# Computing the impulse response of a system with complex poles (Example 1)

This example shows three different ways to compute the impulse response.

## Define system coefficients

Difference equation: y(n) = x(n) - 2.5 x(n-1) + y(n-1) - 0.7 y(n-2)

```b = [1 -2.5];
a = [1 -1 0.7];
```

## Compute impulse response directly from different equation

Use the MATLAB function 'filter' to compute the impulse response

```u = @(n) n >= 0;     % step signal
del = @(n) n == 0;   % impulse signal

n = -10:30;
x = del(n);             % impulse
h1 = filter(b, a, x);   % impulse response

figure(1)
clf
stem(n, h1, 'filled');
title('Impulse response computed using difference equation')
```

## Compute impulse response using partial fraction expansion

Use MATLAB function 'residue' to find the poles and residues.

```[r, p, k] = residue(b,a)

% r =
%    0.5000 + 1.4907i
%    0.5000 - 1.4907i
%
% p =
%    0.5000 + 0.6708i
%    0.5000 - 0.6708i
%
% k =
%      []

h2 = r(1)*p(1).^n .* u(n) + r(2)*p(2).^n .* u(n);

figure(1)
clf
stem(n, h2, 'filled')
title('Impulse response computed using partial fraction expansion')
```
```r =

0.5000 + 1.4907i
0.5000 - 1.4907i

p =

0.5000 + 0.6708i
0.5000 - 0.6708i

k =

[]

```

Verify that h2 is the same as h1

```max(abs(h1 - h2))
% This is zero to machine precision
```
```ans =

8.8818e-16

```

## Compute impulse response using cosine formula

```R = abs(r(1));
theta = angle(r(1));

a = abs(p(1));
om = angle(p(1));

h3 = 2 * R * a.^n .* cos(om * n + theta) .* u(n);

figure(1)
clf
stem(n, h3, 'filled')
title('Impulse response computing using cosine formula')
```

Verify that h3 is the same as h1

```max(abs(h1 - h3))
% This is zero to machine precision
```
```ans =

9.8532e-16

```

## Plot all

```figure(2)
clf
subplot(3, 1, 1)
stem(n, h1, 'filled')
title('Impulse response computed using difference equation')

subplot(3, 1, 2)
stem(n, h2, 'filled')
title('Impulse response computed using partial fraction expansion')

subplot(3, 1, 3)
stem(n, h3, 'filled')
title('Impulse response computing using cosine formula')

orient tall
print -dpdf ComplexPoles_1
```