Another direct approach of LTIL system analysis is to perform in time domain. In continuoustime
cases, the responses can be computed in two ways: direct method of solving the
differential equation and state-variable method. The former is known to be a classical method
which is normally analyzed on paper while the latter can be solved numerically using a digital
computer.
In discrete-time cases, if the system can be described by a difference equation, the response
may be solved directly by recursion. When the system is initially relaxed or only the zero-state
response is significant, the response can be computed by the discrete convolution, which is
given as
[ ] [ ] [ ] [ ] [ ]
i
y k h k u k h k i u i
, (1)
where h[k] is the impulse response sequence of the system. Generally, h[k] can be
determined from the difference equation if available, or directly derived from the real system.