SP-404MKII Web Companion

A browser-based companion suite for the Roland SP-404mkII sampler. Runs entirely client-side — no backend, no install — just open it in a browser. Installable as a PWA for offline use on desktop and mobile.

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What It Does

The SP-404MKII Web Companion gives you a full visual interface for managing your SP-404mkII's sounds, patterns, and projects without touching the hardware's tiny screen. Load audio on your computer, chop it up visually, arrange it into patterns, tweak it with effects, and export everything in the exact folder structure the sampler expects.

Pad Matrix

A 4×4 pad grid that mirrors the SP-404mkII's 16 physical pads. Each pad holds an audio sample with its own volume, pan, pitch, and reverse settings. Features 10 banks (A through J) for 160 sample slots total. Keyboard-mapped (1-4, Q-R, A-F, Z-V) for fast triggering.

Waveform Chopper

Import and slice any audio file visually with draggable slice handles. Offers two modes:

  • Equal Slicing — divides audio into N equal-length slices
  • Transient Detection — uses spectral flux to find natural hit points

Pattern Sequencer

A 16-step grid sequencer synced to a configurable BPM (40–240). Tap steps to program patterns, adjust velocity per step, and use swing to add groove. Plays back using the Web Audio API with sample-accurate timing.

Envelope & Effects

A visual Attack-Hold-Release (AHR) envelope editor with draggable handles. Includes a real-time effects chain:

  • Vinyl Sim — wow and flutter LFO
  • Filter — lowpass, highpass, bandpass
  • Bitcrusher — lo-fi sample rate reduction

Project & Export Manager

Save and load entire projects to your browser's local storage. Export sounds resampled to 48kHz 16-bit WAV bundled in a ZIP file formatted directly for the SP-404MKII SD card structure.

Hardware Integration

Connect an SP-404mkII hardware unit via the Web MIDI API. The app auto-detects devices and maps incoming MIDI notes to pad triggers in real time. Features a hardware-inspired OLED display bar tracking bank, BPM, and MIDI status.

Design Philosophy

The entire app is built to feel like an extension of the SP-404mkII hardware — dark, focused, and functional. The OLED-styled UI with amber/orange accents mirrors the hardware's aesthetic. Every interaction is designed to be fast: keyboard shortcuts for pad triggering, drag-and-drop for audio loading, and one-click actions for common tasks like normalization, fade, and slice assignment.

Everything runs 100% in the browser. Audio processing uses the Web Audio API, file I/O uses the File API and Blob downloads, and MIDI uses the Web MIDI API. There is no server — your audio never leaves your machine.

Tech Stack



Built for the SP-404mkII community. Open source, browser-native, zero dependencies on a server.