths that never terminate and finally fill up processor and memory.

Gated synths are non-standard in Sonic Pi. They use an envelope that is kept open at sustain level at a defined release node until a special parameter, the gate is set to 0. Only then they start the release phase after which they finally terminate. At this point they are cleaned up, just like the standard synths. The problem with gated synths is that you have to have a handle on a synth, so that later you can control the gate to 0. You obtain this handle by assigning the result from play or synth to a variable, like so:

a = play 60

On the contrary, if you miss this assignment, there will be no way to control the synth and set the gate to 0, so it will run forever, and the only way to stop it is to stop the entire run.

However, gated synths can be fun, especially with external MIDI keyboards. The expected behaviour for playing music over a keyboard would be as described above: When the key is hit, the envelope progresses through the attack and decay phases, but it is kept at sustain level until the key is released.

Sonic Pi comes with gated synths, but they are not directly accessible as standard synths, nor are they listed in the built-in help system. If you want to use them, you can find the sources in these directories

etc/synthdefs/designs/overtone/sonic-pi/src/sonic_pi/gated
etc/synthdefs/designs/supercollider/gated

and the compiled ones in

etc/synthdefs/compiled/gated

Using them requires loading them. A simple usage example would be:

load_synthdefs "<<path-to-your-sonic-pi-repo-fork>>/etc/synthdefs/compiled/gated"
use_synth 'sonic-pi-fm_gated'
sth = play 43
sleep 4
control sth, gate: 0

Some final notes:

  • It is perfectly possible to play the standards synths with a MIDI keyboard. Using gated synths is a bit more straightforward, e. g. when trying to get the synth to use the release phase as selected with the release: parameter. Look for "MIDI keyboard" on in-thread for examples of how a MIDI keyboard handler can be made, with or without gated synths.

  • Some synths have been left out on purpose. The :dull_bell and :pretty_bell synths depend on the interrelations of the envelopes of their partials and no reasonable way of adding a gate parameter was found. Also, merely percussive synths like the SC808 group of synths were not enabled to be used with a gate.

  • On some synths, e. g. :kalimba, it is necessary to reduce the default :release in order for the gate to take effect.