KAROUSEL screenshot 1

KAROUSEL

Shipped

Rotary and drive pedal with Leslie cabinet modeling, variable horn/drum speed, and stereo output.

CTO at Keystone Musical Instruments. Sole engineer and technical decision-maker.

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Tech Stack

MCU: STM32H750 (ARM Cortex-M7, 480MHz) via Daisy SeedMemory: 128KB Flash + 480KB SRAM + 64MB SDRAM + 8MB QSPI FlashAudio: 24-bit/96kHz stereo codec, DMA-managed buffersLanguages: C++, ARM assembly (VTOR/FPU setup)Build: GNU Make, ARM GCC toolchainLibraries: libDaisy, DaisySP, CMSIS-DSPI/O: 6 pots, 2 footswitches, 3 rotary switches, MIDI (5-pin DIN + USB), expression pedal, RGB LEDs

Stats

0.1Hz – 4Hz
Slow speed
4Hz – 10Hz
Fast speed
3 (Vintage / Classic / Modern)
Cabinet types
3 (Near / Mid / Far)
Mic placement
128
Presets
9
MIDI CCs

Overview

A hardware rotary and drive pedal built on the STM32H750 (ARM Cortex-M7, 480MHz) via Daisy Seed. Emulates the doppler, amplitude modulation, and tonal coloration of a rotating horn and drum speaker. 128 presets, expression pedal mapping via APEX mode, and configurable I/O modes (mono true bypass, mono-to-stereo, stereo buffered). 24-bit/96kHz stereo audio with custom QSPI bootloader for OTA firmware updates. Part of the Keystone Musical Instruments pedal line, launched January 2026.

Engineering Highlights

  1. 01Leslie cabinet modeling — Physically-informed simulation of rotating horn and drum with independent speed, acceleration curves, and doppler effect.
  2. 02Cross-platform DSP core — Same C++ algorithm compiles for both STM32H750 firmware (ARM) and VST3 plugin (x86_64/ARM64) without modification.
  3. 03Custom QSPI bootloader — Firmware exceeds 128KB internal flash, boots from 8MB QSPI. OTA updates via USB MIDI SysEx.
  4. 04APEX expression pedal learning — Runtime parameter mapping without recompilation. Users assign expression pedal to any parameter with custom min/max/inversion, saved to flash.