ostra

A modular playground for your browser.

Patch a melody. Make it wander. Build the instrument you wish existed.

Get started · Explore the modules · Build a module · Contribute

ostra is a local-first modular synthesizer with 25 instruments and utilities plus a fixed stereo master, tactile controls and an open canvas. Connect audio, control voltages and gates to make anything from a single voice to a self-playing patch. Everything runs on your machine—no accounts, API keys or audio backend.

Experimental alpha. Ready to explore and contribute to; expect the occasional rough edge.

Why ostra?

• A rack you can get lost in. Colourful, individually designed faceplates, draggable modules and patch cables that fade out of the way while you work.
• More than note sequencing. Sequence arbitrary voltages, quantise four voices to a shared scale, route signals, delay CV and build feedback loops.
• A useful palette from the first patch. Oscillators, envelopes, filters, VCAs, random sources, granular sampling, tape delay, distortion and reverb.
• Record a take. Physical transport buttons, fixed L/R output jacks and a master level knob. Finish recording to download a stereo WAV directly to your device.
• Bring your own sounds. Drop a WAV or MP3 onto GRAIN. Decode, play and store it locally.
• Pick up where you left off. Rack, settings and cables save automatically. Refresh returns to your patch with audio paused.
• Make your own modules. One folder contains a module's definition, React panel and audio processor. Shared knobs, switches and jacks handle the familiar parts.

Get started

Requires Node.js 22+ and npm.

git clone https://github.com/NickTikhonov/ostra.git
cd ostra
npm ci
npm run dev

Open http://127.0.0.1:3000. A connected starter rack is ready for your first visit. Press Play in the top bar when you want to hear it.

Gesture	What it does
Click empty space, or press A	Search and add a module
Drag a module's title	Move it; nearby modules snap together
Drag between jacks	Patch an output into an input
Click a connected jack	Reconnect its cable; Shift starts another cable
Right-click a jack or cable	Unplug it
Drag a knob up/down	Change its value; hold Shift for fine control
Hover a jack	Inspect its audio or CV voltage after a short delay
⌘/Ctrl + Z	Undo; add Shift to redo

The rack guide covers patching, persistence, samples and recovery in more detail.

Inside the rack

Make sound	Move it	Shape it	Connect it
ORBIT — VCO	PATH — 8-stage voltage sequencer	SIEVE — filter	VEIL — dual VCA
GRAIN — sampler / granular	PRISM — four-channel quantiser	GRIT — distortion	BRAID — CV mixer / attenuverter
CHANCE — random / sample & hold	BLOOM — voltage-controlled ADSR	ECHO — delay	HARBOUR — stereo mixer / sends
	DRIFT — LFO	SPOOL — tape-style delay	JUNCTION — voltage-controlled switch
	VECTOR — dual function generator	HALO — reverb / freeze	RELAY — clocked CV delay
	TICK — clock · SPLIT — divider		LOGIC — gates / comparator

TINE adds a pluck/bell/wood percussion voice; PULSE provides three independent Euclidean trigger tracks with a shared clock and concentric rhythm display. SPROUT adds two independent attack/decay envelopes, with trigger buttons and end-of-cycle outputs. AMBER ranges from gentle stereo warmth to heavy saturation, with a SCORCH boost and analog drive meter.

All jacks carry sample-rate voltages: the audio/CV labels are guides, not restrictions. Nine archived module types also remain available to existing saved racks. Read the module guide →

Build the missing module

Modules are small, self-contained source contributions:

src/modules/your-module/
├── definition.js     Ports, parameters, defaults and saved-data migrations
├── Panel.tsx         Your layout, built with shared React controls
├── processor.js      Sample-by-sample audio and CV processing
├── panel.module.css  Optional scoped styling
└── index.ts          Connects the pieces

The development watcher discovers new module folders automatically. You can write a module by hand or work with a coding assistant—there is no in-browser AI generation or publishing system yet. That future direction is described in the platform design notes.

Start with the module contributor guide, which includes a complete attenuator example. Module contributions run as trusted application code; they are not sandboxed plugins.

Under the hood

React + Next.js draw the rack. An AudioWorklet processes the signal graph independently of the interface, with parameter smoothing, explicit feedback delays and bounded speaker output. localStorage holds the patch; IndexedDB holds imported samples.

npm run format:check
npm run typecheck     # UI, every module processor and the audio host
npm test              # Silent DSP, persistence and recovery regressions
npm run build
npm start

The build produces a static out/ directory. Serve it at the root of an HTTPS site; localhost works for development. There is no backend to deploy. Browser storage belongs to its origin, so moving between localhost and a hosted URL does not transfer your rack or samples.

Contributing

New modules, thoughtful panel improvements, DSP fixes and small reproducible bug reports are welcome. Read CONTRIBUTING.md for the workflow and AGENTS.md for module design principles.

The panels and processors are original digital instruments inspired by modular synthesis, not circuit-accurate emulations or manufacturer-endorsed products. Third-party notices, including font licenses, are included in the repository and every static build. The project license is awaiting selection; no open-source license has been granted yet.