I recently spent a little time trying to make a version of Terry Riley’s In C using Ableton Live.

The original piece, if you’re not familiar with it, is a landmark of aleatory minimalismāa score with 53 short fragments that are repeated by the players in an ensemble of varying size. Each fragment is repeated until a player decides to move on to the next. The collective result of these individual decisions is a vibrant pulsing mass of sound, with emergent melodies and rhythms coming from the phasing and combination of the different fragments. You can see a bit of it here:
Riley specifies that the players try to stay within about three fragments of each other, and to come into unison periodically, for dramatic effect.
To approximate this in Live, I used “follow actions.” Follow actions let you specify what happens when a clip (in this case one of the 53 fragments) finishes. You can choose two behaviors and assign them probabilities. So my first setup: when a clip finished, there was a 10:1 chance it would repeat. Otherwise, it would advance to the next clip.
The main distinction between a track in Live and a human player here is that the track has no knowledge of what the other tracks are playing. So, Riley’s admonition that “It is important to think of patterns periodically so that when you are resting you areĀ conscious of the larger periodic composite accents that are sounding” is a little tough.
I tried messing around with varying the probabilities throughout the piece, basically to make little speed bumps, where the probability of advancing past fragment 11 is low, so the clips catch up with each other. The problem with this is that it then holds on to a few clips for too long, spreading everyone out again.
I’ve uploaded the Live file for people with Live to check out. It has 35 voices as recommended in the score, all using piano variously panned for easiness; you’ll want to freeze the tracks to play it unless you have a monster processor. The quick fix to the drifting problem seems to be to just go around triggering clips to keep everything in sync. Try experimenting with different follow action probabilities!
I haven’t uploaded an audio file because I haven’t gotten it to a place where it sounds good leaving to computer to its own devices. This is really a job for Max/MSP, which would let me make tracks that are aware of the positions of other tracks, but that’s a project for another procrastinatory night!