# 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. All the
`synthdef`s that ship with Sonic Pi are designed in a Clojure based
library called [Overtone](https://overtone.github.io). You can find the
existing synth designs in the folder

```
etc/synthdefs/designs/sonic_pi/synths/
```

If you wish to contribute synths to Sonic Pi then they need to be in
this format. However, it is also very easy to write and trigger your own
synthdefs in SuperCollider language if you are already familiar with
that.


## Synth design constraints

Also 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.


## 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 (`brew install 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/sonic_pi/synths/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/sonic_pi/synths/core.clj`
  * Evaluate the namespace as you normally would for Clojure in emacs

## The synth design file

Taking the example of a basic synth lets 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.

## Compiling your synths using SuperCollider

If you're just experimenting or wishing to build your own synthdefs and
already know SuperCollider, there's no need to use Overtone. Simply
define your synth 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.


    (
    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\") ;
    )


## Making the synth available in Sonic Pi

To make the synths available simply 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.

However, if you want the synths to be visible to the GUI you'll need to
add the appropriate metadata to
`app/server/sonicpi/lib/sonicpi/synths/synthinfo.rb` and re-compile the
app.

Have fun!