Adventures in Synthesis: Pure Data drum machine + DIY modular

Published on Mar 2, 2016 Chris Beckstrom

"More about this project:
http://www.chrisbeckstrom.com/portfol...

Get the drum machine code here:
https://github.com/chrisbeckstrom/pur...

I've been building a Pure Data - based drum machine / sequencer that runs on iOS using the fantastic MobMuPlat iOS app. Basically it lets you run Pure Data on your device and allows you to build a GUI for it. This is the first patch that uses my new drum machine. It's still a work in progress, but many of the features work.

Why build my own drum machine when there are so many great-sounding iOS apps and other software synths available? Because why not! None of the apps I've tried have the exact features I want, so I'm building my own. In my Pure Data programming I'm keeping to my DIY modular aesthetic: simple modules, analog-style sound, and basic waveforms that can be manipulated and combined.

Here's an early morning jam featuring the CB-100 Sound Sequencer (tentative name) combined with my DIY modular synth.

Patch notes:
The left channel of the iPad audio output is the master output from the drum machine, and the right channel is a very short pulse on every 16th note. This pulse is patched into an amp in the modular and then used to clock the sequencer, clock divider, and various envelopes.

The drum machine goes into a preamp to get it up to modular level, then into a mixer. I'm always amazed at how nice and warm things sound when you run them through even a single analog gain stage! Straight out of the iPad the drum machine sounds pretty digital and cold, but that preamp warms it right up and adds some crunch when turned to 11.

The sequencer is set to open a VCA every 8 16th notes (on 2 and 4); the input of the VCA is the drum machine and the output is patched to the delay. Basically that means the delay only gets signal on 2 and 4, which I think sounds cool. (It's not exactly JUST on 2 and 4, because the VCA is a double vactrol VCA, and when one LED is turned on it has a tendency to bleed through the loads of electrical tape and influence the other VCA. Ah, such is the DIY experience!).

The sequencer modulates the frequency of a single saw oscillator, which goes into the low pass gate before joining the other sounds in the mixer.

The drum machine clock goes into a clock divider, and four of those outputs go into an R/2R ladder to generate CV to modulate the frequency of a square oscillator. That oscillator goes into another VCA, then into the mixer to join the other signals.

I have a few square waves going into a diode-based pseudo-ring modulator which then goes into yet another VCA (you can never have too many VCAs!!). This VCA is opened by me pressing a button on the far left of the modular, which sends 9V to the VCA and lights the vactrol LED. The button also sends a trigger to an A/D envelope generator which controls the pitch of one of the oscillators, which adds some nice pitch variation like some sort of weird drum.

Finally, the output of the whole modular gets split into two paths: one directly into the mixer, and another into an Alesis Microverb II digital reverb unit spitting out 2 channels of delicious lofi digital reverb. These three channels (reverb + modular) are mixed together and head into the computer onto a single stereo track in Bitwig Studio."