A discrete-time signal is represented as a sequence of numbers:
Here n is an integer, and x[n] is the nth sample in the sequence. Discrete-time signals are often obtained by sampling continuous-time signals. In this case the nth sample of the sequence is equal to the value of the analogue signal xa(t) at time t = nT:
The sampling period is then equal to T, and the sampling frequency is fs = 1=T .
For this reason, although x[n] is strictly the nth number in the sequence, we often refer to it as the nth sample. We also often refer to \the sequence x[n]" when we mean the entire sequence. Discrete-time signals are often depicted graphically as follows: