Can you help me with this FFT filter freq response plot?

Development
1

Hi good boys,

I’m discovering FFT and how to plot freq response (following this my old post and suggestion by @kerfuffle).

So I tried using basic tools of JUCE, which works pretty straightforward (searching also code online):

// Create impulse
zeromem(mImpulse, sizeof(mImpulse));
mImpulse[0] = 1;
zeromem(mFilteredImpulse, sizeof(mFilteredImpulse));

// Filter impulse
for (auto i = 0; i < fftSize; ++i)
{
	mFilteredImpulse[i] = filterIIR.processSample(mImpulse[i]);
}

// Perform fft
mFFT.performFrequencyOnlyForwardTransform(mFilteredImpulse);

Now, I’d like to use different library (in this case, pffft).
So I tried to write the same code:

    // Create impulse
zeromem(mImpulse, sizeof(mImpulse));
mImpulse[0] = 1;
zeromem(mFilteredImpulseTemp, sizeof(mFilteredImpulseTemp));

// Initialize the PFFFT library
PFFFT_Setup *fft_setup = pffft_new_setup(fftSize * 2, PFFFT_REAL);

// Perform the FFT of the impulse
pffft_transform_ordered(fft_setup, mImpulse, mFilteredImpulseTemp, nullptr, PFFFT_FORWARD);

// Filter impulse
for (auto i = 0; i < fftSize; ++i)
{
	mFilteredImpulseTemp[i] = filterIIR.processSample(mImpulse[i]);
}

// Perform the inverse FFT to get the filter's response
pffft_transform_ordered(fft_setup, mFilteredImpulseTemp, mFilteredImpulse, nullptr, PFFFT_BACKWARD);

// Normalize the filter response
for (int i = 0; i < fftSize * 2; ++i)
{
	mFilteredImpulse[i] /= fftSize * 2;
}

But, with the same settings:

enum
{
	fftOrder = 14,
	fftSize = 1 << fftOrder
};
float mImpulse[fftSize] = {0.0f};
float mFilteredImpulse[fftSize * 2] = {0.0f};
float mFilteredImpulseTemp[fftSize * 2] = {0.0f};

can’t match the values, and I don’t understand where am I wrong.

With your experience, can you help me to understand where the mismatch come from? Probably a factor scaling when I get back the values?

I could also use fftw if you already have experience with it, or some others libraries (the purpose is to use a common library, which can be used in different platforms in future, if needed).

Thanks to anyone can give to me the right direction, and some tips :slight_smile: