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.
Voltages can be sequenced using a counter, too. In this case, a set of eight control voltages scaled sequenced in a regular loop. A gate output allows both a volume envelope to be controlled as well as the pitch of an oscillator or filter.
A 40106 LFO oscillator is used to drive a 4022 counter, The RES pin of the counter is connected to ground, meaning that a loop of eight is counted. Each output Q0 to Q7 of the 4022 is then connected to an LED for visual feedback (displaying when each step is being played) as well as a potentiometer. Each potentiometer sets the voltage for that step, and all eight steps are connected via diodes to the control voltage output. The signal from the 40106 is used as a gate for envelope control.
If a different length of loop is desired, Q1 - Q6 can be connected to RES instead of ground, achieving loop lengths of 2 to 7 steps.
By combining short sequences with an and logic gate, complex and controlled rhythms can be created. Three interrelated patterns are created - the first sequence, the second sequence, and the points where both intersect against time and repetition.
With the assumption that both sequences are of different lengths, the resulting and logic will yield a rhythmic pattern that is longer than either. For example, take a rhythm that is eight steps long versus one that is only seven steps long. Fifty-six beats are required before the and logic rhythm will perform a perfect repetition.
A 40106-based oscillator is used to drive two 4022 counters. One counter has a loop length of the default eight counts. The other has a loop length of seven counts, which is achieved by connecting the Q7 output to the RES input. Selected Q output pins on the counters are connected via LEDs to the inputs of a 4081 and gate. The Q outputs that are selected form the rhythm of each loop. The selected outputs from the first counter are connected to the the first input of the 4081 gate. The selected outputs from the second counter are connected to the the second input of the 4081 gate. The resulting and logic will produce a rhythm formed from both counters. Three gate / trigger outputs are sent to the modular system - one output for the rhythm from the first counter, one output from the rhythm for the second counter, and the and logic rhythm.
Note that in this case, the LEDs are forming the rhythm, as there are no switches. By connecting the LEDs to different Q outputs on the counters, or by removing LEDs, the rhythm can be changed. By connecting the RES input to different Q outputs, rhythms of varying lengths from 2 - 7 beats can be achieved.
In this video example, the kick drum is from the first counter, the hi hat is from the second counter, and the tom is from the and logic.
LFOs with different frequencies can be used in conjunction with one or more logic gates to produce rhythmic patterns. These patterns are the result of the phase and frequency difference between the LFOs as well as the gate function, and can be used as a modular gate / trigger signal.
Two 40106-based LFO square wave oscillators are used to drive the two inputs of a 4081 quad, 2-input and gate. An and gate will only go high when all of the inputs are also high. The resulting signal from the 4081 and gate is then divided and output as a gate / trigger signal.
Each 40106 chip can be used to make up to 6 LFOs, and each 4081 contains 4 and gates. Multiple and gates can be connected in serial.
Similar functions that could be worthwhile exploring are or and xor gates.
