Geary Yelton: The excellent book Patch & Tweak is one of the most complete sources of information about modular synthesis available. The publisher Bjooks is graciously allowing Synth and Software to present an excerpt from its chapter on random control-voltage sources.
RANDOM By Kim Bjørn and Chris Meyer
In contrast to LFOs that produce predictable, repeatable patterns of modulation, random sources create voltages that seem to either meander or jump from one value to the next without an easily discernable pattern. These have many uses, from random note and gate generation to more natural fluctuations (for example, the sound of wind), to constantly evolving drones, to changing the articulation of a sound from note to note. In this article we’re going to look at some different ways random signals are generated and put to work.
Sample and Hold
One of the most common random voltage sources is the sample and hold (S&H) module. It has a voltage and a gate input. Whenever the gate goes high, it “samples” the voltage at its other input, and “holds” that value. When the gate goes low and then high again, it samples a new value. You will often see S&H included in the same module as noise or an LFO.
A related function is track and hold, which holds onto the sampled voltage as long as the gate is low, but then “tracks” the input voltage (passes it untouched to the output) while the gate is high.
If you’ve heard a synth produce a stream of random notes, you’ve probably heard a S&H at work. Usually, noise is patched into its input, and then some regular clock like the output of an LFO triggers when new notes are sampled, held, and played. If you patch the output into a quantizer, then the notes will be conformed to its scale.
A shift register is a chain of sample and holds strung together. When you first send it a gate or trigger, the first S&H stores a voltage. On the second trigger, the first S&H passes its voltage to the second S&H to hold, and the first one samples a new voltage. This continues down the line for as many stages as you have.
The first analog shift register (ASR) was reportedly custom-built by Fukushi Kawakami for composer Barry Schrader’s Buchla in the early ’70s. It was first commercially produced as a module for Serge systems. There are several shift register modules available, including the analog Snazzy FX Telephone Game that adds feedback and noise to make it less predictable. It is also one of the functions of the digital IntellijelShifty, the Expert SleepersDisting mk4, Ornament & Crime, and Make Noise tELHARMONIC.
Despite what its name implies, “chaos” does not mean “completely random.” Instead, a true chaotic system stays within boundaries, like water inside a boiling pot. However, the path of each bubble inside that pot may appear to be random. A chaos generator module usually allows a smooth transition from cyclic LFO behavior to increasingly random or chaotic behavior. Therefore, they are a good bridge between the predictability of an LFO and pure randomness.
One of the most popular chaos examples is the double well potential equation. A cycle of a normal LFO can be thought of as circling around one point. The double well has two points at different voltage levels. This circuit then decides randomly whether to circle around the first point or the second. A good analogy is a moth flying in between and around two lights on a porch at night. The moth does not stray too far away from (or too close to) the lights, but its exact path is random. A double well module contains at least separate X and Y axis outputs, matching a 2D plot of the moth’s flight path.
Several random source modules are based around the idea of loading a string of random values into memory, and then using a digital shift register to step through this pattern. They are usually called Turing machines because they are loosely based on the work of Alan Turing. His Turing machine could theoretically create any output based on the values and instructions fed into it.
One of the earliest machines to apply this theory to music was the Triadex Muse, designed by Marvin Minsky and Edward Fredkin of MIT in 1972.
Instead of explicitly entering a series of notes, you set a group of parameters that generate patterns that might take anywhere from seconds to years to repeat.
Source of Uncertaincy
One of the most celebrated random source modules is the Buchla266 Source of Uncertainty. A significant evolution of the original Model 265, this packed module includes:
- Three different colors of noise: pink, white, and blue.
- Two fluctuating random voltages—smoothly wandering outputs—with voltage-controlled rate of change.
- Two different quantized random voltages—stepped outputs—that can either take on 1 of 7 different voltage levels (the n+1 output), or 1 of 64 different voltage levels (the 2n output). A new trigger selects a new value.
- Two different stored random voltages that share their own trigger input. The first one is a set of completely random stepped voltages. The second one allows you to choose the probability distribution of whether the voltages tend to be low, high, or in the middle of the range.
- An integrator (slew) to smooth out changes in voltage.
- A sample and hold, with the extra feature that both the trigger in and voltage output can be alternated between two different jacks.
The Wiard Woggle-Bug—created by Grant Richter of Wiard—is a wonderfully mutant variation on the original Buchla 265 Source of Uncertainty. It adds to the equation two internal VCOs driven by its random voltages, plus ring-modulated child tones of those VCOs. This enables it to produce both tones and modulation voltages. The special woggle feature is a stepped random voltage that has decaying sine-wave-like impulses. This creates a bouncing voltage in response to changes in the stepped output.
Grant was constantly evolving the design of the Wiard Woggle-Bug. The original Make NoiseWogglebug and the DIY kit from Erica Synths are very similar to the Wiard Wogglebug #3. The latest Make Noise Wogglebug mk2 is an evolution of Grant’s Wogglebug #5.
Random Gates and Triggers
A lot of rhythm pattern generators have options to use probability or completely random functions to generate new patterns. A few modules create additional bursts of triggers in response to an incoming trigger, which may or may not be in time with a master clock signal.
If you want purely random triggers to be generated, several random voltage generators also have random gate or trigger outputs that are not tied to an external trigger. Some simple examples include the Erica Synths Pico RND and Noise Reap Flux.
Patch your Own
It is also possible to patch your own random gate generator. Patch the output of a random voltage source to the input of a comparator, envelope follower, or any module that outputs a gate or trigger in response to a voltage crossing a certain threshold.