The first thing we have to do, is to obtain signal values from the continuous signal at regular time-intervals. This process is known as sampling. The sampling interval is denoted as Ts and its reciprocal, the samplingfrequency or sample-rate is denoted as fs, where fs = 1/Ts. The result of this process is just a sequence of numbers. We will use n as an index into this sequence and our discrete time signal is denoted as x[n] – it is customary in DSP literature to use parentheses for continuous variables such as the time t and brackets for discrete variables such as our sample index n. Having defined our sampling interval Ts, sampling just extracts the signals value at all integer multiples of Ts such that our discrete time sequence becomes: