 The signals that we encounter in practice are mostly analog signals, these signals which vary continuously in time and amplitude, a process using electrical networks containing active and passive circuit elements. This approach is known as analog signal processing or ASP, for example radio and television receivers. They can also be processed using digital hardware containing adders, multipliers and logic elements or using special purpose microprocessors. However one needs to convert analog signals into a form suitable for digital hardware. This form of the signal is called a digital signal or discrete signal. It takes one of the finite number of values at specific instances in time and hence it can be represented by binary numbers or bits. The processing of digital signals is called DSP. In DSP an analog signal XT is given to an analog to digital converter which takes samples and digitizes it to get a discrete signal XN. XN is passed through a discrete system which produces a discrete signal YN. Discrete system is the heart of DSP. It can have various properties but we are only going to discuss LSI systems which have two very important properties of linearity and shift invariance. In hardware discrete system can represent a general purpose computer or a special purpose processor. After this step YN is given to a digital to analog converter which produces a staircase waveform. We will have to use a post filter to smooth out this staircase waveform in order to get that output analog signal YT.