Racer Session #037 | Brandon Lucia | October 3, 2010
We are all familiar with the concept of chance. Music based on the idea of chance is interesting, and unpredictable for the listener, and challenging for the performer. This week, we are not going to think about randomness. Instead, we’re going to think about a related, but distinct idea: nondeterminism. An example of something nondeterministic in the real world is the location of a traffic jam – it isn’t random, because it is the result of some idiot or an accident or something, but it is very hard to predict in advance where a traffic jam will take place. This example captures the central idea of the concept, and I’m going to explain why in more detail, and how we can use this idea in music.
The name of the piece I am presenting this week is Nondeterminate. Nondeterminate is music that I wrote in a computer programming language, that is interpreted and played by a computer, uniquely, every time the program is run. The key idea of this work is that it explores the idea of nondeterminism in complex systems – in this case computer systems. There are four questions that I think are important to answer about the piece: (1)What is nondeterminism? (2)Why are computers nondeterministic? (3)How can a computer program/piece of music be “interpreted” by a computer “uniquely”? (4)What you should listen for when you hear Nondeterminate. Along the way, I’ll mention some related music you might want to check out, and at the end, I’ll talk about other ideas I have related to this idea of nondeterminism, and how we can use it in our improvisations.
The most important thing to sort out is what I mean when I say “nondeterminism”. We all have some definition of this in our heads, I’m sure. In this context, I consider an event nondeterministic if it is hard to predict, but not random.
The flow of particles in a body of fluid is another example of something nondeterministic – it isn’t random, but we can’t figure out how to precisely model it (it is a 1000 year old open problem in mathematics to tractably model fluid flow without randomness). The idea that randomness closely models nondeterminism allows us to approximately think about the idea, but is somewhat deceptive. Very few things are truly random, and it is important to keep in mind that nondeterministic events have a clear causal chain, but that it is very difficult to understand or predict. In fact this is one of the key ideas of the piece today: In Nondeterminate I quite deliberately did not use randomness.
While I was writing this, I had a hard time thinking of anything that is *actually* random, and isn’t just best understood as a random phenomenon. Even the flip of a coin is the result of, amongst other things, air pressure, wind direction, the flipper’s arm strength, etc. We can use randomness to statistically reason about coin flips (about 50% heads, 50% tails), but for a specific flip, we can actually watch it happen, though it is hard to really understand due to the complexity of the world. Nondeterminism is all around us, and if we look carefully enough, we can see through what appears random and attempt to understand it.
The absence of randomness in Nondeterminate was important to me, because the idea of chance in music has been explored deeply by many people (even at the Racer Sessions, when Ray Larsen and Kristian Garrard curated). One example that is particularly relevant in this discussion is ‘Pithoprakta’ by Iannis Xenakis. This piece is a musical encoding, for a small orchestra, of a mathematical model of the flow of particles in a fluid. This model is approximate, and relies on randomness to simulate nondeterminism – we have all probably heard of 'brownian motion’; that is the model that Xenakis turned into music.
I found out about this piece of music after I performed an experiment with a fluid dynamics simulation of my own. I have a program that models the interaction of particles in a fluid that some physicist somewhere wrote. Using the same technique I used to make Nondeterminate a piece of music (which I will explain in just second), I made this simulation a piece of music. The result was noise. The reason is that the simulation uses a random number generator to emulate particles’ interactions.
So nondeterministic events are non-random, but difficult to predict, and always have a causal chain. Why are computers a good example of this?
Today’s computers are designed to do many things at the same time. The idea of doing things at the same time is called “concurrency”. A computer program is, at its heart, just a list of things for the computer to do. Add 1 + 2, read a value from the computer’s memory, etc. Capturing the idea of concurrency, programmers can create programs made up of individual “threads” of execution. Each thread is, in effect, it’s own program. The threads all run through their lists of things to do concurrently. Threads can run, and ignore the activities of the other threads, but more often, threads must communicate with one another.
The threads are operating independently in a concurrent program. That means if I am thread #1, and you are thread #2, I don’t necessarily know what you’re doing, and you don’t know what I am doing. But occasionally, I need some piece of information that you recently computed, and so I communicate with you to get it, so I can continue doing my list of things. However, threads may also power through their lists of things at different rates, and the rate at which a single thread makes progress is very difficult to predict (in fact, this is an open problem in computer science). Furthermore, from one execution to the next, a thread’s rate of execution may vary from prior and future executions due to environmental factors (the temperature of the air can even have an impact on this). The result is that the order in which the communications that take place between threads is difficult to predict, but not random – it is nondeterministic. This nondeterminism is the nondeterminism I intend to explore.
The next thing to explain is how a computer can transform a non-deterministic pattern of inter-thread communication events into a piece of music. To do this, I wrote a program that takes any arbitrary program as its input. This program is called Audiolyzer (I released it on the web at http://github.com/blucia0a/Audiolyzer). The way Audiolyzer works is by analyzing the code that makes up each thread in the program that was its input. During that analysis, it finds communication events, and marks them with a special marker. This marker generates a tone when the program runs. Each thread has its own unique tone, and so when you run a program using Audiolyzer, you can hear the interleaving of the communication events in the program’s threads. The interleaving is non-deterministic from run to run, so the pattern of sounds you hear each time you run the program is unique. The computer is interpreting the program and generating a new piece of music with each execution. Nondeterminate is effecively a piece of music composed in the medium of C++ code to be performed by a computer. I will be in the audience with you, while my laptop plays for us.
Now I’m going to discuss some things you should listen for when you listen to Nondeterminate. The piece is organized in four movements. Each is intended to explore a different aspect of nondeterminism. When you are first listening, you will hear what sounds like chaotic bleeps and bloops all randomly mixed together. It isn’t the case! Listen carefully and patterns will emerge.
The reason for the patterns is that there is one additional constraint on the interleaving of communication events. That constraint is that every thread eventually needs to get a turn. The computer is being fair. For example, if there are 16 threads (like there are in movement #1), you may hear the tone that corresponds to thread #1 first. After that, you will not hear thread #1’s tone again, until you’ve heard all the other threads’ tones. This ensures that every thread gets to do its communcation without being “starved”. The patterns in this case will be length 16 sequences of tones.
It isn’t a hard and fast rule, however, which is another interesting feature of the piece – you will hear the sequences mutate as the piece unfolds. The computer tries to be fair, but it can only react within the bounds of its environment. Two threads may have their communication order inverted, just because of environmental factors that change the rate of their execution (again: air temperature, other programs, etc). Fairness plus mutation leads to procedurally generated music.
An aesthetically related piece you may have heard is by Steve Reich, called “Tokyo/Vermont Counterpoint”. Roughly, it begins with a simple pattern, which then mutates over the course of the piece. The main difference between Nondeterminate and this work is that Nondeterminate generates a *different* base pattern with each execution (in fact, many times during each movement), whereas Reich’s piece is statically defined by its base pattern (though Reich’s piece has points where the pattern abruptly changes as well, it is still a change to a predefined new pattern).
The movements are structured in the following way: Movement 1 sets the stage, using 16 threads to introduce the concept of nondeterminism, and (hopefully) revealing the pattern concept that is explored in later movements. Movement 2 divides the threads into two groups of 16 – 16 are low, and 16 are high. Within a group, there are nondeterministic interleavings, but the groups interleave, deterministically alternating. The third movement is an attempt, within the confines of programming as a compositional medium, to *eliminate* nondeterminism. This movement is a parody of the other three, creating 8 groups of 4 that typically proceed in sequence, and the tones of which interleave in a much more predictable way. Movement 4 is the culmination of the piece. It creates 64 threads, spanning the spectrum from A3 to C8. The patterns are 64 tones long, and therefore take some time to discern. However, by the end of the piece, it is clear what the order is, and the mingling of nondeterminism, and mutations is illustrated.
I chose to keep this piece simple. I did not vary the rhythmic pattern of the piece. I did not use a particularly interesting voice (it is a simple sine wave). There are two reasons, one more convincing than the other: The less convincing is that I wanted the concept of nondeterminism to be the highlight of the piece (I think that comes across as the piece is), and the more convincing is that writing music in multithreaded C++ is difficult, and it was a challenge to make something that sounded coherent at all.
Finally, I’d like to note that the idea of nondeterminism doesn’t begin and end with computer systems. Lots of other sources of nondeterminism exist, and may have different interesting patterns than those found in computer systems. I think it would be interesting, for example, to create a piece of music that was a continuous tone, the pitch of which corresponded to the density of traffic on a stretch of highway over a period of days. Because of the fact that people go to work in the morning and come home in the evening, patterns would emerge, but to a degree, because people are flaky and whimsical, it is likely that the tone would vary a great deal in smaller, nondeterministic ways as well.
The improvisations will be loosely structured this week. I have one or two ideas that are related to nondeterminism, and after that things will be more free-form, with some loose, abstract constraints. When you’re playing, try to keep the concept of nondeterminism, fairness, and mutation in your head. Start, and let a pattern emerge. Then as a group, work with that idea, without changing it. Slowly allow things to change and mutate to become new things. Your interactions with your fellow improvisors are nondeterministic. Use that fact to make music that embodies the complexity of the world.
I hope you all enjoy the piece, and if you’re saavy with computers, you can download the source code to Audiolyzer, as well as the source code to Nondeterminate from http://github.com/blucia0a/Audiolyzer.
Recommended Listening:
Iannis Xenakis - Pithoprakta (explores randomness in complex systems as music)
Krzystof Penderecki - De Natura Sonoris (uses wide ranges of tones)
Steve Reich - Tokyo/Vermont Counterpoint (explores patterns and mutations)
Terry Riley - In C (composed for simple voice and rhythm, in C)