117 lines
4.7 KiB
Markdown
117 lines
4.7 KiB
Markdown
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## SuperCollider Racket library
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## Goal
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I planned on writing a client for the SuperCollider (SC) music synthesis
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server for use in the Music Programming section of CSC123. Using the
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interface to SuperCollider, students will have access to real-time
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synthesis techniques.
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## Initial steps
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In the first quarter, after examining implementations in various
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languages, I set out on creating a new implementation in Racket from
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scratch. Quickly, however, it appeared that two quarters is not enough
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time, so I looked into porting the scheme implementation of a
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supercollider client, rsc3, to Racket.
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### The Porting process
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I first tried rsc3 in a scheme interpreter, including ikarus and
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DrRacket's r6rs mode (#lang r6rs). rsc3 ran with very little
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modification (one library funciton name was wrong). However, rsc3 could
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not be installed as a Racket package, due to a conflict of the way
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scheme and racket handle libraries. Installing rsc3 as a package is an
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important aspect since that is how students will install rsc3.
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What I did is instead of using "#lang r6rs", which makes racket behave
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like a scheme interpreter confirming to the r6rs scheme standard, I used
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"#lang racket" and imported the rnrs library that has scheme functions
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in a racket environment. This way I could use Racket's provide and
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package functionality works.
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The process of porting was relatively straightforward. I compiled a
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file, looked for errors, and fixed them. Some defined utility functions
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were not necessary because racket had native implementations.
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### Challenges when porting
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One tricky part was a difference in the list representations between
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r6rs scheme and Racket. Racket uses mutable pairs to construct lists,
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while scheme uses immutable pairs. This makes it so that a list created
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in Racket cannot be used in a scheme function that accepts a list. One
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way around this is to construct lists using mcons in Racket then pass it
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to a scheme function.
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## Features added
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It would be hard for first year students to use rsc3 directly, so I
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created simplified abstractions (functions and structures) to perform
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common tasks of creating and controlling sounds.
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### Instruments
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Students could create instruments with parameters that could be changed
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in real-time. Students can use this defined instruments to play notes
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I used a Racket macro to create the instrument creation form.
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An instrument corresponds to a Synthdef in SuperCollider. A Synthdef is
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a composition of functions called ugens. ugens, which stands for unit
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generator,
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Instruments are associated with functions that play notes. A note
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contains the instrument name and frequency. The functions can turn on a
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note, turn it off, and change some instrument parameters, which will
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only affect the playing note, while the note is playing.
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### GUI elements to control parameters
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One interesting feature I implemented is the ability to easily create
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GUI elements to control note parameters. Currently there is a slider and
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checkbox. The slider works by passing a label, start value, end,
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initial, values, and a callback function that accepts the value of the
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slider.
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While this uses a lambda function, it could be easily changed to accept
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a particular note and the parameter string to change.
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I also added a slider funciton that can be used directly in the
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definition of an instrument. This encourages experimentation with
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parameter values.
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### Filters
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I added functions that can be used to chain effects, such as reverb and
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low pass filters. An effect added after a previous effect will act on
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the output of the previous effect. To implement this, I originally
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planned on chaining nodes using SuperCollider's audio buses. However, I
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found a ugen that could do that called replace-out that provides this
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functionality. That is, replace-out can read, modify, and write to the
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same bus.
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### Scheduling notes
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Using the Bundle feature of the OSC protocol, I could schedule events to
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happen in the future. The function accepts the utc seconds and a
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fraction to denote milliseconds, duration, and the note to play.
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## Cross platform issues
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SuperCollider has versions for Windows, Linux and OS X. On Windows and
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OS X, SuperCollider doesn't need dependencies, but on Linux,
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SuperCollider requires a Jack server to be running. Students will have
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to install Jack, which is a different procedure depending on their Linux
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distribution. It's one line in a terminal on a Debian based distribution
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such as Ubuntu.
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I added a function that starts the SuperCollider server if it's not
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already started whenever the library is initialized. Racket has
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functions to return the OS type, and I issue a system command to start
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the server based on the OS. For Linux, I wrote a script to also start
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the Jack audio server and connect SuperCollider's outputs to the system
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audio output.
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