# Focus Room Playlist Renderer

> Mix a playlist of generated ambience scenes with crossfades and master effects into a WAV. Renders a playlist of generated ambience scenes — Focus Room presets such as brown noise, light rain or a surf-like swell, and the generated Soundscape Atlas worlds — one after another with equal-power crossfades, then runs the master effects: low cut, high cut, an 8-band graphic EQ, room reverb, stereo width, volume and a gentle soft limiter. Paste scene lines or a whole Focus Room (JSON or share link). The result is a 16-bit stereo WAV file with a timeline of the scenes and the EQ curve; the Focus Room app uses the same renderer, so both give identical files.

Part of usefulweb.app (https://usefulweb.app/). Part of [Focus Room](https://focusroom.usefulweb.app/) (ownerApp: focusroom). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://focusroomplaylistrender.usefulweb.app/ (interactive tool at https://focusroomplaylistrender.usefulweb.app/tool)
- JSON API: POST https://focusroomplaylistrender.usefulweb.app/api/v1/calculate with a JSON object of inputs (schema: https://focusroomplaylistrender.usefulweb.app/api/v1/tool)
- OpenAPI: https://focusroomplaylistrender.usefulweb.app/openapi.json
- MCP (Streamable HTTP): https://focusroomplaylistrender.usefulweb.app/mcp - tool `focusroom_playlist_render`

## Inputs
- items (textarea): Scenes (one per line: scene id and seconds) - Generated scenes: brown-noise, pink-rain, surf-like-swell, fireside, forest-stream, night-wind, rainy-study-room, submarine, medieval-tavern, spacecraft-hum, cavern-drips, coffee-shop-murmur. Write "brown-noise 30" or "fireside 2 min". Used when no room is pasted below.
- project (textarea): Focus Room (JSON or share link, optional) - Paste a .focusroom.json file or a Focus Room share link to render its playlist (shuffle order included) with its master effects. Scenes with Wikimedia Commons recordings cannot be rendered on the server.
- secondsPerItem (number): Seconds per scene - Length of each scene of a pasted room (a room’s scenes usually last many minutes), and of scene lines without their own length.
- crossfade (number): Crossfade - Equal-power crossfade between scenes (at most half a scene). A pasted room uses its own crossfade.
- fx (select): Master effects preset - A pasted room uses its own master effects.
- sampleRate (select): Sample rate

## Method
- Each scene is a Soundscape Atlas scene rendered by the shared audio engine exactly like a Soundscape WAV export: seamless loops of the noise and ambience generators (seeded), layer gains, equal-power pan, filters, slow automation curves and the scene’s own room. Layer fade-outs are switched off, as in live playback.
- Scenes follow each other on one timeline; scene n + 1 starts one crossfade before scene n ends (at most half of the shorter scene). The overlap uses equal-power gains — outgoing cos(uπ/2), incoming sin(uπ/2) — so the summed power of two unrelated ambiences stays constant. The last scene fades out the same way.
- Master chain in this order: high-pass (low cut) and low-pass (high cut), both 12 dB/octave Butterworth biquads; eight peaking biquads at the ISO octave centres 63 Hz – 8 kHz with Q 1.41 (Robert Bristow-Johnson’s Audio EQ Cookbook, the same formulas as the Web Audio BiquadFilterNode); room reverb (Freeverb-style, preset reverberation time, mono input); mid/side stereo width; volume; and a soft limiter (tanh above 80 % of a 0.98 ceiling).
- The mix is written as 16-bit PCM WAV. The Focus Room app plays the same chain live with Web Audio nodes whose coefficients equal these biquads.

## Limits
- All sounds are synthesized; names such as “surf-like swell” or “coffee-shop-like murmur” describe what filtered noise imitates, not recordings. This is not a medical or hearing device; keep the volume comfortable.