How to use web audio api to get raw pcm audio?

Question

How usergetmedia to use the microphone in chrome and then stream to get raw audio? I need need to get the audio in linear 16.

Answer 1

Unfortunately, the MediaRecorder doesn't support raw PCM capture. (A sad oversight, in my opinion.) Therefore, you'll need to get the raw samples and buffer/save them yourself.

You can do this with the ScriptProcessorNode. Normally, this Node is used to modify the audio data programmatically, for custom effects and what not. But, there's no reason you can't just use it as a capture point. Untested, but try something like this code:

const captureNode = audioContext.createScriptProcessor(8192, 1, 1);
captureNode.addEventListener('audioprocess', (e) => {
  const rawLeftChannelData = inputBuffer.getChannelData(0);
  // rawLeftChannelData is now a typed array with floating point samples
});

(You can find a more complete example on MDN.)

Those floating point samples are centered on zero 0 and will ideally be bound to -1 and 1. When converting to an integer range, you'll want to clamp values to this range, clipping anything beyond it. (The values can sometimes exceed -1 and 1 in the event loud sounds are mixed together in-browser. In theory, the browser can also record float32 samples from an external sound device which may also exceed that range, but I don't know of any browser/platform that does this.)

When converting to integer, it matters if the values are signed or unsigned. If signed, for 16-bit, the range is -32768 to 32767. For unsigned, it's 0 to 65535. Figure out what format you want to use and scale the -1 to 1 values up to that range.

One final note on this conversion... endianness can matter. See also: https://stackoverflow.com/a/7870190/362536

Answer 2

You should look into MediaTrackConstraints.sampleSize property for the MediaDevices.getUserMedia() API. Using the sampleSize constraint, if your audio hardware permits you can set the sample size to 16 bits.

As far as the implementation goes, well that's what the links are and google are for...