Raspberry Pi Pico Polyphonic DCO

This repository contains source code, schematics and PCB for a digitally controlled oscillator (DCO) with up to 6 voices which are driven by a Raspberry Pi Pico. It uses PIO to generate a highly accurate frequency which is controlled by USB or serial MIDI. The analog oscillator part is based on the Juno 106 and generates sawtooth and square wave signal with a 10Vpp amplitude. Amplitude compensation is done by a smoothed PWM signal coming from the Pico.

Key features

• Digitally controlled analog oscillator using a Raspberry Pi Pico
• Up to six voices
• Voice stacking
• Detuning of voices
• Frequency modulation (FM)
• Separate gate signal
• DIN MIDI serial input
• USB MIDI input
• MIDI pitch bend
• Portamento with adjustable speed

This is how it sounds: Ramp sample Pulse sample Polyphonic sample

I use the DCO in my Polykit-6 polyphonic synthesizer, see https://www.youtube.com/watch?v=vj-DSh6yfM0 and https://www.youtube.com/watch?v=go1mErOdtsU

Schematics and PCB

Gerber files for the PCB can be found here.

Installation (simple)

Press BOOTSEL button on the Pico while powering it with USB. Copy file build/pico-dco.uf2 onto the USB mass storage device.

Usage

After installing the Pico should register as USB MIDI device. Alternatively serial MIDI input is available. The DCO listens to note on/note off messages on MIDI channel 1. Pitch bend is also supported. Portamento can be enabled by MIDI CC message #65, portamento time can be adjusted by CC message #5.

Detuning, stacking and FM

The build-in analog-to-digital converter can be used to modulate these parameters. You can add an external control voltage (0-3.3V) to that input pins. These features can be disabled by commenting (//) the following lines:

#define USE_ADC_STACK_VOICES // gpio 28 (adc 2)
#define USE_ADC_DETUNE       // gpio 27 (adc 1)
#define USE_ADC_FM           // gpio 26 (adc 0)

References

https://blog.thea.codes/the-design-of-the-juno-dco/

https://electricdruid.net/roland-juno-dcos/

https://www.youtube.com/watch?v=yYnQYF_Xa8g

https://github.com/raspberrypi/pico-examples/tree/master/pio/pwm

https://github.com/raspberrypi/pico-examples/tree/master/pio/pio_blink

https://github.com/raspberrypi/pico-examples/tree/master/pwm/hello_pwm

https://qiita.com/jamjam/items/f2fdd5c072ff348fd809

https://github.com/infovore/pico-example-midi

https://learn.sparkfun.com/tutorials/midi-tutorial/all

http://www.music-software-development.com/midi-tutorial.html

http://midi.teragonaudio.com/tech/midispec/run.htm