Adventures in Synthesis: Glitchy Shift Register Feedback

Published on Dec 19, 2016 Chris Beckstrom

"I'm finishing up a techno album at the moment (which we be available for free at chrisbeckstrom.bandcamp.com soon!), and thought it would be refreshing to make some nonrhythmic music with my modular. This is a patch that some might not even consider music- and that's ok. We can agree to disagree.

These glitchy, distorted blips and beeps are a pretty good aural expression of how I feel about 2016.

PATCH NOTES
This patch is based on a bunch of feedback: things modulating other things modulating other things which eventually find their way back and modulate the first thing. When I try to do this on the computer I end up crashing everything. With analog circuits there is nothing to crash: it sounds awesome.

The pitch of an oscillator is modulated by the output of an R/2R resistor ladder, which is receiving gate signals from a 4-step shift register. The shift register is clocked by a square oscillator. The input of the shift register- the thing that gets it going, so to speak- is the output of a clock divider which is listening to the triangle wave coming out of the original oscillator. (It's not really a triangle wave- it's more of a rounded square wave, so the clock divider has no trouble spitting out divisions.) In this way there is a feedback loop: the oscillator speed is controlling its own rate of modulation! This accounts for the periods of silence or relative stasis. A few times throughout the video I make the oscillator go faster, which in turn modulates itself more, causing more activity. Turning the big oscillator knob to the left slows everything down.

Two wave outputs of the main oscillator- square and triangle- are routed into three inputs of a sequential switch. The third input to this sequential switch is the output of a saw wave oscillator. This saw wave oscillator's pitch is modulated by a 10 step sequencer, which in turn is clocked by the pulse output of the main oscillator. The sequential switch is clocked by another divided output of the main oscillator, again causing a feedback loop. The fourth input to the switch is the output of a module I call "space noise," which is really just a bunch of square wave oscillators making clicking sounds. I often use it for random modulation at slower rates; at faster rates, it's just noisy clicking.

As the sequential switch steps through each of its four channels, we're basically hearing each input in order: first the "space noise" clicking, then the main square wave oscillator, then the saw wave, then finally the triangle from the main oscillator. When the sequential switch is clocked slowly, we can hear each wave as if we were muting and unmuting four channels on a mixer in order. At fast speeds (audio rates) we hear the four outputs as a single wave. This phenomenon- things sounding completely different depending how quickly we hear them- never ceases to amaze and entertain me.

Finally, the saw oscillator (step three in the sequential switch) is being modulated by another 10 step sequencer, which is in turn clocked with yet another square wave oscillator. The clock divider which is clocking the sequential switch is receiving reset messages (gates) from the "space noise" module, which causes the clock division to become a bit random and glitchy.

The mono output of the modular goes into a mixer and some of the modular is sent to a vintage (90's!) Alesis Microverb II digital reverb, which is brought back into the mixer on two channels for some nice stereo. I like the combination of gritty analog sounds and gritty digital reverb. The Microverb came from a church, and I can guarantee they never put anything like these sounds through it."