Twitch Channel Volume

Chrome extension (MV3) that auto-balances per-channel volume on Twitch using real-time ITU-R BS.1770 LUFS measurement of the playing audio. Separate gain is applied during ad breaks.

Twitch does not publish any loudness metadata (no loudnessDb-equivalent API, no HLS audio metadata), so the extension measures the playing <video> directly via Web Audio API and persists the resulting gain per channel × media kind (Live / VOD / Clip).

Features

• Real-time LUFS metering: Momentary (400 ms), Short-term (3 s), Integrated (BS.1770 gated)
• Per-channel auto gain: Save gain per Live / VOD / Clip kind; applied automatically on revisit
• Ad-break handling: Detects EXT-X-DATERANGE CLASS="twitch-stitched-ad" in HLS manifests and [data-a-target="video-ad-countdown"] in the DOM; applies a separate gain offset during ads
• 0–600 % gain range via Web Audio GainNode (HTML5 video.volume would cap at 1.0)
• No external dependencies — pure JavaScript, no bundler

How it works

<video>
  ├─→ MediaElementSource ─→ GainNode ─→ destination          (playback)
  └─→ MediaElementSource ─→ K-pre ─→ K-RLB ─→ AudioWorklet   (measurement)
                                              │
                                              └─→ port.postMessage
                                                  (per 100 ms mean-square)

The measurement path applies the BS.1770-4 K-weighting (high-shelf pre-filter + RLB high-pass), accumulates mean-square in an AudioWorklet, and the main thread aggregates 400 ms momentary, 3 s short-term, and gated integrated LUFS in real time.

Install (developer mode)

1. Clone or download this repository
2. Open chrome://extensions/
3. Enable Developer mode
4. Click Load unpacked and select this folder

Usage

1. Open a Twitch stream, VOD, or clip
2. Wait a few seconds for integrated LUFS to stabilize
3. Click the extension icon → Apply to channel
4. The computed gain is saved per channel × kind and re-applied on revisit
5. Use the manual slider (0–600 %) for one-off adjustments
6. Ad break gain is configurable in Settings (default −6 dB)

Settings

• Target LUFS: Reference loudness used to compute gain (default −18 LUFS, range −30 to −6)
• CM Gain: Extra gain applied during ad breaks (default −6 dB)
• Display unit: % or dB
• Saved channels: Table view with delete / clear-all

Development

# Run tests
node test.js

# Regenerate icons (requires Pillow)
python3 gen_icons.py

# Build Chrome Web Store zip
python3 pack.py

Why not just use a compressor?

A static compressor (like FrankerFaceZ's option) clips loud peaks but does nothing about median loudness drift across channels. This extension targets a constant integrated loudness across channels, the way YouTube does for VODs — except Twitch refuses to do it server-side.

Background

• Twitch does not perform loudness normalization on either streams or ads
• Loudness varies wildly between channels (often 20 + dB)
• California SB 576 (enforcement 2026-07-01) may force changes for ads
• Other major platforms normalize: YouTube (-14 LUFS), Spotify (-14 LUFS), Apple Music (-16 LUFS)

Privacy

• No external network requests; all data stays on your device in chrome.storage.local
• Audio/video data itself is never recorded or transmitted — only computed LUFS values and your gain settings are kept
• See PRIVACY_POLICY.md (日本語) for details

License

MIT