## SuperCollider Racket library


## Goal
I planned on writing a client for the SuperCollider (SC) music synthesis server for use in the Music Programming section of CSC123. Using the interface to SuperCollider, students will have access to real-time synthesis techniques.

## Initial steps
In the first quarter, after examining implementations in various languages, I set out on creating a new implementation in Racket from scratch. Quickly, however, it appeared that two quarters is not enough time, so I looked into porting the scheme implementation of a supercollider client, rsc3, to Racket.

### The Porting process

I first tried rsc3 in a scheme interpreter, including ikarus and DrRacket's r6rs mode (#lang r6rs). rsc3 ran with very little modification (one library funciton name was wrong). However, rsc3 could not be installed as a Racket package, due to a conflict of the way scheme and racket handle libraries. 
Installing rsc3 as a package is an important aspect since that is how students will install rsc3.

What I did is instead of using "#lang r6rs", which makes racket behave like a scheme interpreter confirming to the r6rs scheme standard, I used "#lang racket" and imported the rnrs library that has scheme functions in a racket environment. This way I could use Racket's provide and package functionality works.

The process of porting was relatively straightforward. I compiled a file, looked for errors, and fixed them. Some defined utility functions were not necessary because racket had native implementations.

### Challenges when porting
One tricky part was a difference in the list representations between r6rs scheme and Racket. Racket uses mutable pairs to construct lists, while scheme uses immutable pairs. This makes it so that a list created in Racket cannot be used in a scheme function that accepts a list. One way around this is to construct lists using mcons in Racket then pass it to a scheme function.



## Features added

It would be hard for first year students to use rsc3 directly, so I created simplified abstractions (functions and structures) to perform common tasks of creating and controlling sounds.

### Instruments


Students could create instruments with parameters that could be changed in real-time. Students can use this defined instruments to play notes

I used a Racket macro to create the instrument creation form.

An instrument corresponds to a Synhtdef in SuperCollider. A Synthedef is a composition of functions called ugens. ugens, which stands for unit generator,

Instruments are associated with functions that play notes. A note contains the instrument name and frequency. The functions can turn on a note, turn it off, and change some instrument parameters, which will only affect the playing note, while the note is playing.

### GUI elements to control parameters

One interesting feature I implemented is the ability to easily create GUI elements to control note parameters. Currently there is a slider and checkbox. The slider works by passing a label, start value, end, initial, values, and a callback function that accepts the value of the slider.

While this uses a lambda function, it could be easily changed to accept a particular note and the parameter string to change.

I also added a slider funciton that can be used directly in the definition of an instrument. This encourages experimentation with parameter values.


### Filters

I added functions that can be used to chain effects, such as reverb and low pass filters. An effect added after a previous effect will act on the output of the previus effect. To implement this, I originally planned on chaining nodes using SuperCollider's audio buses. However, I found a ugen that could do that called replace-out that provides this functionality. That is, replace-out can read, modify, and write to the same bus.

### Scheduling notes

Using the Bundle feature of the OSC protocol, I could schedule events to happen in the future. The function accepts the utc seconds and a fraction to denote milliseconds, duration, and the note to play.

## Cross platform issues

SuperCollider has versions for Windows, Linux and OS X. On Windows and OS X, SuperCollider doesn't need dependencies, but on Linux, SuperCollider requires a Jack server to be running. Students will have to install Jack, which is a different procedure depending on their Linux distribution. It's one line in a terminal on a Debian based distribution such as Ubuntu.

I added a function that starts the SuperCollider server if it's not already started whenever the library is initialized. Racket has functions to return the OS type, and I issue a system command to start the server based on the OS. For Linux, I wrote a script to also start the Jack audio server and connect SuperCollider's outputs to the system audio output.