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Tuesday, September 27, 2016

Adventures in Synthesis: Backyard Fall Afternoon Modular

Published on Sep 27, 2016 Chris Beckstrom

"I'm on a mission to challenge some ideas about modular synthesis:

That is has to be expensive, or that you can't build it yourself, or that it has to look pretty, that it's too complicated, that you need an electrical engineering degree to play it, or that you can't take it outside.

After all, it's a musical instrument not totally unlike a guitar. Now that I've reconfigured it a bit, why not bring it out to the back yard and play a bit?

This simple patch was inspired by the work of Kaitlyn Aurelia Smith. If you haven't heard her magnificent music, check it out!

Patch Notes

There are 4 voices in this patch: The bassline, a 3-note repeated figure by saw waves, the delay of that figure, and another saw oscillator.

The bassline is a sort of triangle oscillator whose pitch is being modulated by the output of a 3-input mixer. Into the mixer are patched 3 outputs from a 4-step shift register clocked by the main clock. The shift register is listening to the output of and AND gate listening to a pair of unrelated square wave oscillators. Basically every now and then different steps of the shift register are HIGH, and when sent to the mixer their voltages are converted to analog (i.e. not just HIGH and LOW) voltages. I'm pretty sure the best way to describe this is using the mixer as a digital to analog converter (ADC) or a R/2R ladder. Normally I'd use my 10-step sequencer for something like this, but right before I started recording, the wind blew my modular over and did something weird to the sequencer.

The triangle bassline heads out into a low pass gate which is triggered by the main clock (so it pings every beat). That in turn goes into one channel of a mixer, which is routed out into my portable recorder.

The 3-note saw wave figure actually comes from 3 separate saw oscillators tuned to 3 different notes. Each note goes into a different step in a 4-step sequential switch, which like the bassline shift register, is listening to the output of an AND gate. Basically this makes the switch step at fairly random times, playing the 3 pitches in order, then resting (since there is nothing plugged into the 4th input of the switch). The output of the switch goes directly into a mixer (no VCA) as well as the input of the delay unit.

The delay, very simply, goes back into the mixer. This means there is a channel of delay which can be adjusted independently from the original saw wave sounds. Sometimes I mix so you can only hear the delay, sometimes only the original, and sometimes both. If I had a fancier mixer I might automate this with CV, but then again I like actually *playing* the modular.

The 4th voice uses my newest module, an 8-bit stepped saw wave. (I'm not entirely sure it's actually 8-bit as I don't have a full understanding of what that even means, but if you look at it on an oscilloscope it certainly looks lofi digital!) The saw wave is built by sending a square wave into a 4040 divider, then taking 3 of the 4040 outputs and sending them through a R/2R ladder (an ADC) which spits out a stepped rising saw wave/ramp. I got this idea from the fantastic Logic Noise series on Hackaday.com - https://hackaday.com/tag/logic-noise/ . I built the module because I love that sound and I have a million square wave oscillators. I thought it would be nice to be able to easily change them to saw waves sometimes. Plus, chiptunes!

Nothing is modulating the pitch other than the shiny knob (and my hands). The output of the digital saw wave goes into a vactrol-controlled VCA, which then goes into the mixer. I discovered my "vactrol" circuits (where CV is implemented using light-dependent resistors and LEDs) do not work so great outside on a sunny day! That's why toward the end of the video I put my hand inside the case to cover up the LDR. Because of all the light bleed, instead of muting the digital saw when the VCA is "off," it mostly just attenuates it. Either way I like the effect, and it's definitely not something you'd get from a plugin on your computer.

The mono output of the mixer goes into my Roland Edirol portable recorder, which I then slightly tweaked in Bitwig Studio on Linux before mixing it with the video (in Kdenlive, also on Linux).

During the patch I'm mostly manipulating the 4 levels in the mixer, changing the delay time (which changes the pitch in that fantastic lofi digital way), and playing a melody on the digital saw wave using just the pitch knob... sort of like a fretless bass, violin, or oud."

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