As an example, consider the case of an analog-to-digital (AD) converter. The input to the AD converter is an analog signal while the output is a digital signal obtained by taking the samples of the analog signal at periodic intervals of time and approximating the sampled values by a discrete set of values.
Figure 3 Analog-to-digital (AD) converter |
is defined for a continuum of values of domain parameter 
and it can take a continuous range of values. 
is defined at discrete points and also takes a discrete set of values. 