source: gs3-extensions/web-audio/trunk/js-dsp/dsp-custom.js@ 30667

// FFT based on dsp.js

// 1. Cutdown to just what is needed
// 2. Messed around with => added in array for the powerSpectrum


var FFTCustom = function(bufferSize, sampleRate) {
    this.bufferSize   = bufferSize;
    this.sampleRate   = sampleRate;
    this.powerSpectrum  = new Float32Array(bufferSize/2);
    this.spectrum       = new Float32Array(bufferSize/2);
    this.real         = new Float32Array(bufferSize);
    this.imag         = new Float32Array(bufferSize);
    this.reverseTable = new Uint32Array(bufferSize);
    this.sinTable     = new Float32Array(bufferSize);
    this.cosTable     = new Float32Array(bufferSize);
    
    var limit = 1,
    bit = bufferSize >> 1;

    while ( limit < bufferSize ) {
        for ( var i = 0; i < limit; i++ ) {
            this.reverseTable[i + limit] = this.reverseTable[i] + bit;
        }

        limit = limit << 1;
        bit = bit >> 1;
    }

    for ( var i = 0; i < bufferSize; i++ ) {
        this.sinTable[i] = Math.sin(-Math.PI/i);
        this.cosTable[i] = Math.cos(-Math.PI/i);
    }
};

FFTCustom.prototype.forward = function(buffer) {
    var bufferSize   = this.bufferSize,
    cosTable     = this.cosTable,
    sinTable     = this.sinTable,
    reverseTable = this.reverseTable,
    real         = this.real,
    imag         = this.imag,
    spectrum          = this.spectrum;
    powerSpectrum     = this.powerSpectrum;

    if ( bufferSize !== buffer.length ) {
        throw "Supplied buffer is not the same size as defined FFT. FFT Size: " + bufferSize + " Buffer Size: " + buffer.length;
    }

    if (_dspdebug) {
    var line = "sample: [";
    for (var ii=0; ii<500; ii+=100) {
        line += buffer[ii] + " ";
    }
    line += "]";
    console.log(line);
    }

    for ( var i = 0; i < bufferSize; i++ ) {
        real[i] = buffer[reverseTable[i]];
        imag[i] = 0;
    }

    var halfSize = 1,
    phaseShiftStepReal, 
    phaseShiftStepImag,
    currentPhaseShiftReal,
    currentPhaseShiftImag,
    off,
    tr,
    ti,
    tmpReal,    
    i;

    while ( halfSize < bufferSize ) {
        phaseShiftStepReal = cosTable[halfSize];
        phaseShiftStepImag = sinTable[halfSize];
        currentPhaseShiftReal = 1.0;
        currentPhaseShiftImag = 0.0;

        for ( var fftStep = 0; fftStep < halfSize; fftStep++ ) {
            i = fftStep;

            while ( i < bufferSize ) {
        off = i + halfSize;
        tr = (currentPhaseShiftReal * real[off]) - (currentPhaseShiftImag * imag[off]);
        ti = (currentPhaseShiftReal * imag[off]) + (currentPhaseShiftImag * real[off]);

        real[off] = real[i] - tr;
        imag[off] = imag[i] - ti;
        real[i] += tr;
        imag[i] += ti;

        i += halfSize << 1;
            }

            tmpReal = currentPhaseShiftReal;
            currentPhaseShiftReal = (tmpReal * phaseShiftStepReal) - (currentPhaseShiftImag * phaseShiftStepImag);
            currentPhaseShiftImag = (tmpReal * phaseShiftStepImag) + (currentPhaseShiftImag * phaseShiftStepReal);
        }

        halfSize = halfSize << 1;
    }

    i = bufferSize/2;
    while(i--) {
        spectrum[i] = 2 * Math.sqrt(real[i] * real[i] + imag[i] * imag[i]) / bufferSize;
        powerSpectrum[i] = 2 * (real[i] * real[i] + imag[i] * imag[i]) / bufferSize; // done to match fftExtract
    }

    
    if (_dspdebug) {
    for (i=0; i<bufferSize/2; i+=80) {
        console.log("{re: " + real[i] + ", im:" + imag[i] + "}");
    }
    console.log("-----");
    }

};