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:

• JS copies ArrayBuffer into WebAssembly's input buffer.
• WebAssembly decodes it to planar float values on the output buffer.
• JS copies the output buffer as Float32Arrays into an AudioBuffer.
• 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