Paul - 4 months ago 19

Python Question

I am struggling with

`numpy`

`import numpy as np`

from matplotlib import pyplot as plt

signal = array([[ 0.], [ 0.1667557 ], [ 0.31103874], [ 0.44339886], [ 0.50747922],

[ 0.47848347], [ 0.64544846], [ 0.67861755], [ 0.69268326], [ 0.71581176],

[ 0.726552 ], [ 0.75032795], [ 0.77133769], [ 0.77379966], [ 0.80519187],

[ 0.78756476], [ 0.84179849], [ 0.85406538], [ 0.82852684], [ 0.87172407],

[ 0.9055542 ], [ 0.90563205], [ 0.92073452], [ 0.91178145], [ 0.8795554 ],

[ 0.89155587], [ 0.87965686], [ 0.91819571], [ 0.95774404], [ 0.95432073],

[ 0.96326252], [ 0.99480947], [ 0.94754962], [ 0.9818627 ], [ 0.9804966 ],

[ 1.], [ 0.99919711], [ 0.97202208], [ 0.99065786], [ 0.90567128],

[ 0.94300558], [ 0.89839004], [ 0.87312245], [ 0.86288378], [ 0.87301008],

[ 0.78184963], [ 0.73774451], [ 0.7450479 ], [ 0.67291666], [ 0.63518575],

[ 0.57036157], [ 0.5709147 ], [ 0.63079811], [ 0.61821523], [ 0.49526048],

[ 0.4434457 ], [ 0.29746173], [ 0.13024641], [ 0.17631683], [ 0.08590552]])

sinus = np.sin(np.linspace(0, np.pi, 60))

plt.plot(signal)

plt.plot(sinus)

The blue line is my signal, the green line is the sinus.

`transformed_signal = abs(np.fft.fft(signal)[:30] / len(signal))`

transformed_sinus = abs(np.fft.fft(sinus)[:30] / len(sinus))

plt.plot(transformed_signal)

plt.plot(transformed_sinus)

The blue line is

`transformed_signal`

`transformed_sinus`

Plotting only

`transformed_signal`

Can someone explain to me what's going on here?

I was indeed a problem of calling the FFT. This is the correct call and the correct result:

`transformed_signal = abs(np.fft.fft(signal,axis=0)[:30] / len(signal))`

Answer

Numpy's `fft`

is by default applied over rows. Since your `signal`

variable is a column vector, `fft`

is applied over the rows consisting of one element and returns the one-point FFT of each element.

Use the axis option of `fft`

to specify that you want FFT applied over the columns of `signal`

, i.e.,

```
transformed_signal = abs(np.fft.fft(signal,axis=0)[:30] / len(signal))
```