tauri-plugin-system-audio

Capture what your app hears and what the computer plays — with echo cancellation — in Tauri 2.

Microphone + system audio (WASAPI loopback) dual capture for Windows, with WebRTC AEC3 echo cancellation, anti-aliased resampling to 16 kHz mono PCM, and 10 Hz level metering. Extracted from the production desktop app of SubcueAI, where it feeds live dual-stream speech-to-text during video calls.

  ┌─────────────┐    ┌─────────────┐    ┌───────────┐
  │ mic capture │───▶│  resampler  │───▶│   APM     │──▶ Pcm { source: "mic" }
  └─────────────┘    │  to 16k f32 │    │ near-end  │
                     └─────────────┘    └───────────┘
  ┌─────────────┐    ┌─────────────┐    ┌───────────┐
  │ loopback*   │───▶│  resampler  │───▶│   APM     │──▶ Pcm { source: "loopback" }
  └─────────────┘    │  to 16k f32 │    │ reverse   │
     *Windows only    └─────────────┘    └───────────┘

Why this exists

Capturing system audio in a Tauri app is a recurring pain point: browsers can't do it, getDisplayMedia audio is unreliable, and most examples stop at "open a mic stream". This plugin packages the hard parts:

• WASAPI loopback via cpal 0.16 — capturing the default output device as an input stream (AUDCLNT_STREAMFLAGS_LOOPBACK), so you hear Zoom/Meet/Teams/whatever the machine plays. No virtual audio driver, no Stereo Mix.
• Echo cancellation that actually works — the loopback feed doubles as WebRTC AEC3's far-end reference, so your own speakers are subtracted from the mic before your app sees it. Without this, speaker bleed re-enters the mic and wrecks downstream STT/recording.
• Dual independent streams — mic and loopback are emitted as separate tagged PCM streams (not premixed), so you can route them to two STT sessions and label "local speaker" vs "remote party". A sample-and-hold Mixer is included if you want one combined stream.
• STT-grade signal path — f32 end to end (no precision-eating i16 round-trips), 129-tap windowed-sinc anti-alias filter on downsample, i16 quantisation only at the serialisation boundary, 20 ms frames for continuous interim STT partials.
• The boring-but-vital details — Windows mic-consent registry preflight (otherwise you capture silence forever), permission-aware error categories for UI deep-links, allocation-free hot path, drift-tolerant frame pairing, clean stream teardown order.

Platform behavior: Windows = full pipeline. macOS/Linux = mic-only (loopback and APM compile to stubs; on macOS, Apple's VPIO already does AEC at the OS level, and system-audio capture there is ScreenCaptureKit's job — out of scope for this plugin).

Install

Rust side (src-tauri/Cargo.toml):

cargo add tauri-plugin-system-audio

[dependencies]
tauri-plugin-system-audio = "0.1"

Register the plugin:

fn main() {
    tauri::Builder::default()
        .plugin(tauri_plugin_system_audio::init())
        .run(tauri::generate_context!())
        .expect("error while running tauri application");
}

Allow the commands in your capability file (src-tauri/capabilities/default.json):

{ "permissions": ["system-audio:default"] }

Echo cancellation dll (Windows)

AEC needs webrtc-apm/webrtc-apm.dll (a C-ABI build of WebRTC's AudioProcessing module, BSD-3 licensed — ABI reference). Copy it into src-tauri/resources/ and bundle it:

// tauri.conf.json
{ "bundle": { "resources": ["resources/webrtc-apm.dll"] } }

The plugin resolves the bundled dll automatically (dev and build). Missing dll degrades gracefully: capture keeps working, just without echo cancellation — a warning is logged.

Use (JavaScript)

npm i @subcueai/tauri-plugin-system-audio

import { start, stop, decodePcm } from '@subcueai/tauri-plugin-system-audio';

await start((event) => {
  switch (event.kind) {
    case 'pcm': {
      const samples = decodePcm(event.samples_base64); // Int16Array, 16 kHz mono, 20 ms
      if (event.source === 'mic') sttLocal.send(samples);
      else sttRemote.send(samples); // system audio: the remote party
      break;
    }
    case 'level': // 10 Hz meter: event.mic_rms / event.loopback_rms (0..1)
      break;
    case 'failure': // category: 'permission' | 'device' | 'io' | 'lifecycle'
      break;
  }
});

// later
await stop();

Options (all optional): start(onEvent, { loopback: true, processing: true, levelOnly: false })

• loopback: false — mic only.
• processing: false — skip WebRTC APM entirely.
• levelOnly: true — no PCM at all, just 10 Hz levels (~0% upload, <1% CPU) for an idle "mic check" meter.

Pre-flight the mic permission for your Settings UI:

const status = await permissionStatus(); // 'allowed' | 'denied' | 'unknown'

Event reference

| Event | Payload | Notes | |---|---|---| | pcm | seq, source (mic|loopback), sample_rate (16000), channels (1), samples_base64 | 20 ms frames; i16 LE, base64 | | level | mic_rms, loopback_rms (0..1) | ≤10 Hz, deduped below 0.005 delta | | failure | category, message | Worker exited; permission → deep-link OS Settings |

FAQ

Why 16 kHz mono? It's the native rate of speech models and WebRTC APM's processing band. Capturing at device rate and downsampling once (with a proper anti-alias FIR) beats letting each downstream consumer resample.

Why base64 over a Tauri Channel instead of raw buffers? One ordered channel carries tagged heterogeneous events (pcm/level/failure) with backpressure, ~160 KB/s per stream — trivial for the IPC. Decode cost is one atob per 20 ms.

Can it capture a specific app's audio only? No — WASAPI loopback captures the default render endpoint mix. Per-process capture needs the Windows 10 2004+ AUDIOCLIENT_PROCESS_LOOPBACK path, which cpal doesn't expose yet.

Does AEC work if the user wears headphones? There's simply no echo to cancel — AEC3 idles. Loopback capture still works and is unaffected.

macOS system audio? Use ScreenCaptureKit from native code (that's what SubcueAI's own macOS app does); this plugin intentionally stays cpal-only.

Provenance & license

This is the audio pipeline that ships in SubcueAI's Windows desktop app (Tauri 2 + React), extracted with its production comments and tests intact.

MIT © 2026 Subcue AI LLC. webrtc-apm.dll is built from BSD-3-Clause WebRTC — see THIRD-PARTY.md.