StupidSynth/DESIGN.md

Stupid Synth Design Document

Overview

Stupid Synth is a minimalist, 4-voice polyphonic synthesizer designed for the RP2040. It prioritizes zero external component count (other than the sound transducer itself) and code simplicity.

Hardware Design

• MCU: Raspberry Pi Pico (RP2040).
• Audio Output:
  • Method: High-speed PWM (Pulse Width Modulation).
  • Pin: GP28.
  • Circuit: Connect a piezo buzzer directly between GP28 and GND, or a speaker/headphones in series with a 220Ω resistor.
• Input: USB Serial Console (no physical buttons/pots required).

Software Architecture

Audio Engine

• Carrier Frequency: ~490 kHz (125MHz sys clock / 255 wrap). This pushes PWM noise far above human hearing, minimizing the need for an analog filter.
• Sample Rate: 22,050 Hz.
• Bit Depth: 8-bit.

Synthesis Engine

• Waveform: Sawtooth (generated via simple phase accumulation).
• Polyphony: 4 simultaneous voices.
• Envelope: Simple Decay (percussive/plucked sound).
• Mixing: Tanh saturation (soft clipping) to prevent digital distortion when voices sum.

Interface

• Control: Keyboard characters sent via Serial Monitor trigger notes.
  • Keys: a, s, d, f, g, h, j, k (Scale C4 - C5).

Execution Plan

1. Configure RP2040 Hardware PWM on GP28.
2. Setup a repeating timer interrupt at 22.05kHz.
3. Implement the mixing loop and voice management in the interrupt handler.
4. Process Serial input in the main loop.