MIXI-CUT Hardware Decoder Guide

Build a standalone MIXI-CUT decoder that converts vinyl timecode to MIDI.

Architecture

[Turntable] → [Phono Preamp] → [ADC] → [MCU] → [USB MIDI]
                                         │
                                    MIXI-CUT C Decoder
                                         │
                              Speed → MIDI CC #1
                              Position → MIDI Note
                              Beat → MIDI Clock

Bill of Materials

Component	Part	Cost
MCU	STM32F446RE (Cortex-M4, 180 MHz, FPU)	~8 EUR
ADC	PCM1808 (stereo, 96 kHz, 24-bit)	~5 EUR
Phono preamp	Custom or TI OPA2134	~3 EUR
USB connector	Micro-USB or USB-C	~0.50 EUR
PCB	2-layer, 50x30mm	~2 EUR
Passives	Capacitors, resistors	~1 EUR
Total		~20 EUR

Alternative: ESP32-S3

Component	Part	Cost
MCU	ESP32-S3-WROOM-1 (with I2S ADC)	~4 EUR
ADC	INMP441 MEMS or external I2S ADC	~3 EUR
Total		~10 EUR

Firmware Overview

The firmware runs the C decoder from decoder_c/:

#include "mixi_decoder.h"

// DMA buffer: double-buffered, 128 stereo samples
#define BLOCK_SIZE 128
static float dma_left[BLOCK_SIZE];
static float dma_right[BLOCK_SIZE];

static mixi_decoder_t decoder;

void audio_callback(void) {
    mixi_result_t result;
    mixi_decoder_process(&decoder, dma_left, dma_right,
                         BLOCK_SIZE, &result);

    // Map speed to MIDI CC
    uint8_t cc_speed = (uint8_t)((result.speed + 2.0f) / 4.0f * 127.0f);
    midi_send_cc(1, cc_speed);

    // Map position to MIDI note (one per beat)
    static float last_beat = 0;
    float beat = result.position * (bpm / 60.0f);
    if ((int)beat > (int)last_beat) {
        midi_send_note(60, 127);  // Middle C
    }
    last_beat = beat;
}

int main(void) {
    mixi_decoder_init_default(&decoder);
    adc_start_dma(dma_left, dma_right, BLOCK_SIZE, audio_callback);
    while (1) { __WFI(); }  // Sleep between interrupts
}

MIDI Mapping

MIDI Message	Parameter	Range
CC #1 (Mod Wheel)	Speed	0=−2x, 64=0x, 127=+2x
CC #2	Lock quality	0=unlocked, 127=locked
Note C4 (60)	Beat pulse	Velocity=127
Clock	Tempo sync	24 PPQN from vinyl speed
Start/Stop	Play state	When speed crosses ±0.1

Performance Requirements

Metric	Requirement	Achieved
CPU usage	<5% @ 44.1 kHz	<1% (Cortex-M4 @ 180 MHz)
Latency	<5 ms	2.9 ms (128 samples)
Memory	<1 KB	~200 bytes (decoder state)
Power	<100 mA @ 3.3V	~50 mA (typical)

PCB Design

Full KiCad files will be published in the hardware/ directory.

Pin assignment (STM32F446RE)

Pin	Function
PA4	I2S WS (word select)
PA5	I2S SCK (bit clock)
PA7	I2S SD (serial data)
PA11	USB D-
PA12	USB D+
PB6	Status LED

Power supply

• USB-powered (5V → 3.3V LDO)
• Separate analog/digital ground planes
• Ferrite bead on USB VBUS

Testing

1. Generate a test WAV: mixi-cut generate --preset test-cut --output test.wav
2. Play through speakers or line output
3. Connect hardware decoder's audio input
4. Monitor MIDI output with a MIDI monitor tool
5. Verify speed tracks 1.000x ±0.01

Open Hardware

This project will release:

• KiCad schematic and PCB layout
• Gerber files for direct manufacturing
• BOM with LCSC/Mouser part numbers
• 3D-printable enclosure (STL)