resampler.js

"use strict";
//JavaScript Audio Resampler
//Copyright (C) 2011-2015 Grant Galitz
//Released to Public Domain
export function Resampler(fromSampleRate, toSampleRate, channels, inputBuffer) {
    //Input Sample Rate:
    this.fromSampleRate = +fromSampleRate;
    //Output Sample Rate:
    this.toSampleRate = +toSampleRate;
    //Number of channels:
    this.channels = channels | 0;
    //Type checking the input buffer:
    if (typeof inputBuffer != "object") {
        throw(new Error("inputBuffer is not an object."));
    }
    if (!(inputBuffer instanceof Array) && !(inputBuffer instanceof Float32Array) && !(inputBuffer instanceof Int16Array)) {
        throw(new Error("inputBuffer is not an array or a float32 or a int16 array."));
    }
    this.inputBuffer = inputBuffer;
    //Initialize the resampler:
    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;
            this.outputBuffer = this.inputBuffer;
		}
		else {
            this.ratioWeight = this.fromSampleRate / this.toSampleRate;
			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.initializeBuffers();
		}
	}
	else {
		throw(new Error("Invalid settings specified for the resampler."));
	}
}
Resampler.prototype.compileLinearInterpolationFunction = function () {
	var toCompile = "var outputOffset = 0;\
    if (bufferLength > 0) {\
        var buffer = this.inputBuffer;\
        var weight = this.lastWeight;\
        var firstWeight = 0;\
        var secondWeight = 0;\
        var sourceOffset = 0;\
        var outputOffset = 0;\
        var outputBuffer = this.outputBuffer;\
        for (; weight < 1; weight += " + this.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 + "; 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 += " + this.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 outputOffset;";
	this.resampler = Function("bufferLength", toCompile);
}
Resampler.prototype.compileMultiTapFunction = function () {
	var toCompile = "var outputOffset = 0;\
    if (bufferLength > 0) {\
        var buffer = this.inputBuffer;\
        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 currentPosition = 0;\
        do {\
            if (alreadyProcessedTail) {\
                weight = " + this.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 + " / " + this.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);\
    }\
    return outputOffset;";
	this.resampler = Function("bufferLength", toCompile);
}
Resampler.prototype.bypassResampler = function (upTo) {
    return upTo;
}
Resampler.prototype.initializeBuffers = function () {
	//Initialize the internal buffer:
    var outputBufferSize = (Math.ceil(this.inputBuffer.length * this.toSampleRate / this.fromSampleRate / this.channels * 1.000000476837158203125) * this.channels) + this.channels;
	var fl = false; if(this.inputBuffer instanceof Float32Array) fl = true;
	try {
		this.outputBuffer = new (fl?Float32Array:Int16Array)(outputBufferSize);
		this.lastOutput = new (fl?Float32Array:Int16Array)(this.channels);
	}
	catch (error) {
		this.outputBuffer = [];
		this.lastOutput = [];
	}
}