abcjs/src/synth/download-buffer.js

Renderer for abc music notation

var downloadBuffer = function(buffer) {
	return window.URL.createObjectURL(bufferToWave(buffer.audioBuffers));
};

// Convert an AudioBuffer to a Blob using WAVE representation
function bufferToWave(audioBuffers) {
	var audioBuffer = audioBuffers[0];
	var numOfChan = audioBuffer.numberOfChannels;
	var length = audioBuffer.length * numOfChan * 2 + 44;
	var buffer = new ArrayBuffer(length);
	var view = new DataView(buffer);
	var channels = [];
	var i;
	var sample;
	var offset = 0;
	var pos = 0;

	// write WAVE header
	setUint32(0x46464952);                         // "RIFF"
	setUint32(length - 8);                         // file length - 8
	setUint32(0x45564157);                         // "WAVE"

	setUint32(0x20746d66);                         // "fmt " chunk
	setUint32(16);                                 // length = 16
	setUint16(1);                                  // PCM (uncompressed)
	setUint16(numOfChan);
	setUint32(audioBuffer.sampleRate);
	setUint32(audioBuffer.sampleRate * 2 * numOfChan); // avg. bytes/sec
	setUint16(numOfChan * 2);                      // block-align
	setUint16(16);                                 // 16-bit (hardcoded in this demo)

	setUint32(0x61746164);                         // "data" - chunk
	setUint32(length - pos - 4);                   // chunk length

	// write interleaved data
	for(i = 0; i < numOfChan; i++)
		channels.push(audioBuffer.getChannelData(i));

	while(pos < length) {
		for(i = 0; i < channels.length; i++) {             // interleave channels
			sample = Math.max(-1, Math.min(1, channels[i][offset])); // clamp
			sample = (0.5 + sample < 0 ? sample * 32768 : sample * 32767)|0; // scale to 16-bit signed int
			view.setInt16(pos, sample, true);          // write 16-bit sample
			pos += 2;
		}
		offset++; // next source sample
	}

	// create Blob
	return new Blob([buffer], {type: "audio/wav"});

	function setUint16(data) {
		view.setUint16(pos, data, true);
		pos += 2;
	}

	function setUint32(data) {
		view.setUint32(pos, data, true);
		pos += 4;
	}
}

module.exports = downloadBuffer;