October 2009: Twelve hours later...
February 2010: Finally, a real parser.

Original Twelve hours later... Finally, a real parser. Editable
version 1 & 2 of 2

Two years ago, I was bit by the procedural audio bug and cranked out CFA. Today I felt like doing it again. CFA was a mess because you had to build all of your samples out of sine waves and envelopes. It was pretty slow, requiring minutes of render time to get a few seconds of sound.

Once again, I am taking the two really good parts of ContextFreeArt (the inspiration for ContextFreeAudio) for this project. First, infinite recursion is a good thing. In fact it is the only way to get anything done. Second, non-deterministic function calls. If you have three functions with the same name, each is called one third of the time.

MIDI seemed like a much better way to go. FluidSynth is probably the easiest of all MIDI synths to control. Just dump every little command into standard in! This also means you have to take care of every little thing, too. There is nothing to help keep notes in synchronization. So your layer above fluidsynth needs to be fairly realtime.

Making a an initial wrapper for FluidSynth was pretty easy, but a lot of features were dropped. It only supports channels, notes and durations. Dynamics, (non-default) instruments and a slew of tunables were ignored. Now it is much more complete. Support for dynamics is pretty weak though.

On top of this went a second wrapper, the music engine. It looks at the programmed rules and tell Fluidsynth what notes to play. This layer new about things like chords and durations, durations and channels. More features were dropped, such as channels. There are three major components within it. One is for soft realtime operation. Another is to keep the infinite recursion in check. The final component process an abstract syntax tree which contains the entire composition.

Finally, a third wrapper provides a crude interface. You specify a file on the command line. When the file is edited, the changes are read in, parsed and dynamically loaded into the syntax tree. The new rules take effect immediately.

Here is how the rules work:

    chord 80 for 1
    next Single
    chord 32 37 39 44 for 8
    next Several
    chord 80 for 2
    chord 85 for 2
    chord 87 for 2
    next Tune
    chord 32 for 1
    chord 33 for 1
    next Forkbomb Forkbomb
    chord 80 for 1
    next MutualB
    chord 85 for 1
    next MutualB
    chord 87 for 1
    next MutualA

These are crude examples and are missing channel set up. "Single" plays the same note every beat, looping forever. "Several" plays the same chord every eight beats, looping forever. "Tune" plays a pattern of three notes over and over again. "Forkbomb" will try to destroy your computer, while playing the Jaws theme. After a few forks, it will catch on and stop it from doing damage. The "Mutuals" are mutually recursive and play a slightly random pattern. Primitive, but this is just the proof of concept.

Originally, I had spec'ed out a huge an complex language to replace this. It was fun, but lead to death by architecture astronaut. Now I am building it incrementally, only adding a feature if it reduces the size of a composition by half. Here are some of the current meta operations:

channel 2
    inst 0-48
    volume 50
    chord 0 for 4 on 2
    next a
    chord 40 for 1 on 2
    next a
    chord 42 for 1 on 2
    next a

You can set instrument bank and volume for each channel. A chord of zero counts as a rest. The "on 2" means to play the chord on channel 2. To bias probability towards a particular rule, append the name with "*N", making that rule N times more probable. Current pain points: poor dynamic control, no way to manipulate lists of chords, no tempo changes, falls apart above 300 BPM. Those last two will probably need a huge rewrite to the engine. Also considering replacing floating point with fractions.


Now, something a program that needed soft realtime, infinite recursion and hot code deployment should be written in Erlang. But that would be too easy. So here it is in Python, along with a simple sample: ts.py ts.txt Maybe I'll post some audio clips tomorrow. No audio clips yet. It seems at some point I lost my sense of pitch and forgot what makes a good chord. If you want to use this yourself, you'll need install funcparserlib, you'll need and to edit the path to the soundfont and the fluidsynth launch command. I swear I did not choose funcparserlib just because the homepage had a big Arch logo.

Kyle (2009-10-12-08-31-01-719)

A bug: Don't give it a blank text file.

Kyle (2009-10-14-20-55-01-984)