Funkin/source/audiovis/dsp/FFT.hx

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package audiovis.dsp;
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import audiovis.dsp.Complex;
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using audiovis.dsp.OffsetArray;
using audiovis.dsp.Signal;
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// these are only used for testing, down in FFT.main()
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/**
Fast/Finite Fourier Transforms.
**/
class FFT
{
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/**
Computes the Discrete Fourier Transform (DFT) of a `Complex` sequence.
If the input has N data points (N should be a power of 2 or padding will be added)
from a signal sampled at intervals of 1/Fs, the result will be a sequence of N
samples from the Discrete-Time Fourier Transform (DTFT) - which is Fs-periodic -
with a spacing of Fs/N Hz between them and a scaling factor of Fs.
**/
public static function fft(input:Array<Complex>):Array<Complex>
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return do_fft(input, false);
/**
Like `fft`, but for a real (Float) sequence input.
Since the input time signal is real, its frequency representation is
Hermitian-symmetric so we only return the positive frequencies.
**/
public static function rfft(input:Array<Float>):Array<Complex>
{
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final s = fft(input.map(Complex.fromReal));
return s.slice(0, Std.int(s.length / 2) + 1);
}
/**
Computes the Inverse DFT of a periodic input sequence.
If the input contains N (a power of 2) DTFT samples, each spaced Fs/N Hz
from each other, the result will consist of N data points as sampled
from a time signal at intervals of 1/Fs with a scaling factor of 1/Fs.
**/
public static function ifft(input:Array<Complex>):Array<Complex>
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return do_fft(input, true);
// Handles padding and scaling for forwards and inverse FFTs.
private static function do_fft(input:Array<Complex>, inverse:Bool):Array<Complex>
{
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final n = nextPow2(input.length);
var ts = [for (i in 0...n) if (i < input.length) input[i] else Complex.zero];
var fs = [for (_ in 0...n) Complex.zero];
ditfft2(ts, 0, fs, 0, n, 1, inverse);
return inverse ? fs.map(z -> z.scale(1 / n)) : fs;
return fs;
}
// Radix-2 Decimation-In-Time variant of CooleyTukey's FFT, recursive.
private static function ditfft2(time:Array<Complex>, t:Int, freq:Array<Complex>, f:Int, n:Int, step:Int, inverse:Bool):Void
{
if (n == 1)
{
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freq[f] = time[t].copy();
}
else
{
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final halfLen = Std.int(n / 2);
ditfft2(time, t, freq, f, halfLen, step * 2, inverse);
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ditfft2(time, t + step, freq, f + halfLen, halfLen, step * 2, inverse);
for (k in 0...halfLen)
{
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final twiddle = Complex.exp((inverse ? 1 : -1) * 2 * Math.PI * k / n);
final even = freq[f + k].copy();
final odd = freq[f + k + halfLen].copy();
freq[f + k] = even + twiddle * odd;
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freq[f + k + halfLen] = even - twiddle * odd;
}
}
}
// Naive O(n^2) DFT, used for testing purposes.
private static function dft(ts:Array<Complex>, ?inverse:Bool):Array<Complex>
{
if (inverse == null)
inverse = false;
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final n = ts.length;
var fs = new Array<Complex>();
fs.resize(n);
for (f in 0...n)
{
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var sum = Complex.zero;
for (t in 0...n)
{
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sum += ts[t] * Complex.exp((inverse ? 1 : -1) * 2 * Math.PI * f * t / n);
}
fs[f] = inverse ? sum.scale(1 / n) : sum;
}
return fs;
}
/**
Finds the power of 2 that is equal to or greater than the given natural.
**/
static function nextPow2(x:Int):Int
{
if (x < 2)
return 1;
else if ((x & (x - 1)) == 0)
return x;
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var pow = 2;
x--;
while ((x >>= 1) != 0)
pow <<= 1;
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return pow;
}
// testing, but also acts like an example
static function main()
{
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// sampling and buffer parameters
final Fs = 44100.0;
final N = 512;
final halfN = Std.int(N / 2);
// build a time signal as a sum of sinusoids
final freqs = [5919.911];
final ts = [for (n in 0...N) freqs.map(f -> Math.sin(2 * Math.PI * f * n / Fs)).sum()];
// get positive spectrum and use its symmetry to reconstruct negative domain
final fs_pos = rfft(ts);
final fs_fft = new OffsetArray([for (k in -(halfN - 1)...0) fs_pos[-k].conj()].concat(fs_pos), -(halfN - 1));
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// double-check with naive DFT
final fs_dft = new OffsetArray(dft(ts.map(Complex.fromReal)).circShift(halfN - 1), -(halfN - 1));
final fs_err = [for (k in -(halfN - 1)...halfN) fs_fft[k] - fs_dft[k]];
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final max_fs_err = fs_err.map(z -> z.magnitude).max();
if (max_fs_err > 1e-6)
haxe.Log.trace('FT Error: ${max_fs_err}', null);
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// else for (k => s in fs_fft) haxe.Log.trace('${k * Fs / N};${s.scale(1 / Fs).magnitude}', null);
// find spectral peaks to detect signal frequencies
final freqis = fs_fft.array.map(z -> z.magnitude)
.findPeaks()
.map(k -> (k - (halfN - 1)) * Fs / N)
.filter(f -> f >= 0);
if (freqis.length != freqs.length)
{
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trace('Found frequencies: ${freqis}');
}
else
{
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final freqs_err = [for (i in 0...freqs.length) freqis[i] - freqs[i]];
final max_freqs_err = freqs_err.map(Math.abs).max();
if (max_freqs_err > Fs / N)
trace('Frequency Errors: ${freqs_err}');
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}
// recover time signal from the frequency domain
final ts_ifft = ifft(fs_fft.array.circShift(-(halfN - 1)).map(z -> z.scale(1 / Fs)));
final ts_err = [for (n in 0...N) ts_ifft[n].scale(Fs).real - ts[n]];
final max_ts_err = ts_err.map(Math.abs).max();
if (max_ts_err > 1e-6)
haxe.Log.trace('IFT Error: ${max_ts_err}', null);
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// else for (n in 0...ts_ifft.length) haxe.Log.trace('${n / Fs};${ts_ifft[n].scale(Fs).real}', null);
}
}