Here's a video of me playing around with a MIDI script sent to me by Robby Kilgore. This script is a "harmonic rotator" that emulates the voice rotation function of the classic Oberheim Xpander analog synth. This rotator function is most notably used to its fullest extent by Saxophonist and EWI-ist Michael Brecker.

What is a "harmonic rotator" you ask?

I first became interested in random harmonic generation when I heard some early computer music while in college. I found it interesting and unique in that while it was unpredictable, when the programming was handled in the correct way, the resulting “music” - if randomly generated sound can be called that - had the potential to be mesmerizing. In 1987 Michael Brecker released his album “Michael Brecker” and one of the hallmark sounds on that disc was his use of the EWI (Electronic Wind Instrument) and synthesis to create an amazing palette of sounds. One of the most interesting things he did was to use a feature of the Oberheim Xpander module to randomly rotate through a series of harmonies. It works something like this:

The Xpander had a setting called “Rotate Mode” with 6 voice polyphony. Michael was able to turn that into an amazing harmonic tool by creating two zones on the synth, one with two “static” voices set to re-trigger and one with four rotating voices in rotate mode.

Oberheim Xpander Rotate Mode:

Screenshot 2018-01-29 01.38.20

Voice 1 & 2 are static intervals set in semitones above or below the input note. Assuming a C3 is played, the static output would be C3 and G3 (seven semitones above C3 or +7). Every time C3 is pressed, the static voices will sound the parallel 5th C & G.

The rotating voices are set (relative to C3) to -10 (D2), -7 (F2), -8 (E2) and -2 (Bb2). Every time C3 is pressed the static 5th plays as well as one of the rotating voices in sequence.

So:

Play middle C (C3) four times:
Voices 1 and 2 always play a parallel 5th C & G
Voices 3, 4, 5, and 6 play in rotation D, F, E, Bb Output would be [DCG], [FCG], [ECG], [BbCG]

And there ya have it, harmonic rotation. This can be an extremely powerful performance tool and when done randomly can be an amazingly creative a powerful compositional device. With patience and time spent working out harmonies, you can get some amazingly fresh sounding harmonic progressions out of this thing.

Playing around with Robby Kilgore's rotator script in Logic