---
name: PCM bridge (AudioWorklet)
type: architecture-map-page
slice: 03-audio-transcription
last_verified: 2026/05/09
---

# PCM bridge (AudioWorklet)

> **Where you are:** [Architecture map](../README.md) → [Audio + Transcription](README.md) → PCM bridge

**Plain English summary.** `static/pcm-processor.js` is a tiny browser-side AudioWorklet. It runs inside the Svelte frontend's `AudioContext`, naively downsamples the device's native rate to 16 kHz, and posts ~0.5 s batches of f32 samples back to the main thread. It only matters on a code path that goes browser-mic → frontend → Tauri command rather than the native cpal capture; today's primary path is `MicrophoneCapture` (Rust). Treat this file as a minimal fallback / web-context stub.

## At a glance

- Runtime: browser AudioWorkletProcessor (not a Rust crate).
- Path: `static/pcm-processor.js`
- LOC: 44
- External deps: none (Web Audio API).
- Internal callers (best effort, slice 1 reconciles): the Svelte frontend instantiates it with `audioContext.audioWorklet.addModule("/pcm-processor.js")` and listens for `port.message` events. The native cpal path in `magnotia-audio` is the production path; this exists for parity in dev/web contexts.

Public surface: registers the AudioWorklet processor name `pcm-processor`.

## What's in here

The whole file:

```js
class PcmProcessor extends AudioWorkletProcessor {
  constructor() {
    super();
    this.buffer = [];
    this.ratio = sampleRate / 16000;
    this.needsResample = Math.abs(this.ratio - 1.0) > 0.01;
    this.resamplePos = 0;
  }

  process(inputs) {
    const input = inputs[0];
    if (!input || input.length === 0) return true;
    const samples = input[0]; // First channel (mono)
    if (!samples) return true;

    if (this.needsResample) {
      // Simple downsampling to 16kHz
      for (let i = 0; i < samples.length; i++) {
        this.resamplePos += 1;
        if (this.resamplePos >= this.ratio) {
          this.buffer.push(samples[i]);
          this.resamplePos -= this.ratio;
        }
      }
    } else {
      for (let i = 0; i < samples.length; i++) {
        this.buffer.push(samples[i]);
      }
    }

    if (this.buffer.length >= 8000) {
      this.port.postMessage({ type: "pcm", samples: this.buffer });
      this.buffer = [];
    }
    return true;
  }
}
registerProcessor("pcm-processor", PcmProcessor);
```

`sampleRate` is the AudioWorklet global (the context's rate). The file picks the first channel and ignores the rest.

## Data flow

```
mediaStream → MediaStreamAudioSourceNode → AudioWorkletNode("pcm-processor")
   └─ process(inputs)
       ├─ if needsResample (ratio != 1): drop-sample downsample to 16 kHz
       └─ else: passthrough
   └─ buffer to ~0.5 s (8000 samples)
   └─ postMessage({ type: "pcm", samples: f32[] })
       └─ frontend receiver (slice 1) → Tauri command (slice 2)
```

## Watch-outs

- **Drop-sample downsampling, no anti-alias filter.** Above ~8 kHz the result will alias. For ASR this is rarely a problem because Whisper expects 16 kHz inputs (so anything above 8 kHz Nyquist is already discarded), but if anyone repurposes this for non-ASR work it will show.
- **Mono only.** Reads channel 0, ignores channels 1+.
- **`Math.abs(ratio - 1) > 0.01` decides passthrough.** A 16001 Hz context (rare but legal) would skip the resample loop.
- **`port.postMessage` clones the array.** No transferables in this implementation; large session lengths magnify the cost.
- **The native cpal path is the primary path.** This worklet matters when the frontend captures audio itself (web build, dev preview without the Tauri shell). Slice 1 will know which routes mount it.
- **Build artefacts mirror the source.** Copies appear under `build/` and `.svelte-kit/output/client/` after a build. The source-of-truth file is `static/pcm-processor.js`.

## See also

- [Audio capture pipeline](audio-capture-pipeline.md) — the production path using cpal, not this worklet.
- [Audio resampling](audio-resampling.md) — the (much higher quality) sinc resampler used on the native path.
- Frontend Audio plumbing (slice 1) — the consumer of `port.postMessage`.