# Impulse response of a causal system from transfer function in z-domain

The transfer function is $$H(z)=(z+1)/(z^2+z+0.5)$$ I need to find the impulse response h[n] of a causal system with x[n] as unit impulse.

I have tried to find the impulse response by the following methods:

1. partial fraction decomposition and then inverse z transform of transfer function.
2. homogeneous solution of differential equation.
3. Matlab iztransf() function I get different solutions from the above methods and i don't know how to remove iota from the impulse response.What is the correct procedure to find the impulse response?

Let's follow option 1. I'm using Python but same is directly available in matlab. The partial fraction expansion can be found via residue function

>>> import scipy.signal as sig
>>> b = [1, 1]
>>> a = [1, 1, 0.5]
>>> r, p, k = sig.residue(b,a)
>>> r
array([0.5-0.5j, 0.5+0.5j])

>>> p
array([-0.5+0.5j, -0.5-0.5j])

>>> k
array([0.])


This gives us $$H(z) = \frac{0.5-0.5i}{z+0.5-0.5i} + \frac{0.5+0.5i}{z+0.5+0.5i}$$

Now either by computing manually, or using a Z-transform table, or using another computational tool you can show that these are two complex terms can be inversed transformed as

$$\mathcal{Z}^{-1}\left[\frac{a}{z+a}\right] = \mathcal{Z}^{-1}\left[\frac{az^{-1}}{1+az^{-1}}\right] = -(-a)^k \mathbf{1}(k - 1)$$ Here $\mathbf{1}(k)$ is the discrete time step function. But since we have complex numbers this looks like our response should be a complex function. However this is not possible since our transfer function has real valued coefficients. Hence even before I compute anything, I know already that imaginary parts will cancel out.

And since $|a| < 1$ this is going to be a decaying sinusoidal. Indeed if we check this with say sampling period $T=1$,

>>> import harold as har
>>> G = Transfer([1, 1], [1, 1, 0.5], dt=1)
>>> impulse_response_plot(G);


As expected, there is a one sample delay coming from $\mathbf{1}(k-1)$