audio-encoder/src/encodeWav.js

encode audioBuffer to Wave or mp3

var HEADER_LENGTH = 44;
var MAX_AMPLITUDE = 0x7FFF;

function encodeWav(audioBuffer, cb) {
	var nChannels = audioBuffer.numberOfChannels;

	if (nChannels !== 1 && nChannels !== 2) {
		throw new Error('Expecting mono or stereo audioBuffer');
	}

	var bufferLength = audioBuffer.length;

	// Creating the array buffers (2 bytes per samples * 1 channel)
	var arrayBuffer = new ArrayBuffer(HEADER_LENGTH + 2 * bufferLength * nChannels);

	// Creatiing views on the array buffer
	var int16 = new Int16Array(arrayBuffer);
	var uint8 = new Uint8Array(arrayBuffer);


	// WAV header
	// http://soundfile.sapp.org/doc/WaveFormat/
	// 52 49 46 46     R I F F
	// 24 08 00 00     chunk size
	// 57 41 56 45     W A V E

	// 66 6d 74 20     F T M █
	// 10 00 00 00     subchunk size
	// 01 00           audio format
	// 02 00           number of channels
	// 44 AC 00 00     sample rate
	// 88 58 01 00     bitrate
	// 04 00           block align
	// 10 00           bit per sample
	// 64 61 74 61     d a t a
	// 00 08 00 00     subchunk2 size

	var sr = audioBuffer.sampleRate;
	var l2 = bufferLength * nChannels * 2; // subchunk2 = numSamples * numChannels * BitsPerSample / 8
	var l1 = l2 + 36; // chunkSize = subchunk + 36
	var br = sr * nChannels * 2 // bitrate = SampleRate * NumChannels * BitsPerSample / 8

	uint8.set([
		// "RIFF" chunk descriptor
		0x52, 0x49, 0x46, 0x46, // R I F F
		l1 & 255, (l1 >> 8) & 255, (l1 >> 16) & 255, (l1 >> 24) & 255, // chunk size
		0x57, 0x41, 0x56, 0x45, // W A V E

		// "ftm" sub-chunk
		0x66, 0x6D, 0x74, 0x20, // F T M █
		0x10, 0x00, 0x00, 0x00, // sub chunk size = 16
		0x01, 0x00, // audio format = 1 (PCM, linear quantization)
		nChannels, 0x00, // number of channels
		sr & 255, (sr >> 8) & 255, (sr >> 16) & 255, (sr >> 24) & 255, // sample rate
		br & 255, (br >> 8) & 255, (br >> 16) & 255, (br >> 24) & 255, // byte rate
		0x04, 0x00, // block align = 4
		0x10, 0x00, // bit per sample = 16

		// data sub-chuk
		0x64, 0x61, 0x74, 0x61, // d a t a
		l2 & 255, (l2 >> 8) & 255, (l2 >> 16) & 255, (l2 >> 24) & 255 // sub chunk 2 size
	]);

	// Append sample data
	var buffers = [];

	for (var channel = 0; channel < nChannels; channel++) {
		buffers.push(audioBuffer.getChannelData(channel));
	}

	for (var i = 0, index = HEADER_LENGTH / 2; i < bufferLength; i++) {
		for (var channel = 0; channel < nChannels; channel++) {
			var sample = buffers[channel][i];

			// clamp and convert to 16bit number
			sample = Math.min(1, Math.max(-1, sample));
			sample = Math.round(sample * MAX_AMPLITUDE);

			int16[index++] = sample;
		}
	}

	var blob = new Blob([uint8], { type: 'audio/x-wav' })

	setTimeout(function () {
		return cb(blob);
	}, 30);
}

module.exports = encodeWav;