This is my 8-bit synthesizer project for the Arduino UNO. It'll probably work with minimal modification on any 8-bit AVR with at least 2k of RAM at 16MHz. I compile with avr-gcc and upload with a SPI programmer, but it doesn't take up a ton of flash space so you should in theory be able to keep Optiboot if you tweak the makefile.

Features:

• 8 simultaneous independent channels at 44.1kHz
• Input over MIDI and 7x8 key matrix with separate assignable voices
• Square wave, triangle wave, and noise
• Square wave duty cycle adjustable to 50%, 25%, or 12.5%
• Attack-decay-sustain envelopes
• Vibrato, arpeggio, and echo effects
• Preprogrammed percussion using sliding pitches and shaped noise
• "Burst note" setting that plays hi-hat with notes
• 8-beat record and playback (work in progress)

This project is inspired by Linus Åkesson's incredible Chipophone. My source code is written from scratch and is hopefully a little more reusable (I aim for MIDI and Arduino compatibility). It uses the same MCP49x1 DAC for output and can be wired up with a few external components:

For the audio output:

• MCP4921 DAC (can substitute 4911 or 4901)
• 470uF electrolytic capacitor to block DC on the output
• 100nF electorlytic capacitor to filter out noise on the volume knob
• 10kOhm linear potentiometer to select the Vref level
• 56kOhm volume limiting resistor
• Audio jack

For MIDI input:

• 6N138 optoisolator
• 1N4148 diode
• 220, 470, and 10kOhm resistors
• MIDI socket

For key-matrix input:

• 74HC164 or 74HC595 shift register