db-synth

A MIDI-controlled mono-voice digital synthesizer built on top of the AVR DB microcontroller series.

Overview

db-synth is a digital synthesizer that runs entirely on an AVR DB series microcontroller, taking advantage of the integrated DAC and operational amplifiers available in this chip family. It implements a single synthesizer voice with a band-limited oscillator, an ADSR envelope generator, and a first-order digital filter -- all controllable via standard MIDI messages.

The design is bare-metal, with no operating system or HAL layer. The audio pipeline -- oscillator, envelope, amplifier, and filter -- runs in the main loop at a 48 kHz sample rate with 10-bit resolution, driven by the internal DAC and shaped through the on-chip opamps that implement a second-order reconstruction filter before reaching the audio output.

Key highlights

• Standard MIDI control -- all synthesizer parameters are accessible via MIDI Control Change messages over a hardware MIDI interface with thru output
• Band-limited oscillator -- four waveforms (square, sine, triangle, saw) using pre-computed wavetables with band limiting to reduce aliasing
• ADSR envelope generator -- attack, decay, sustain, and release with both linear and exponential (AS3310-style) curves, ranging from 2 ms to 20 s
• First-order digital filter -- low-pass and high-pass modes with cutoff from 20 Hz to 20 kHz
• On-chip signal path -- internal 10-bit DAC through two integrated opamps (unity gain buffer and second-order reconstruction filter)
• OLED parameter display -- SSD1306-based display showing current synthesizer settings in real time over I2C
• EEPROM preset storage -- current settings can be saved to internal EEPROM and automatically restored on power-up
• Open source -- firmware licensed under BSD-3-Clause, hardware under CERN-OHL-S-2.0

Explore further

• Hardware -- PCB design, connectors, enclosure, and display
• Firmware -- building, flashing, and architecture overview
• MIDI implementation -- complete MIDI implementation chart
• Source Code -- full project repository on GitHub