====== PicoMic — Noise-Cancelling USB Microphone ======

Noise-cancelling USB microphone built on the Raspberry Pi Pico 2 (RP2350). Multiple I2S MEMS mics + adaptive filtering = clean voice on a €12 budget.

===== Concept =====

<code>
[INMP441 #1 (voice)]  ──┐
                         ├── PIO I2S RX (1 state machine, stereo)
[INMP441 #2 (noise ref)]┘            │
                                  DMA → Audio Buffer
                                      │
                                Core 0: LMS adaptive filter (5% CPU!)
                                      │
                                TinyUSB UAC → appears as USB mic
</code>

LMS (Least Mean Squares) adaptive filter uses the reference mic to model ambient noise, then subtracts it from the voice mic signal in real-time. 10-15 dB noise reduction with 2 mics.

===== Why RP2350? =====

  * **Cortex-M33 with DSP extension** — single-cycle MAC, SIMD for 16-bit math
  * **CMSIS-DSP** built-in: ''arm_lms_f32'', ''arm_lms_norm_f32'', FFT, FIR filters
  * **PIO** handles I2S with zero CPU overhead
  * At 16kHz / 256-tap LMS: **5.5% of one core**. Absurd headroom.
  * RP2040 (Cortex-M0+) works too at ~41% of one core, but no hardware MAC

===== Parts List =====

^ Part ^ Price ^ Notes ^
| Pico 2 (RP2350) | €5 | Or Pico 2 W for wireless monitoring |
| 2-4x INMP441 breakout | €3-6 | I2S MEMS mic, 61 dB SNR |
| PCM5102A DAC (optional) | €3 | I2S output for monitoring processed audio |
| **Total** | **~€12-14** | |

Alternative mics: ICS-43434 (65 dB SNR, better linearity), SPH0645LM4H (Adafruit breakout).

===== Mic Placement =====

The hard part is not computation — it's **physical placement**.

=== 2-mic ANC (recommended start) ===

  * Voice mic: pointing at mouth, ~30 cm away
  * Reference mic: **pointing opposite direction**, back-to-back
  * Spacing: doesn't matter much for LMS — **orientation** and **acoustic isolation** matter
  * A foam barrier or PCB between mics helps
  * LMS filter adapts to any delay

=== 3-mic hybrid (ANC + beamforming) ===

  * Voice mic: center, facing user
  * 2 reference mics: flanking at **40-60 mm** spacing
  * Gives both adaptive NC AND spatial rejection
  * 40-60 mm avoids spatial aliasing up to ~4 kHz

=== Reference spacing from products ===

^ Device ^ Mics ^ Spacing ^ Application ^
| ReSpeaker 2-Mic HAT | 2 | 58 mm | Voice capture |
| ReSpeaker 4-Mic Linear | 4 | 65 mm | Far-field beamforming |
| Amazon Echo | 7 | ~35 mm circular | Far-field 360° |
| AirPods Pro | 3/ear | ~8 mm | Feedforward + feedback ANC |
| Laptop built-in | 2 | 60-80 mm | Near-field beamforming |

=== Rules ===

  * For beamforming: spacing must be < λ/2 at highest frequency (4 kHz → < 4.25 cm)
  * For ANC/LMS: spacing barely matters, orientation and shielding matter
  * More reference mics = diminishing returns. Sweet spot: 1 voice + 3 reference (covers 3 noise axes)

===== Computational Budget =====

<code>
LMS at 16kHz, N=256 taps:
  Per sample: 2*256+1 = 513 MACs
  Per second: 8.2 million MACs/s
  RP2350: 5.5% of one core
  RP2040: 41% of one core

LMS at 48kHz, N=512 taps:
  Per second: 49.2 million MACs/s
  RP2350: 33% of one core — still comfortable
</code>

Memory: ~16 KB total (audio buffers + filter coefficients + FFT workspace). Non-issue on either chip.

===== Key Resources =====

  * [[https://github.com/ArmDeveloperEcosystem/microphone-library-for-pico|microphone-library-for-pico]] — Arm's I2S/PDM mic library, includes USB mic example
  * [[https://arm-software.github.io/CMSIS-DSP/latest/|CMSIS-DSP]] — LMS, FIR, FFT functions for Cortex-M33
  * PineBuds Pro — open-source TWS earbuds with ANC (VM ''pinebuds'' on data has the dev environment)

===== Wiring =====

Two INMP441s on one I2S bus (3 wires + power):

<code>
Pico GPIO → INMP441 #1 (L/R=GND) + INMP441 #2 (L/R=VDD)
  SCK  → pin X ──┬── BCLK #1
                  └── BCLK #2
  WS   → pin Y ──┬── WS #1
                  └── WS #2
  SD   → pin Z ──┬── SD #1
                  └── SD #2
  3V3  → VDD both
  GND  → GND both
</code>

One PIO state machine, one DMA channel. Samples arrive interleaved L(voice)/R(noise).

===== Status =====

**Phase:** Backlog. Synergizes with [[picoshell|PicoShell]] (same Pico 2 platform) and turing voice satellite.