NN is doing matrix multiplications anyway. That's what it does, basically. The property of matrix multiplications is that they are composable, i.e. `X * (Y * z

NN is doing matrix multiplications anyway. That's what it does, basically. The property of matrix multiplications is that they are composable, i.e. `X * (Y * z) = (X * Y) * z` that is, in the end you only need one matrix. So what this means in practice is that you have FFT for free. NN is doing a matrix multiplication anyway. Discrete time Fourier Transform is a matrix multiplication. Thus it can simply fold DTFT together with whatever other transform it is doing - it doesn't cost anything.