Geist

  USB-C ──► AP2112K (3.3V LDO) ──► ESP32-S3 ──► PCM5102A DAC ──► 2× OPA1656 ──► Headphones
       └──► LM27762 (±4.5V Charge Pump) ─────────────────────────────┘

Step   Location                  Signal State                        Domain
─────────────────────────────────────────────────────────────────────────────
 1     PC audio engine           24-bit integer: 0x3FFFFF            Software
 2     USB cable (D+/D-)         NRZI-encoded differential packet    Digital
 3     ESP32 USB DMA buffer      Bytes in RAM                        Digital
 4     ESP32 I2S peripheral      Serial bitstream at 3.072 MHz       Digital (3.3V)
 5     PCM5102A delta-sigma      1-bit stream at 6.144 MHz           Digital
 6     PCM5102A OUTL pin         +2.05V (audio + 1.55V DC bias)      Analog
 7     OPA1656 buffer output     +2.05V → OUT_L+ (hot phase)         Analog
 8     OPA1656 inverter output   −0.50V → OUT_L− (cold phase)        Analog
 9     After 10Ω resistors       +2.04V / −0.49V                     Analog
 10    Headphone driver          ΔV = 2.53V differential → sound     Acoustic

Single-Ended:                      Balanced:
Signal ── ╱╲╱~~╲╱╲ ── +V+noise     HOT  ── ╱╲╱~~╲╱╲ ── +V+noise
Ground ── ──────── ── 0V            COLD ── ╲╱╲~~╱╲╱ ── −V+noise

Amp sees: signal + noise            Amp subtracts: (+V+noise) − (−V+noise)
→ noise passes through ✗            = 2V signal, noise cancelled ✓

◄──── DIGITAL ZONE ──────────►│◄──────── ANALOG ZONE ──────────►

┌──────────┐  ┌──────────────┐ │  ┌───────────┐  ┌───────────┐
│ ESP32-S3 │  │ Display,     │ │  │ OPA1656×2 │  │ Audio     │
│          │  │ Encoder,     │ │  │           │  │ Jacks     │
│          │  │ Buttons      │ │  │           │  │           │
└──────────┘  └──────────────┘ │  └───────────┘  └───────────┘
                               │
                         ┌─────┴─────┐
                         │ PCM5102A  │ ← sits on the border
                         └───────────┘

Bottom layer: UNBROKEN ground copper pour — no splits.

geist-firmware/
├── src/
│   ├── main.rs                ← Embassy entry, spawns all tasks
│   ├── usb/
│   │   ├── descriptors.rs     ← USB composite device (UAC1 + HID + CDC)
│   │   ├── audio.rs           ← Isochronous audio reception, buffer management
│   │   ├── hid.rs             ← Media key HID reports
│   │   └── cdc.rs             ← Serial link to companion app
│   ├── audio/
│   │   ├── i2s.rs             ← I2S DMA driver for PCM5102A
│   │   ├── buffer.rs          ← Lock-free ring buffer (USB → I2S)
│   │   └── dsp.rs             ← Biquad EQ engine + FFT analysis
│   ├── ui/
│   │   ├── display.rs         ← ST7789 SPI driver
│   │   ├── encoder.rs         ← EC11 rotary encoder with debounce
│   │   ├── buttons.rs         ← Debounced media buttons
│   │   └── renderer.rs        ← Track info view + spectrum visualizer
│   └── app/
│       └── state.rs           ← Global state machine

// One EQ band — runs for every sample, every channel
fn process(&mut self, x: f32) -> f32 {
    let y = self.b0 * x + self.b1 * self.x1 + self.b2 * self.x2
          - self.a1 * self.y1 - self.a2 * self.y2;
    self.x2 = self.x1; self.x1 = x;
    self.y2 = self.y1; self.y1 = y;
    y
}

Core 0 (Real-time audio — never blocked):
├── usb_audio_task()       ← Isochronous USB reception
├── i2s_feed_task()        ← Drains ring buffer → I2S DMA → PCM5102A
└── usb_hid_cdc_task()     ← HID reports + serial I/O

Core 1 (UI + DSP — lower priority):
├── dsp_task()             ← Biquad EQ processing + FFT analysis
├── display_task()         ← ST7789 rendering @ 30fps
└── input_task()           ← Encoder + button polling + debounce

Geist/
├── hardware/                    ← KiCad project
│   ├── Geist.kicad_pro
│   ├── Geist.kicad_sch          ← Schematic
│   ├── Geist.kicad_pcb          ← PCB layout
│   └── fabrication/             ← Gerber files for manufacturing
│
├── firmware/                    ← ESP32-S3 Rust firmware
│   ├── Cargo.toml
│   └── src/
│
├── companion/                   ← Tauri desktop app
│   ├── src-tauri/               ← Rust backend
│   ├── src/                     ← Svelte frontend
│   └── package.json
│
├── enclosure/                   ← 3D-printable case (STEP/STL)
│
├── docs/
│   ├── hardware-signal-trace.md ← Complete electrical signal walkthrough
│   └── usb-audio-architecture.md← USB Audio Class deep dive
│
└── README.md

# Prerequisites: Rust toolchain + ESP32 target
rustup target add xtensa-esp32s3-none-elf
cargo install espflash

# Build and flash
cd firmware
cargo build --release
espflash flash target/xtensa-esp32s3-none-elf/release/geist-firmware

# Prerequisites: Rust, Node.js 18+, pnpm
cd companion
pnpm install
pnpm tauri dev     # development
pnpm tauri build   # production binary