System Classification - Signal Processing of Random Physiological Signals - page 9

parameter system by virtue of being an ordinary differential equation. It implies that
the physical size is of no concern, since the excitation propagates through the system
instantaneously, which is valid only if the largest physical dimension of the system is
small compared to the wavelength of the highest significant frequency considered.
A distributed parameter system is defined as a system whose dimensions are large
( NOT Small ) compared to the shortest wavelength of interest. Distributed-parameter sys-
tems are generally represented by partial differential equations. For example, waveguides,
microwave tubes, and transmission lines (telephone and power lines) are all distributed
parameter systems, because of their physical lengths; that is, Hoover Dam to Los Angeles
are much larger than the highest frequency or shortest wavelength of interest.

Note : As the frequency increases (higher frequency), the wavelength decreases.

2.6 CONTINUOUS -TIME VS. DISCRETE TIME
What is a Continuous or Discrete time system? “Continuous-Time Systems” are those
that can be represented by continuous data or differential equations, whereas “Discrete
Time Systems” are represented by sampled digital data.
It should be noted that all natural physical systems are inherently continuous-
time, since time is continuous. In addition, it should be noted that both Continuous-
and Discrete time systems can also be

1. linear or nonlinear,

2. fixed or time varying, and

3. lumped or distributed.

2.7 INSTANTANEOUS VS. DYNAMIC
What is an Instantaneous or a Dynamic System? An Instantaneous system is defined as a
system that has no memory, which means that the system is not dependent on any future
or past value of excitation. The response at some point in time ( t 1 ) of an Instantaneous
system depends only on the excitation at time ( t 1 ). For example, a fully resistive circuit