what is fft code?

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nur yusof
nur yusof on 19 Jun 2015
Hi all,
I'm new to FFT and I want to ask about calculating FFT in Matlab. The following is my coding guidance from Matlab help:-
clc;
Fs = 512; % Sampling frequency
T = 1/Fs; % Sample time
L = 8100; % Length of signal
t = (0:L-1)*T; % Time vector
% Sum of a 50 Hz sinusoid and a 120 Hz sinusoid
x = 0.7*sin(2*pi*50*t) + sin(2*pi*120*t);
y = x + 2*randn(size(t)); % Sinusoids plus noise
plot(Fs*t(1:50),y(1:50));
title('Signal Corrupted with Zero-Mean Random Noise')
xlabel('time (milliseconds)');
NFFT = 2^nextpow2(L); % Next power of 2 from length of y
Y = fft(y,NFFT)/L;
f = Fs/2*linspace(0,1,NFFT/2);
% Plot single-sided amplitude spectrum.
plot(f,2*abs(Y(1:NFFT/2)));
title('Single-Sided Amplitude Spectrum of y(t)');
xlabel('Frequency (Hz)');
ylabel('|Y(f)|');
Can someone verify my code because the fft graph unconvincing. Here the figure of the graph
Thank you.
  5 Comments
Afshin Loghmani M Toussi
Afshin Loghmani M Toussi on 1 Dec 2020
Because you are plotting half of the frequency range( because of the symmetry). However, to show the same amount of energy you are multiplying the values by 2.

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Answers (1)

Andreas Goser
Andreas Goser on 19 Jun 2015
Frequent puzzlese with interpreting FFT:
2 peaks instead of 1 peak
Frequency "off" by a factor of 6.
Both effects are in the nature of the algorithm and "6" is actually "2*Pi". Next steps depend on whether you want to just use the results, then this will help. If you want to understand, read a text book about signal processing.
  1 Comment
nur yusof
nur yusof on 29 Jun 2015
thank you so much

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