Matlab Matrix Multiplication Query in an fft application

[1alph1]

Hey everyone, first of all id just like to clarify this isn't a homework or coursework related problem, its part of my personal MatLab learning for future years. I obtained a nice little siganls and systems question off a friend, it looked fairly simple but I've come across a problem which i can't find a solution to. Here goes...

Investigate the frequency content X(f) of a single 10 Hz sinewave, lasting for a time T_end. Set the sample frequency to be fs=1000, and T_end to be 2. Plot the absolute value of the function X(f) as a against frequency f.

Here is my code for this part.

function [x, t] = sinewave(T_end)
fs = 1000;
ts = 1/fs;
T = T_end;
t = 0:ts:T_end-ts;
f= -fs/2 : 1/T : fs/2-1/T; % sets the frequecncy axis, between -500 and (500-0.5).
x = sin(2.*pi.*f.*t);
X = fft(x);
Y = fftshift(X); % shifts right hand end to its alias below f=0, as graph is periodic this just centers the plot about 0.

stem(f,abs(Y),'k')
xlabel('frquency Hz');
ylabel('Abs(X)');
title('frequency content at T end = 2');

I believe this to be correct, and can explain the results obtained at different T_end values.

The next part of the question is where I get stuck.

Replace the line that defines x(t) with the code,
t1= 0:ts:1.0-ts;
x=0*t;
x(1:length(t1))=sin(2*pi*f*t1);
figure(1)
plot(t,x)
figure(2)

in order to create a pulse of length 1.

When this code is entered i get the standard, Error using ==> times
Matrix dimensions must agree.

Error in ==> sinwav at 9
x(1:length(t1))=sin(2*pi*f*t1);

I know this error occurs due to matrix multiplication, but simply .*'ing does not solve it,
the matrix sizes are apparaently as follows, x[1 x 2000] , t1[1 x 1000], t[1x2000],f[1 x 2000]. And I am not actually sure if i should add the .'s and this would change the outcome.

Any comments or information would be greatly appericated, and hopefully this will help others as well.

Many Thanks

 

[NateD]

.You can solve this problem by reshaping the matrices so that they have the same dimensions. For example, you could use the reshape() function to make sure the matrices are both 1x1000. You could also use the transpose operator (') to transpose a matrix, if necessary. Once the matrices have the same dimensions, you can use the .* operator to multiply them together.

 

[oswaler]

Hi there,

It looks like you are trying to create a pulse of length 1 using the code provided. The reason you are getting the "matrix dimensions must agree" error is because you are trying to assign a vector of length 1000 (t1) to a section of a vector of length 2000 (x). This is a mismatch in dimensions and cannot be done.

To solve this, you can either change the length of t1 to match the length of x, or you can use a different method to create the pulse. One option is to use the function "rectpuls" which creates a rectangular pulse of specified length. You can then use this pulse as the input to your frequency content analysis.

Another option is to use the "conv" function to convolve the sinewave with a rectangular pulse of length 1. This will essentially create the same result as the code provided, but without the mismatch in dimensions.

I hope this helps and good luck with your learning in Matlab!