Problems with Octave impz function, in the moment I plot results

Question

I'm very new to the Matlab and Octave "World". I've been suffering for some hours to make a simple script run. Problem is, my teacher wrote it on Matlab, and I can't find a way to make it work on Octave. The script finds the impulse answer for the system and plots the curve. Here it is:

b = [1]; a = [1, -1, 0.9]; n = [0:100];
h = impz(b,a,n);
stem(n,h);
subplot(1,1,1);
title('Impulse Response'); xlabel('n'); ylabel('h(n)');

The error is:

error: stem: inconsistent sizes for X and Y error: called from stem >check_stem_arg at line 276 column 11 stem at line 37 column 40 stem at line 127 column 8 questao6_lab2 at line 4 column 1

I understand it is because of the axis difference. Though, I don't understand why octave gives only a single value for h. The function Impz should describe a curve all along thee values of n, but i doesn't.

Thanks

Answer 1

The issue comes from impz returning a scalar in this case

octave-gui:26> h = impz (1, [1, -1, 0.9], 0:100)
h =  1

while you are expecting a vector with 100 elements (which is what Matlab does). In Matlab, the third argument ( N ) can be a scalar stating the number of samples of the impulse response, or a vector specifying the values where to compute the impulse response. In Octave, you can only specify a number of samples. So do this instead:

h = impz (1, [1, -1, 0.9], 101);

In addition, it seems that Octave returns a row vector instead of a column vector so do this:

h = impz (1, [1, -1, 0.9], 101)(:);

The reason why I am using (:) instead of .' (transpose) is so that it continues working in the next version of the signal package where this will be fixed . Alternatively, run which impz to find where is the source for Octave's impz function, and fix it there for yourself now.