resampler.js

// JavaScript Audio Resampler (c) 2011 - Grant Galitz
// https://github.com/grantgalitz/XAudioJS/blob/0a233b743f11371d97b72153a5a223ac16a88748/resampler.js
function Resampler(fromSampleRate, toSampleRate, channels, outputBufferSize, noReturn) {
	this.fromSampleRate = fromSampleRate;
	this.toSampleRate = toSampleRate;
	this.channels = channels | 0;
	this.outputBufferSize = outputBufferSize;
	this.noReturn = !!noReturn;
	this.initialize();
}
Resampler.prototype.initialize = function () {
	//Perform some checks:
	if (this.fromSampleRate > 0 && this.toSampleRate > 0 && this.channels > 0) {
		if (this.fromSampleRate == this.toSampleRate) {
			//Setup a resampler bypass:
			this.resampler = this.bypassResampler;		//Resampler just returns what was passed through.
			this.ratioWeight = 1;
		}
		else {
			if (this.fromSampleRate < this.toSampleRate) {
				/*
					Use generic linear interpolation if upsampling,
					as linear interpolation produces a gradient that we want
					and works fine with two input sample points per output in this case.
				*/
				this.compileLinearInterpolationFunction();
				this.lastWeight = 1;
			}
			else {
				/*
					Custom resampler I wrote that doesn't skip samples
					like standard linear interpolation in high downsampling.
					This is more accurate than linear interpolation on downsampling.
				*/
				this.compileMultiTapFunction();
				this.tailExists = false;
				this.lastWeight = 0;
			}
			this.ratioWeight = this.fromSampleRate / this.toSampleRate;
			this.initializeBuffers();
		}
	}
	else {
		throw(new Error("Invalid settings specified for the resampler."));
	}
}
Resampler.prototype.compileLinearInterpolationFunction = function () {
	var toCompile = "var bufferLength = buffer.length;\
	var outLength = this.outputBufferSize;\
	if ((bufferLength % " + this.channels + ") == 0) {\
		if (bufferLength > 0) {\
			var ratioWeight = this.ratioWeight;\
			var weight = this.lastWeight;\
			var firstWeight = 0;\
			var secondWeight = 0;\
			var sourceOffset = 0;\
			var outputOffset = 0;\
			var outputBuffer = this.outputBuffer;\
			for (; weight < 1; weight += ratioWeight) {\
				secondWeight = weight % 1;\
				firstWeight = 1 - secondWeight;";
	for (var channel = 0; channel < this.channels; ++channel) {
		toCompile += "outputBuffer[outputOffset++] = (this.lastOutput[" + channel + "] * firstWeight) + (buffer[" + channel + "] * secondWeight);";
	}
	toCompile += "}\
			weight -= 1;\
			for (bufferLength -= " + this.channels + ", sourceOffset = Math.floor(weight) * " + this.channels + "; outputOffset < outLength && sourceOffset < bufferLength;) {\
				secondWeight = weight % 1;\
				firstWeight = 1 - secondWeight;";
	for (var channel = 0; channel < this.channels; ++channel) {
		toCompile += "outputBuffer[outputOffset++] = (buffer[sourceOffset" + ((channel > 0) ? (" + " + channel) : "") + "] * firstWeight) + (buffer[sourceOffset + " + (this.channels + channel) + "] * secondWeight);";
	}
	toCompile += "weight += ratioWeight;\
				sourceOffset = Math.floor(weight) * " + this.channels + ";\
			}";
	for (var channel = 0; channel < this.channels; ++channel) {
		toCompile += "this.lastOutput[" + channel + "] = buffer[sourceOffset++];";
	}
	toCompile += "this.lastWeight = weight % 1;\
			return this.bufferSlice(outputOffset);\
		}\
		else {\
			return (this.noReturn) ? 0 : [];\
		}\
	}\
	else {\
		throw(new Error(\"Buffer was of incorrect sample length.\"));\
	}";
	this.resampler = Function("buffer", toCompile);
}
Resampler.prototype.compileMultiTapFunction = function () {
	var toCompile = "var bufferLength = buffer.length;\
	var outLength = this.outputBufferSize;\
	if ((bufferLength % " + this.channels + ") == 0) {\
		if (bufferLength > 0) {\
			var ratioWeight = this.ratioWeight;\
			var weight = 0;";
	for (var channel = 0; channel < this.channels; ++channel) {
		toCompile += "var output" + channel + " = 0;"
	}
	toCompile += "var actualPosition = 0;\
			var amountToNext = 0;\
			var alreadyProcessedTail = !this.tailExists;\
			this.tailExists = false;\
			var outputBuffer = this.outputBuffer;\
			var outputOffset = 0;\
			var currentPosition = 0;\
			do {\
				if (alreadyProcessedTail) {\
					weight = ratioWeight;";
	for (channel = 0; channel < this.channels; ++channel) {
		toCompile += "output" + channel + " = 0;"
	}
	toCompile += "}\
				else {\
					weight = this.lastWeight;";
	for (channel = 0; channel < this.channels; ++channel) {
		toCompile += "output" + channel + " = this.lastOutput[" + channel + "];"
	}
	toCompile += "alreadyProcessedTail = true;\
				}\
				while (weight > 0 && actualPosition < bufferLength) {\
					amountToNext = 1 + actualPosition - currentPosition;\
					if (weight >= amountToNext) {";
	for (channel = 0; channel < this.channels; ++channel) {
		toCompile += "output" + channel + " += buffer[actualPosition++] * amountToNext;"
	}
	toCompile += "currentPosition = actualPosition;\
						weight -= amountToNext;\
					}\
					else {";
	for (channel = 0; channel < this.channels; ++channel) {
		toCompile += "output" + channel + " += buffer[actualPosition" + ((channel > 0) ? (" + " + channel) : "") + "] * weight;"
	}
	toCompile += "currentPosition += weight;\
						weight = 0;\
						break;\
					}\
				}\
				if (weight == 0) {";
	for (channel = 0; channel < this.channels; ++channel) {
		toCompile += "outputBuffer[outputOffset++] = output" + channel + " / ratioWeight;"
	}
	toCompile += "}\
				else {\
					this.lastWeight = weight;";
	for (channel = 0; channel < this.channels; ++channel) {
		toCompile += "this.lastOutput[" + channel + "] = output" + channel + ";"
	}
	toCompile += "this.tailExists = true;\
					break;\
				}\
			} while (actualPosition < bufferLength && outputOffset < outLength);\
			return this.bufferSlice(outputOffset);\
		}\
		else {\
			return (this.noReturn) ? 0 : [];\
		}\
	}\
	else {\
		throw(new Error(\"Buffer was of incorrect sample length.\"));\
	}";
	this.resampler = Function("buffer", toCompile);
}
Resampler.prototype.bypassResampler = function (buffer) {
	if (this.noReturn) {
		//Set the buffer passed as our own, as we don't need to resample it:
		this.outputBuffer = buffer;
		return buffer.length;
	}
	else {
		//Just return the buffer passsed:
		return buffer;
	}
}
Resampler.prototype.bufferSlice = function (sliceAmount) {
	if (this.noReturn) {
		//If we're going to access the properties directly from this object:
		return sliceAmount;
	}
	else {
		//Typed array and normal array buffer section referencing:
		try {
			return this.outputBuffer.subarray(0, sliceAmount);
		}
		catch (error) {
			try {
				//Regular array pass:
				this.outputBuffer.length = sliceAmount;
				return this.outputBuffer;
			}
			catch (error) {
				//Nightly Firefox 4 used to have the subarray function named as slice:
				return this.outputBuffer.slice(0, sliceAmount);
			}
		}
	}
}
Resampler.prototype.initializeBuffers = function () {
	//Initialize the internal buffer:
	try {
		this.outputBuffer = new Float32Array(this.outputBufferSize);
		this.lastOutput = new Float32Array(this.channels);
	}
	catch (error) {
		this.outputBuffer = [];
		this.lastOutput = [];
	}
}