audio-decode-wasm

audio-decode-wasm

Note: This doesn't work quite yet, but the structure is drafted out.

A stream that decodes ArrayBuffers into AudioBuffers. The decoders are made in WebAssembly so they are portable (Node.js and browser) and decent speed.

const decoder = require('audio-decode-wasm')

// Obtain codec module and initialize decoder:
const mod = await fetch('wav.wasm').then(req => WebAssembly.compileStreaming(res))
const decode = await decoder(mod)

// Decode ArrayBuffers:
decode(arrayBuf, (err, audioBuf) => {
  // ...
})

Install

npm i -D audio-decode-wasm

Usage

decoder(mod) -> Promise<decode>

Initializes a decoder from a given [WebAssembly.Module]. The available ones can be seen in src/.

const mod = new WebAssembly.Module(...)

decoder(mod).then(decode => {
  // ...
})

decode(arrayBuffer, done)

Decodes arrayBuffer and calls done(err, audioBuffer) when finished.

To stop or "reset" the stream send decode(null).

const decode = await decoder(mod)

fetch('foo.wav')
.then(res => res.arrayBuffer())
.then(buf => {
  decode(buf, (err, audio) => {
    // ...
  })
})

Using multiple modules

Promise.all([
  decoder(...),
  decoder(...)  
]).then(([ wav, mp3, ... ]) => {
  // ...
})

How does it work?

The decoders are wrote in C and compiled with Emscripten. The code is more restricted than a normal Emscripten runtime so it's cheap to load.

Each C modules has the functions

Context* open(unsigned char* input, float* output)
void process(Context* context, int amount)

From JS you can create the context with _open(input, output), where the parameters and return values are pointers on WebAssembly's memory, which JS can access and modify.

The Context from C looks like:

typedef struct {
  unsigned char* input;
  float* output;
  uint16_t number_of_channels;
  uint32_t sample_rate;
  unsigned char params;
  // ...
} Context;

To construct an AudioBuffer you need numberOfChannels and sampleRate, so JS imports a set_params(int, int) function which C can call.

The stream routine would look like this:

1. JS copies ArrayBuffer into WebAssembly's input buffer.
2. WebAssembly decodes it to planar float values on the output buffer.
3. JS copies the output buffer as Float32Arrays into an AudioBuffer.
4. Repeat until stream is done.

Building

Requires Emscripten, Binaryen, and WABT. Then, using make:

• make to create dist/
• make debug to produce dist/*.wat
• make clean to remove output