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DESIGN.md

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-# 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.