Change sample rate of AudioContext (getUserMedia)

Question

Im trying to record a 48000Hz recording via getUserMedia. But without luck. The returned audio MediaStream returns 44100Hz. How can i set this to 48000Hz?

Here are snippets of my code:

var startUsermedia = this.startUsermedia;

            navigator.getUserMedia({ 
                audio: true, 
                //sampleRate: 48000 
            }, startUsermedia, function (e) {
                console.log('No live audio input: ' + e);
            });

The startUsermedia function:

startUsermedia: function (stream) {
            var input = audio_context.createMediaStreamSource(stream);
            console.log('Media stream created.');
            // Uncomment if you want the audio to feedback directly
            //input.connect(audio_context.destination);
            //__log('Input connected to audio context destination.');

            recorder = new Recorder(input);
            console.log('Recorder initialised.');
        },

I tried changing the property sampleRate of the AudioContext, but no luck.

How can i change the sampleRate to 48000Hz?

EDIT : We are also now okay with a flash solution that can record and export wav files at 48000Hz

Answer 1

As far as I know, there is no way to change the sample rate within an audio context. The sample rate will usually be the sample rate of your recording device and will stay that way. So you will not be able to write something like this:

var input = audio_context.createMediaStreamSource(stream);
var resampler = new Resampler(44100, 48000);
input.connect(resampler);
resampler.connect(audio_context.destination);

However, if you want to take your audio stream, resample it and then send it to the backend (or do sth. else with it outside of the Web Audio API), you can use an external sample rate converter (e.g. https://github.com/taisel/XAudioJS/blob/master/resampler.js).

   var resampler = new Resampler(44100, 48000, 1, 2229);

   function startUsermedia(stream) {
        var input = audio_context.createMediaStreamSource(stream);
        console.log('Media stream created.');


        recorder = audio_context.createScriptProcessor(2048);
        recorder.onaudioprocess = recorderProcess;
        recorder.connect(audio_context.destination);
    }

    function recorderProcess(e) {
        var buffer = e.inputBuffer.getChannelData(0);
        var resampled = resampler.resampler(buffer);
        //--> do sth with the resampled data for instance send to server
    }

Answer 2

It looks like there is an open bug about the inability to set the sampling rate:

https://github.com/WebAudio/web-audio-api/issues/300

There's also a Chrome issue:

https://bugs.chromium.org/p/chromium/issues/detail?id=432248

I checked the latest Chromium code and there is nothing in there that lets you set the sampling rate.

Edit: Seems like it has been implemented in Chrome, but is broken currently - see the comments in the Chromium issue.

Answer 3

You can't. The sample rate of the AudioContext is set by the browser/device and there is nothing you can do to change it. In fact, you will find that 44.1kHz on your machine might be 48kHz on mine. It varies to whatever the OS picks by default.

Also remember that not all hardware is capable of all sample rates.

Answer 4

You can use an OfflineAudioContext to essentially render your audio buffer to a different sample rate (but this is batch operation).

So you would record your recording using the normal audio context, and then use an OfflineAudioContext with a different sample rate to render your buffer. There is an example on the Mozilla page.

Answer 5

it's been added to chrome:

var ctx = new (window.AudioContext || window.webkitAudioContext)({ sampleRate:16000});

https://developer.mozilla.org/en-US/docs/Web/API/AudioContext/AudioContext

Answer 6

It is now in the spec but not yet implemented in Chromium. Also in bugs.chromium.org, "Status: Available" does not mean it is implemented. It just means that nobody is working on it and that it is available for anyone who wants to work on it. So "Available" means "Not assigned".