Reversible FFT?

General JUCE discussion
10

Hey guys. So my goal is to:

  • load a single cycle wave
  • run an FFT to deconstruct the wave into arrays of level and phase
  • manipulate those levels and phases in normalized space
  • then recreate a waveform

Thanks to your help, I have most of this working.

const int fftOrder = 11;
const int numBins = 1 << fftOrder;     // 2048
const int waveSize = numBins;          // just for clarity
const int fftBufferSize = waveSize * 2;

float fftBuffer[fftBufferSize];
float levels[numBins];
float phases[numBins];

auto fft = new dsp::FFT(fftOrder);
bufferClear(fftBuffer, fftBufferSize);  

loadWaveIntoBuffer();
fft->performRealOnlyForwardTransform(fftBuffer, true);

// zero out the DC and Nyquist bins
fftBuffer[0] = 0.0f;
fftBuffer[fftBufferSize - 1] = 0.0f;

// convert the complex buffer into floats and find the max value
auto* realBuffer = reinterpret_cast<std::complex<float>*> (fftBuffer);
float maxValue = -1.f;
for(int i = 1; i &lt; numBins; ++i)
{
    float mag = std::abs(realBuffer[i]);
    float phase = std::arg(realBuffer[i]);
    maxValue = jmax(mag, maxValue);
    levels[i] = mag;
    phases[i] = phase;
}

// normalize levels
for(int i = 0; i < numBins; ++i)
{
    levels[i] /= maxValue;
    // anything required to normalize phase values?
}

// edit levels and phases using normalized values......................

// reconstruct complex number from levels and phases and put back into fftBuffer
??????

// run fft to create new waveform

fft->performRealOnlyInverseTransform(fftBuffer);

So, given that, my three questions are:

  • Do I need to do anything to the phase values to normalize them after getting them from std::arg(realBuffer[i]); ?
  • How do I reconstruct complex numbers from the normalized levels and phases?
  • Did I miss anything?

Thanks again, gents.

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