Intelligent EQ Auto-mastering — 100% in the browser
Rack4Master mini is a web-based auto‑mastering application that runs entirely in the browser, with no external servers or installation. It analyzes the spectrum and dynamics of any audio file, suggests intelligent corrections, and applies a professional processing chain in real time, with exportable WAV results.
Supports drag & drop or file selection for MP3, WAV, OGG, FLAC and AAC.
Interactive waveform player based on WaveSurfer.js v6. The playback head is clickable and draggable to navigate through the track.
- Play / Pause / Stop with keyboard shortcuts (
Space,S) - Current time and duration display
- Loop with adjustable region: the S (Start) and E (End) handles are draggable on the waveform to define exactly which section to repeat
The audio engine implements a wet/dry chain with click‑free crossfade:
| Stage | Description |
|---|---|
| EQ Low Shelf | Boost or cut lows (150 Hz) |
| EQ High Shelf | Boost or cut highs (5 kHz) |
| EQ Peak Mid | Vocal range control (1.5 kHz) |
| EQ Peak Harsh | Presence / sibilance control (3.5 kHz) |
| Compressor | DynamicsCompressor with ratio, threshold, attack, release and makeup gain |
| Saturation | WaveShaper with configurable soft‑knee curve |
| Stereo Width | Independent M/S side channel control |
| Limiter | High‑ratio compressor at -0.5 dB, configurable |
The ON/OFF button performs a complete bypass with smooth crossfade between the dry and processed signal.
Analyzes the decoded buffer using a custom FFT (Cooley‑Tukey radix‑2) and generates a report with up to 8 automatic diagnostics:
- Excessive or deficient treble (spectral brightness)
- Excessive or deficient bass (energy ratio 60–150 Hz)
- Harshness in the 2–5 kHz range (listening fatigue)
- Sunken or dominant mids (vocal zone 500–3000 Hz)
- Excessive or normal dynamic range (suggests appropriate compression)
- Aggressive transients (crest factor)
- Lack of harmonic warmth (suggests saturation)
- Stereo field too narrow or excessively wide
Each diagnostic includes a toggle on/off and a fine‑tuning slider to modify the suggested parameter in real time while listening to the track.
General Neutral · Smooth Mastering · Broadcast
Genres Rock · Pop · Jazz · Blues · Ballad · Folk · Country · Urban/Hip‑Hop · Latino
Style Warm Vintage · Bright & Open · Punchy · Lo‑Fi
Each preset shows a thumbnail with EQ bars and compression dots. When selected, it is applied instantly and a fine‑tuning panel opens with sliders for the relevant parameters (neutral parameters are omitted). Compression is exposed as a single Intensity slider that interpolates between the neutral value and the preset’s target.
- WAV: offline rendering via
OfflineAudioContextusing the same processing chain, exported as 16‑bit or 24‑bit stereo PCM WAV. - Preset JSON: save and load the complete parameter configuration in JSON format to reuse across sessions.
| Key | Action |
|---|---|
Space |
Play / Pause |
S |
Stop |
L |
Loop on/off |
B |
Bypass on/off |
index.html — Interfaz principal
style.css — Estilos (dark theme, variables CSS)
main.js — Punto de entrada, atajos de teclado
audioEngine.js — Cadena Web Audio, exportación WAV, save/load preset
analysis.js — FFT propia, métricas espectrales y dinámicas
reportUI.js — Renderizado del informe de análisis y sliders
presetsUI.js — Tarjetas de preset, selector y editor de parámetros
presets.js — Definición de los 16 presets
uiControls.js — Controlador principal: WaveSurfer, transporte, modos, loop
- Modern browser with Web Audio API and OfflineAudioContext support
- Local web server (XAMPP, Live Server, etc.) to avoid CORS restrictions
- No Node.js, bundler or npm dependencies required
- Serve the folder from a local web server
- Open
index.htmlin your browser - Drag an audio file or click the upload area
- Choose Analysis for automatic correction or Presets for a predefined style
- Adjust the sliders while listening to the track in real time
- Press the green WAV button to export the result
Rack4Master mini — professional audio processing, right from your browser.