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Matlab interpolation problem

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richaphy

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I have a signal whose length is 7500 samples.
I process this signal and which then downsamples and now the length is 1937.
Now I wanted to reconstruct this signal back to original, for this I used interpolation function in matlab interp1.
For comparing the reconstructed signal (for error) I want to subtract the reconstructed signal from original. But this new reconstructed signal has 7502 samples. 2 samples extra matrices dont match, so cannot subtract.
I want to understand why it is happening, and how to get it correct, which means same no of signal as the original.
Thanks
Richa
 

From Mathworks:
vq = interp1(v,xq,method,extrapolation)
Click to expand...

I'm not sure, but I believe parameter 'xq' defines the points at which the evaluation takes place, so you would define xq to be a vector of 7500 points.
 

I have defined xq to be of same time period and length as the initial signal.
Therefore I expect the reconstructed signal to have exactly same no of points as the original but here I am getting 2 more points.
 

It might have something to do with the fact that your downsampled signal has an irrational relationship to the original. How are you downsampling?
 

in = input(0:t);
Ts = sampling time of input
processed_in = function(in); % here I have done some processing which reduces the number of samples
length of procesed_in is 'tn'
xx = 0:Ts:tn(end); %xx = start:interval:end;
yip = interp1(tn,decimated,xx); % Reconstruct using function interp1
after this 'in' and 'yip' should be of same length, but yip has 2 samples more.
or we can say length of vector 'in' and 'xx' must be same
 

That doesn't really tell us anything, since "function" is a black box. As I said, the problem is possibly due to the irrational relationship. I don't know enough about the inner workings of the interp1 function to explain why it's giving you more samples than you ask for.
 

Sorry I cannot explain what is function.
However, here I have seen something extraordinary.
In this function along with many things I am also filtering the signal.
With one input if I filter the input then in the end I get equal XX and qip, if I dont filter it, qip is one sample more than xx!!!
probably you are right, the problem is something within this function.
Thanks for your time
 

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