# what is fft code?

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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.
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.

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.
nur yusof on 29 Jun 2015
thank you so much