MATLAB: How to perform fft in matlab using a set of data from excel sheet

fft of data from excel sheet

Fs=3800;
Ts=1/Fs;
t=0:Ts:1;
L=97;
Y=abs(fft(VarName1)); %varname1 is the filename of the excel sheet
P1=Y/L;
P2=P1(1:(L+1)/2);
P2(2:end-1)=2*P2(2:end-1);
f=Fs*(0:(L/2))/L;
plot(f,P2)
xlabel('frequency')
ylabel('magnitude of P1')

Is this correct?

Best Answer

I doubt if your code would work.

Try mine:

[d,s] = xlsread(VarName1);                              % Data in ‘d’
Fs=3800;                                                % Sampling Frequency
Ts=1/Fs;                                                % Sampling Interval
Fn = Fs/2;                                              % Nyquist Frequency
L = size(d,1);                                          % Length Of Arrayt
t = linspace(0, 1, L);                                  % Time Vector
FTd = fft(d)/L;                                         % Complex Fourier Transform Of ‘d’
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                     % Frequency Vector
Iv = 1:length(Fv);                                      % Matching Index Vector (For ‘FTd’ Addressing)
figure(1)
plot(Fv, abs(FTd(Iv))*2)
xlabel('frequency')
ylabel('magnitude of P1')

NOTE — This is UNTESTED CODE. It should work, but may require modification.

Related Solutions

MATLAB: Single sided spectrum of vector y obtained using fft

The way I usually calculate and plot it:

t = linspace(0, 1, 1E+4);
s = sum(sin([0; 100; 200; 300]*2*pi*t*10)) .* randn*10;
Ts = mean(diff(t));                                         % Sampling Interval
Fs = 1/Ts;                                                  % Sampling Frequency
Fn = Fs/2;                                                  % Nyquist Frequency
L1 = numel(t);
FTs1 = fft(s)/L1;                                           % FFT Of Original Signal
Fv1 = linspace(0, 1, fix(L1/2)+1)*Fn;                       % Frequency Vector

Iv1 = 1:numel(Fv1);                                         % Index Vector

L2 = 2^nextpow2(L1);
FTs2 = fft(s,L2)/L1;                                        % FFT Of Zero-Padded Signal
Fv2 = linspace(0, 1, fix(L2/2)+1)*Fn;                       % Frequency Vector
Iv2 = 1:numel(Fv2);                                         % Index Vector
figure
subplot(2,1,1)
plot(Fv1, abs(FTs1(Iv1))*2)
title('FFT of Original Signal')
xlabel('Frequency (Hz)')
ylabel('Amplitude')
grid
subplot(2,1,2)
plot(Fv2, abs(FTs2(Iv2))*2)
title('FFT of Zero-Padded Signal')
xlabel('Frequency (Hz)')
ylabel('Amplitude')
grid

This also demonstrates a way to increase the frequency resolution of the fft result by zero-padding it.

MATLAB: Audio fft shape is too strange.

Hi eunbae,

The code is pretty close. The fft of a real signal has positive and negative frequencies, and what you are seeing at the right end of the frequency plot is just the negative frequencies. After [1] using the fftshift function to swap halves of the array and [2] making a frequency grid with 0 in the center, you get the plot in fig. 2 which better displays positive and negative frequency contributiions.

The amplitudes for pos and neg frequency have the same absolute value, so what people do for plotting purposes is double the size of the positive amplitudes and throw away the negative amplitudes. Except the first point in the array, which is frequency 0, is not doubled.

Not having your signal file I made up some data. The example assumes tha N is even.

% make up some data
Fs = 48000;
N = 30*Fs;
ts=1/Fs; 
t=0:ts:N*ts-ts;
data = .8*sin(2*pi*200*t).*exp(-1.5*t);
% shift the data, show positive and negative frequencies
Fdat=abs(fft(data)/N);
F_shift=(0:Fs/N:Fs-Fs/N)-Fs/2;
Fdat_shift=fftshift(Fdat);
figure(2)
plot(F_shift,Fdat_shift);
xlim([-500 500])
% existing code modified
N=length(data); 
ts=1/Fs; 
t=0:ts:N*ts-ts;
F=0:Fs/N:Fs-Fs/N;
Fdat=abs(fft(data)/N);
% take positive frequencies only, multiply by 2
F = F(1:N/2);
Fdat=Fdat(1:N/2);
Fdat(2:end) = 2*Fdat(2:end);
figure(1)
subplot(1,2,1)
plot(t,data);
xlabel('time[sec]')
ylabel('Amplitude')
title('1st pipe sound')
subplot(1,2,2)
plot(F,Fdat)
xlabel('Frequency')
ylabel('Amplitude')
title('1st sound fft result')
xlim([0 500])

Best Answer

Related Solutions

MATLAB: Single sided spectrum of vector y obtained using fft

MATLAB: Audio fft shape is too strange.

Related Question