A signal is a time or space varying quantity that conveys information. It may take the form of a sound wave, a voltage, a current, a light wave, a magnetic field, a displacement, a velocity, an acceleration or a host of other physical quantities. These are examples of continuous signals (the signal is present at all instances of time or space). As a matter of convenience we often convert continuous signals into discrete signals by sampling the phenomena at intervals (Figure 3.1).