usage: [x, t] = impz(b [, a, n, fs])

Generate impulse-response characteristics of the filter. The filter
coefficients correspond to the the z-plane rational function with
numerator b and denominator a.  If a is not specified, it defaults to
1. If n is not specified, or specified as [], it will be chosen such
that the signal has a chance to die down to -120dB, or to not explode
beyond 120dB, or to show five periods if there is no significant
damping. If no return arguments are requested, plot the results.

See also: freqz, zplane