CODEWITHSUNDEEP
pythonlambdasignal-processingfft
Pandora
Pandora

Reputation: 213

Computing the energy of a signal over 30 second periods in Python

I'm new both to Python and to signal processing, so pardon possible misuse of jargon.

I have the discrete values of a signal in a Pandas dataframe x, spaced 1 second apart. It looks like this:

2017-08-02 16:42:00  0.363657
2017-08-02 16:42:01  0.282907
2017-08-02 16:42:02  0.155929
...
2017-08-02 16:43:14  0.178522
2017-08-02 16:43:15  0.488507
2017-08-02 16:43:16  0.194987
...

I would like to compute the energy of the signal over 30 second periods (by energy I mean the sum of squares of the discrete Fourier coefficients, normalized by the number of summands). I would like the output to be of the form

2017-08-02 16:42:00  x_1
2017-08-02 16:42:30  x_2
2017-08-02 16:43:00  x_3
...

where each x_i is a scalar corresponding to the energy for that 30s period. I'm also fine with getting a numpy array, as eventually I only need the energy values and not the time stamps.

I tried doing this:

energy = x.resample('30S').apply(lambda x: (numpy.absolute(numpy.fft.fft(x))**2)/30)

however, I get something of the form

2017-08-02 16:42:00    [[0.422450491863], [0.482244793857], [0.514463...
2017-08-02 16:42:30    [[0.345172558059], [0.554558388074], [0.461898...
2017-08-02 16:43:00    [[0.689816890284], [0.613620822242], [0.389962...

What did I do wrong, and how would be best to correct it?

Thanks!

EDIT: I changed the time a bit as I needed to copy only a small portion of the output, so don't mind the values themselves.

**EDIT 2: The code appears exactly as written here, namely:

x = 2017-08-02 16:42:00  0.363657
    2017-08-02 16:42:01  0.282907
    2017-08-02 16:42:02  0.155929
    ...
    2017-08-02 16:43:14  0.178522
    2017-08-02 16:43:15  0.488507
    2017-08-02 16:43:16  0.194987
    ...
#This is imported as a dataframe, and indeed shows its type as pandas.core.frame.DataFrame.

energy = x.resample('30S').apply(lambda x: (numpy.absolute(numpy.fft.fft(x))**2)/30)

Out: energy = 2017-08-02 16:42:00    [[0.422450491863], [0.482244793857], [0.514463...
2017-08-02 16:42:30    [[0.345172558059], [0.554558388074], [0.461898...
2017-08-02 16:43:00    [[0.689816890284], [0.613620822242], [0.389962...
...
#type(energy) = object

Upvotes: 3

Views: 14723

Answers (1)

fstop_22
fstop_22

Reputation: 1032

import scipy as sp

# Create input of real sine wave
fs = 1.0
fc = 0.25
n = sp.arange(0, 300)
x = sp.cos(2*sp.pi*n*fc/fs)

# Rearrange x into 10 30 second windows
x = sp.reshape(x, (-1, 30))

# Calculate power over each window [J/s]
p = sp.sum(x*x, 1)/x.size

# Calculate energy [J = J/s * 30 second]
e = p*x.size

Upvotes: 5

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