# 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.