Auduino Wavetable Granular Synthesis

The original Auduino used a clever trick to efficiently calculate a triangle wave to use as the “grain” for the granular synthesis, but I always wanted to try it with a wavetable to see if that would work.  This project gives that a go.

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

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

• Arduino Touchscreen Auduino
• Arduino PWM Sound Output

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

Parts list

• Arduino Uno
• 6x 10kΩ potentiometers
• Optional: Arduino PWM Output Filter Circuit – Part 2
• Amplification and speaker

The Circuit

This is the basic Auduino circuit but with the addition of a 6th potentiometer to select the waveform for the “grains”.

I used my Arduino PWM Output Filter Circuit shield to provide some additional output filtering and my potentiometer breakout from my Arduino Multi-pot Mozzi FM Synthesis project as shown below.

The Code

The basic idea is to take the original Auduino code and instead of calculating the waveform to use “on the fly” (as with the original code), read the values from a wavetable instead.  I’ve lifted the code for wavetable generation straight out of my Arduino PWM Sound Output code and added optional code in the main loop() to select one of the four waveforms based on the last potentiometer reading as follows.

#ifdef WAVE_CONTROL
  int newwave = analogRead(WAVE_CONTROL) >> 8; // 0 to 3
  if (newwave != wave) {
    wave = newwave;
    setWavetable(wave);
  }
#endif

In terms of reading from the wavetable rather than calculating the samples, I can use the same phase accumulator as before, and simply use the top 8-bits to give me an index between 0 and 255 to use to read values out of the wavetable.  This all happens in the PWM_INTERRUPT routine under some conditional compilation.  Notice that the grain amplitude calculation does not need to change.

#ifdef WAVETABLE
  // Use top 16 bits as the index into the wavetable
  value = wavetable[grainPhaseAcc >> 8];
#else
  // Convert phase into a triangle wave
  value = (grainPhaseAcc >> 7) & 0xff;
  if (grainPhaseAcc & 0x8000) value = ~value;
#endif
  // Multiply by current grain amplitude to get sample
  output = value * (grainAmp >> 8);

This has to be repeated for both grains, using grainPhaseAcc and grain2PhaseAcc.

I’ve used the MIDI version of my enhanced Auduino code, but for this project I’m using it in non-MIDI mode, so I’ve enabled the “sync frequency” potentiometer on A5.

The full list of potentiometers and their uses is as follows:

• A0 – Grain 1 frequency
• A1 – Grain 1 decay
• A2 – Grain 2 frequency
• A3 – Grain 2 decay
• A4 – Sync frequency
• A5 – Wavetable selection: 0=Sine; 1=Saw; 2=Triangle; 3=Square

Find it on GitHub here.

Closing Thoughts

In the video, right at the start, you can just about see the four different waveforms being selected – sine, saw, triangle and square.  This has the sync frequency right down, and grain 2 effectively “off”, allowing us to almost see grain 1 in action.

You can certainly hear the difference when the waveform changes.

This would be something that would be quite nice to run on my Auduino Universal Synthesizer Panel which has six pots ready and waiting to go!  In fact, I might rebuild the code in the MIDI variant and give that a go!

Now I can use a wavetable as the “grain” there are some other possibilities too.  I might be able to draw the waveform to use or possible grab a short sample from an audio input in some capacity.  There are certainly a few other things to try now.

Kevin