How to render the audio from a synth to a buffer (array of PCM values) with the Web Audio API

Question

I have a simple synth that plays a note for some length of time:

// Creating audio graph
var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
var oscillator = audioCtx.createOscillator();
var gainNode = audioCtx.createGain();
oscillator.connect(gainNode);
gainNode.connect(audioCtx.destination);

// Setting parameters
oscillator.type = "sine";
oscillator.frequency.value = 2500;

// Run audio graph
var currentTime = offlineCtx.currentTime;
oscillator.start(currentTime);
oscillator.stop(currentTime + 1);

How can I get the PCM data of the sound the synthesiser makes? I've managed to do this with audio samples by using decodeAudioData, but I can't find an equivalent for an audio graph that isn't based on loading a sample.

I specifically want to render the audio graph with the OfflineAudioContext since I only care about retrieving the PCM data as fast as possible.

Thanks!

Answer 1

You say you want to use an offline context and then you don't actually use an offline context. So you should do

var offlineCtx = new OfflineAudioContext(nc, length, rate)

where nc = number of channels, length is the number of samples, and rate is the sample rate you want to use.

Create your graph, start everything and then do

offlineCtx.startRendering().then(function (buffer) {
  // buffer has the PCM data you want. Save it somewhere, 
  // or whatever
})

(I'm not sure all browsers support promises from an offline context. If not, use offlineCtx.oncomplete to get the data. See the spec.)