Confusion with FFT for a specific part of the whole signal

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Ancalagon8 2017-2-4

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评论： Star Strider 2021-1-27

As a newbie i am working on a signal from an accelerometer and i am trying to produce an fft for a specific part of the whole signal. I am a little bit confused about which code for the fft is the appropriate to use for my case.

here is my code:

clear all

close all

load signal

tvec=noact(:,1);

acc=noact(:,2);

%% ALL SIGNAL

L=length(signal); % Length of signal

t_max=max(signal(:,1)); % Tmax

Fs=L/t_max; % Sampling Rate

T = 1/Fs; % Sample time

%% PART OF SIGNAL

trace1=signal(Fs*25:Fs*32,:); % select part of signal

matrice1=trace1(:,2); % accelerations column

L1=length(matrice1); % Length of trace (number of samples)

t1 = (0:L1-1)*T; % Time vector

% Frequency vector

f1=(0:Fs/length(matrice1):(length(matrice1)-1)*Fs/length(matrice1));

FrecSign=abs(fft(matrice1))*T;

figure

plot(FrecSign);

xlabel Frequency(Hz)

ylabel Amplitude

title 'FFT'

NFFT = 2^nextpow2(L); % Next power of 2 from length of matrice1

Y = fft(matrice1,NFFT)/L;

f = Fs/2*linspace(0,1,NFFT/2+1); % create frequency vector

figure % Plot single-sided amplitude spectrum.

plot(f,2*abs(Y(1:NFFT/2+1)))

title('Single-Sided Amplitude Spectrum of y(t)')

xlabel('Frequency (Hz)')

ylabel('|Y(f)|')

Thanks in advance

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采纳的回答

Star Strider 2017-2-4

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I cannot follow our code.

However, you cannot ‘produce an fft for a specific part of the whole signal’, since you have to take the fft (link) of the entire signal if you want valid data.

If you want to view only part of the signal, use the axis function to limit the frequency (here x) axis to the range you want.

If you want to eliminate specific frequencies from your accelerometer signals (for example a constant offset, low-frequency baseline variations, high-frequency noise), use a discrete filter. There are several ways to to this, with the designfilt function being the easiest to use (in my opinion).

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Brad 2021-1-27

Hi Star

I have a question regarding your answer here, I would very much appreciate it if you answer my question.

https://uk.mathworks.com/matlabcentral/answers/728853-why-is-fft-of-one-part-and-the-entire-signal-the-same

Thank you

Brad

Star Strider 2021-1-27

Done!

And thank you for accepting it!

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更多回答（2 个）

Ancalagon8 2017-2-6

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https://ww2.mathworks.cn/matlabcentral/answers/323365-confusion-with-fft-for-a-specific-part-of-the-whole-signal#answer_253572

One last question. In [PSDx,fx] = periodogram(x,[],[],Fs,'power');

x should be

i) the original signal (acc over time)?

ii) the original signal (only the acc column)

iii) the fft?

Thanks in advance!

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Star Strider 2017-2-6

I’m not sufficiently certain that I understand your data well enough to address that. I would analyse one variable at a time in each call to periodogram, so most likely (ii).

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Ancalagon8 2017-2-6