For his 1965 composition “It's Gonna Rain”, Steve Reich designed a generative mechanical system.
Steve Reich: It's Gonna Rain (1965)
It was a system of two tape recorders.
A system of two tape recorders
Reich had made field recordings of speech on tape.
With recorded speech
He made two short identical loops from his recordings. One for each tape recorder.
Two identical loops
The tape recorders were set to slightly different speeds.
One would play a little faster than the other.
With slightly different speeds
This started a phasing process. A continuously changing soundscape was generated from the short loops.
A generative phasing process
Though I may have the pleasure of discovering musical processes and composing the musical material to run through them, once the process is set up and loaded it runs by itself.
Steve Reich, Music as a Gradual Process, 1968.