Project philosophy :
Arduino Due driven dual XR-2206 MIDI Analog Synthesizer Projected to use modular approach based on standard eurorack for housing instead of a monolythic PCB. All control done through Digital Potentiometers driving analog components, or DACs with a selection of wavetables or custom user loadable wavetables based on math functions. All control wrapped into standard MIDI CC control.
Project Manifest and current support :
Dual XR2206 VCO With 4 DAC acting as pitch/pwm LFOs (controlling the fine frequency of oscillation of the individual suboscillators in self-keying mode) Coarse frequency control is achieved thru 2 X9C104 (100K ohms digital potentiometers) by suboscillator and a 20k trim potentiometer for base (C3 Note frequency) Fine tuning precision is projected to be remain under 2 musical cents. Frequency Range is from C3 to C7 at least for both oscillators. Total fine tuning range is fixed at +/- 225 Hz around coarse note frequency.
Uses Arduino MIDI USB library. Uses MCP23017 GPIO expander. Uses AD333 and a 4 channel XOR gate for control signal routing (tri/saw or square output to the output stage) and hardsync. Uses an autotune helper and frequency counting based on PulseIn (to measure frequency and pulse width). 555 timer analog Attack/Decay enveloppe generator and OTA based VCA.
Projected support : Adding (short-term roadmap) FreeRTOS based task scheduling (rationale : for concurrent DAC updates to overcome potential DAC update delays causing LFO aliasing/lags artifacts and general real-time responsiveness). Additional 4 general purpose DAC based LFOs/ADSR Digital control 4pole LP,BP,HP filters (one per XR2206). Mixing stage of both oscillators post-filter. WaveShapping from sine to triangle based on vactrol control. (resistive opto-isolator) using XR-2206 waveshapping pins. THD/Symmetry control based on vactrol control. (using XR-2206 symmetry adjust pins) AM/vibrato/Gate control using XR2206 native AM pin Diode/transformer based RINGMOD.
Files :
src/synth1.cpp (Main Arduino firmware file) src/synth1-dds.cpp.bak (DAC DDS driven by secondary arduino) : DEPRECATED src/tune.py (Python helper script for tuning performance measurement and individual notes WAV export) lib/* (Additional Arduino libraries) doc/ (In-depth electronic aspect of project R&D)
For the PCB board prototype advancement part of the project, please refer to : https://easyeda.com/rodv92/synth1