Is there a way to use the Web Audio API to sample audio faster than real-time?

Question

I'm playing around with the Web Audio API & trying to find a way to import an mp3 (so therefore this is only in Chrome), and generate a waveform of it on a canvas. I can do this in real-time, but my goal is to do this faster than real-time.

All the examples I've been able to find involve reading the frequency data from an analyser object, in a function attached to the onaudioprocess event:

processor = context.createJavascriptNode(2048,1,1);
processor.onaudioprocess = processAudio;
...
function processAudio{
    var freqByteData = new Uint8Array(analyser.frequencyBinCount);
    analyser.getByteFrequencyData(freqByteData);
    //calculate magnitude & render to canvas
}

It appears though, that analyser.frequencyBinCount is only populated when the sound is playing (something about the buffer being filled).

What I want is to be able to manually/programmatically step through the file as fast as possible, to generate the canvas image.

What I've got so far is this:

$("#files").on('change',function(e){
    var FileList = e.target.files,
        Reader = new FileReader();

    var File = FileList[0];

    Reader.onload = (function(theFile){
        return function(e){
            context.decodeAudioData(e.target.result,function(buffer){
                source.buffer = buffer;
                source.connect(analyser);
                analyser.connect(jsNode);

                var freqData = new Uint8Array(buffer.getChannelData(0));

                console.dir(analyser);
                console.dir(jsNode);

                jsNode.connect(context.destination);
                //source.noteOn(0);
            });
        };
    })(File);

    Reader.readAsArrayBuffer(File);
});

But getChannelData() always returns an empty typed array.

Any insight is appreciated - even if it turns out it can't be done. I think I'm the only one the Internet not wanting to do stuff in real-time.

Thanks.

Answer 1

There is a really amazing 'offline' mode of the Web Audio API that allows you to pre-process an entire file through an audio context and then do something with the result:

var context = new webkitOfflineAudioContext();

var source = context.createBufferSource();
source.buffer = buffer;
source.connect(context.destination);
source.noteOn(0);

context.oncomplete = function(e) {
  var audioBuffer = e.renderedBuffer;
};

context.startRendering();

So the setup looks exactly the same as the real-time processing mode, except you set up the oncomplete callback and the call to startRendering(). What you get back in e.redneredBuffer is an AudioBuffer.