# Simple question about plotting a convolution

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aurc89 el 25 de Mzo. de 2015
Editada: Bruno Luong el 26 de Sept. de 2019
I have two sets of data (x,y1) and (x,y2).I need to calculate the convolution between y1 and y2 and plot it vs x. Is it correct to do simply like this?
convol=conv(y1,y2);
plot(x,convol)
I've never used the command conv and I'm not sure about it. Thanks!
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Hamza OUDICH el 25 de Sept. de 2019
Be careful, the amplitude given by conv is not correct, it should be multiplied by the step difference in x since conv is an summation approximation of the integral in Matlab. Besides, 'same' removes the first elements to get a list with the same length as x..... so you lose SO MUCH information which can give you complete false result.
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Bruno Luong el 25 de Sept. de 2019
Editada: Bruno Luong el 26 de Sept. de 2019
+1, if the data does not have constant step, using CONV gives incorrect result.
You probably need to interpolate the y data with a constant step, THEN apply conv.
Can't believe such answer was accepted.
Hamza OUDICH el 25 de Sept. de 2019
true! i forgot that one as well thanks!

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### Más respuestas (1)

Andrew Newell el 25 de Mzo. de 2015
Editada: Andrew Newell el 25 de Mzo. de 2015
Your code will give you a vector of length length(y1)+length(y2)-1, so you'll get an error if you plot it against x (which is presumably the same length as y1 and y2). Instead, use
convol = conv(y1,y2,'same')
which gives you the central part of the convolution of the same size as y1. Then you'll be able to plot it.
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aurc89 el 25 de Mzo. de 2015
Thanks a lot!

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