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Training neural network to implement discrete Fourier transform (DFT/FFT)

My second neural network experiment (first was FIR filter). DFT output is just a linear combination of inputs, so it should be implementable by a single layer with no activation function.

Topology of a 4-point complex DFT

Animation of weights being trained:

Neural network weights heatmap

Red are positive, blue are negative. The black squares (2336 out of 4096) are unused, and could be pruned out to save computation time (if I knew how to do that).

Even with pruning, it would be less efficient than an FFT, so if the FFT output is useful, probably best to do it externally and provide it as separate inputs?

This at least demonstrates that neural networks can figure out frequency content on their own, though, if it's useful to the problem.

The loss goes down for a while but then goes up. I don't know why:

loss vs epoch

Train a neural network to implement the discrete Fourier transform
from keras.models import Sequential
from keras.layers import Dense
import numpy as np
import matplotlib.pyplot as plt
N = 32
batch = 10000
# Generate random input data and desired output data
sig = np.random.randn(batch, N) + 1j*np.random.randn(batch, N)
F = np.fft.fft(sig, axis=-1)
# First half of inputs/outputs is real part, second half is imaginary part
X = np.hstack([sig.real, sig.imag])
Y = np.hstack([F.real, F.imag])
# Create model with no hidden layers, same number of outputs as inputs.
# No bias needed. No activation function, since DFT is linear.
model = Sequential([Dense(N*2, input_dim=N*2, use_bias=False)])
model.compile(loss='mean_squared_error', optimizer='adam'), Y, epochs=100, batch_size=100)
# Confirm that it works
data = np.arange(N)
def ANN_DFT(x):
if len(x) != N:
raise ValueError(f'Input must be length {N}')
pred = model.predict(np.hstack([x.real, x.imag])[np.newaxis])[0]
result = pred[:N] + 1j*pred[N:]
return result
ANN = ANN_DFT(data)
FFT = np.fft.fft(data)
print(f'ANN matches FFT: {np.allclose(ANN, FFT)}')
# Heat map of neuron weights
plt.imshow(model.get_weights()[0], vmin=-1, vmax=1, cmap='coolwarm')
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masip85 commented Sep 9, 2020

I guess this DNN execution is not faster than original fft? Isn't it?

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endolith commented Sep 9, 2020

@masip85 No, not at all. It's inaccurate and highly inefficient, and you need to train it on white data if you want it to be correct at all points of the spectrum. It was just an experiment. I'm a signal processing engineer and was trying to learn the basics of ANNs.

If you think that your neural network would benefit from FFT features, then I would suggest calculating the FFT and feeding it to the network separately.

I also tested if it could learn the absolute value of the FFT output and it did not do very well with ReLUs, they were not good at approximating the absolute value function.

Again, I would feed that information directly to the network if I thought it would benefit from it.

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rajb245 commented Sep 21, 2020

Very nice experiment. For people interested in ML for "fast transforms" of this type, let me leave a link to this work here:

In the arXiv paper, they show that you can indeed learn fast-transforms that get to the same order of efficiency as a hand-tuned fast implementation. And the results go beyond that, implying that all matrices admit a representation as a composition of only sparse butterfly matrices and permutations.

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endolith commented Sep 21, 2020

@rajb245 So are they replacing the dense connection layer with a butterfly connection layer?

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rajb245 commented Sep 21, 2020

@endolith That's one idea they explored in the paper, yes. They showed you can learn some underlying sparse structure of otherwise "fully connected" layers using their method. Using it like that is a new kind of matrix/model compression, and it contrasts a lot with the other approach that's popular in DL, pruning.

They also use the ideas of learning the weights of matrices with butterfly-sparsity-patterns and learning a permutation out a discrete set of choices to show that Adam does indeed find solutions that are the usual "fast" linear transforms from signal processing (FFT, fast DCT, fast DST, etc.).

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msaad1311 commented Nov 18, 2020

Hello. I tried to use your code and tried to find the difference between in computing the FFT using numpy and neural network and there was a big difference. Can you provide some insights on how to solve that? I saw multiple papers that are being published in which the authors mention that using the neural network as FFT reduces the time complexity greatly.

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endolith commented Nov 19, 2020


Yes, it's highly inefficient, as I said in the description and the comments. Even more so than a direct DFT because of all the zero weights being calculated unnecessarily. This isn't something that you should be doing. It was just an experiment while teaching myself neural nets.  If your neural net would benefit from frequency domain information, it's better to just do a numpy FFT and pass the output to the net (possibly adding a magnitude function afterward, since that nonlinearity was much harder to learn in my tests).

If you need the net to learn FFT-like transforms in general, look at the comment above about butterfly networks

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sebraun-msr commented Jul 8, 2021

Can you shed some light on the motivation on this? Why do you want to approximate an existing algorithm with a (likely less accurate and less efficient) black box approach?

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endolith commented Jul 8, 2021

@sebraun-msr It was just an experiment. I thought that ANN nodes could be equivalent to signal processing nodes and tried implementing some things that way. I've said multiple times that you shouldn't actually do this. :)

For example, I learned that:

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endolith commented Mar 31, 2022

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