Which statement best describes a discrete-time signal?

• A. A A signal defined for all real time
• B. B A signal defined only at discrete time instants
• C. C A digital code sequence
• D. D A signal with continuous amplitude

Answer: (B)

A discrete-time signal is defined only at discrete time instants, usually at t = kT where k is an integer and T is the sampling period. In practice we think of it as a sequence x[k], with values corresponding to those specific time points, rather than a function that exists for every real time. The sampling process turns a continuous-time signal into a discrete-time one by selecting those regularly spaced samples, and between samples there isn’t a defined value unless we interpolate. This is why the statement that best describes a discrete-time signal is that it is defined only at discrete time instants. The other descriptions refer to signals that are defined for all real time or describe amplitude properties rather than the discrete-time nature.