Tuesday, August 22, 2017

Modular Sequencing: 06 - Isorhythmic Sequence

An isorhythm can be created by sequencing a set of pitches using a rhythmic pattern with a different number of pitches than rhythmic steps. For example, there is a set of four repeating pitches, played by a set of three repeating time lengths. A total of twelve steps is required to play through the pattern completely. The outcome can create an interesting interplay of rhythmic and pitched material.

A 40106 LFO oscillator is used to drive a 4022 counter. This counter is used in a loop of eight to create a rhythm of three lengths, by the use of three diodes from outputs Q0, Q3 and Q6. This is used as both the gate output, and as a driving signal for a second 4022 counter, which - in turn - has a loop of four counts. Each of these outputs (Q0, Q1, Q2 and Q3) are used to create a sequence of four control voltages that are eventually mapped to pitch.

Many variations of this circuit can be created, by varying the lengths of the loops, changing the rhythm of the first counter using more or less diodes, and by scaling the output voltage of the second counter.