### Fast Fourier Transform in Actionscript

As part of the beat detection required for The Rhythm Game With No Name, I had to put together an FFT function that could be used on any portion of the song. Flash does have a function to do this but AFAIK it can only work on the portion of the song that is playing. Anyways here's a function that can be used on any array of floats

And here is how you use it. First we load extract the song into a ByteArray. Then we read in 1024 samples from the ByteArray (using readFloat) and call the FFT function on those arrays. To perform the FFT on a different portion of the song, simply set the read position of the ByteArray accordingly.

private function FFT(dir:Number, m:Number, x:Array, y:Array):void

{

//trace('Getting FFT');

var n:Number; var i:Number; var i1:Number;

var j:Number; var k:Number; var i2:Number;

var l:Number; var l1:Number; var l2:Number;

var c1:Number; var c2:Number; var tx:Number; var ty:Number; var t1:Number;

var t2:Number; var u1:Number; var u2:Number; var z:Number;

n = 1;

for (i=0;i<m;i++)

n *= 2;

// n = m;

/* Do the bit reversal */

//trace("Bit Reversal");

i2 = n >> 1;

j = 0;

for (i=0;i<n-1;i++) {

if (i < j) {

tx = x[i];

ty = y[i];

x[i] = x[j];

y[i] = y[j];

x[j] = tx;

y[j] = ty;

}

k = i2;

while (k <= j) {

j -= k;

k >>= 1;

}

j += k;

}

/* Compute the FFT */

c1 = -1.0;

c2 = 0.0;

l2 = 1;

for (l = 0; l < m; l++) {

// trace('FFTing at ' + l + ' of ' + m);

l1 = l2;

l2 <<= 1;

u1 = 1.0;

u2 = 0.0;

for (j=0;j<l1;j++) {

for (i=j;i<n;i+=l2) {

i1 = i + l1;

t1 = u1 * x[i1] - u2 * y[i1];

t2 = u1 * y[i1] + u2 * x[i1];

x[i1] = x[i] - t1;

y[i1] = y[i] - t2;

x[i] += t1;

y[i] += t2;

}

z = u1 * c1 - u2 * c2;

u2 = u1 * c2 + u2 * c1;

u1 = z;

}

c2 = Math.sqrt((1.0 - c1) * 0.5);

if (dir == 1)

c2 = -c2;

c1 = Math.sqrt((1.0 + c1) * 0.5);

}

/* Scaling for forward transform */

if (dir == 1) {

for (i=0;i<n;i++) {

x[i] /= n;

y[i] /= n;

}

}

}

And here is how you use it. First we load extract the song into a ByteArray. Then we read in 1024 samples from the ByteArray (using readFloat) and call the FFT function on those arrays. To perform the FFT on a different portion of the song, simply set the read position of the ByteArray accordingly.

s = new Sound(soundToLoad);

var extractLength:Number = Math.floor ((s.length * 88.2));

s.extract(bytes, extractLength);

fftx = new Array(1024);

ffty = new Array(1024);

for (j = 0; j < 1024; j++)

{

if (bytes.bytesAvailable)

{

fftx[j] = bytes.readFloat();

ffty[j] = bytes.readFloat();

}

}

FFT(1,10,fftx,ffty)

Labels: Actionscript, beat detection, Fast Fourier Transform, FFT, rhythm game