Auduino Granular Synthesis

The Auduino project (not a typo – AUduino as in “audio arduino”) by Peter Knight has been around a while (over 10 years!) and is many people’s introduction to digital synthesis on the Arduino, so inspired by reading about someone else’s recent build I thought it was time to take a look myself.

• In part 2, I delve deeper into the software to really see what is going on.
• In this part I show how to add MIDI.
• Here is a version that is controlled with a touchscreen.
• And here is a version that uses a wavetable as the “grain”.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments or gear!

These are the key Arduino tutorials for the main concepts used in this project:

• Notes and Volts – Build the Auduino Granular Synth
• Charis Cat – Building the Auduino Synthesiser
• Instructables – Auduino – Lo-Fi Synth for Arduino

If you are new to Arduino, see the Getting Started pages.

Parts list

• Arduino Uno
• 5x 5k to 10k potentiometers (the original project used 4.7k pots)
• 8 ohm speaker or old headphone speaker or alternative amplification
• Breadboard and jumper wires

The Circuit

The simplest way to use the Auduino is to link up five potentiometers to A0 to A4 and a speaker to pin 3 as shown above.  One way is to use a breadboard – alternatively you could link the potentiometers together as shown.

This should be enough to get you started making some noises – see the code for details of what the potentiometers do – or just experiment!

The output is a 0 to 5V wave with a 2.5V DC offset, which is fine for a salvaged speaker – but if you want a more usable “line out” then you really ought to add some extra components.  There are a few options:

• Repurpose the Mozzi Output Circuit – the basic principles should be the same here.
• Follow the Notes and Volts tutorial for a line-level output here.

The basic idea is to create a simple RC filter to filter out any high frequencies due to the PWM output; include some kind of potential divider to bring the voltage down from a 5V range; and include a coupling capacitor on the output to remove the 2.5V DC bias.

But none of that is necessary if you just want to experiment.  I still haven’t really decided what is best, but I used the Notes and Volts circuit (a 33nF capacitor, 2.2K resistor and in my case a 10uF non-polar capacitor for the output) on a small solderless breadboard for my own experiments right now.

I’ve also used my six-potentiometer board I put together for the Arduino Multi-pot Mozzi FM Synthesis project, just because I happened to have it handy already and it is easy to wire up to an Arduino.

The Code

The Auduino code can be found here: https://code.google.com/archive/p/tinkerit/downloads

You need the auduino_v5.pde file which needs putting in your Arduino src directory.  I created an Auduino folder and renamed the file to auduino.ino, but you could just open the “pde” file directly by the Arduino environment if you wish (Arduino files used to be called .pde files before they were .ino files).

There are three modes in the code, selected by uncommenting one of the “synPhaseInc = ” lines in the main loop() function.  They relate to how the “sync” potentiometer controls the base frequency of the note:

• “smooth frequency mapping” allows a continuous sweep of frequency in response to the pot.
• “stepped MIDI mapping” moves between discrete notes in semitone steps.
• “stepped pentatonic” cycles between D, E, G, A and B over a couple of octaves.

I really like the smooth mapping but you’ll need to experiment more to find some nice pitches.  The pentatonic mapping (the default) will get you up and running and making more harmonious sounds more quickly.

Closing Thoughts

This is all relatively straight forward to get up and running, but I’m not convinced I’ve got a decent output from the thing yet, so I’ll experiment a bit more with some filtering, but it certainly shows some promise.

I’m also not totally understanding the details of how granular synthesis is really working with the pot controls, so that is my next step too to dig into the code a bit more and see if I can work it out.  Looks like the folks at Sparkfun have already taken a look, so I’ll probably start there.

I might also add MIDI at some point (although Notes and Volts and been there and done that already too).

Kevin