Introduction: A discrete-time system ( digital system) can be represented by 1) difference equation, 2) transfer function H(z), 3) impulse response, 4) frequency response, 5) pole-zero plot, and 6) realization structure diagram. Generally, continuous-time (Analog) systems are described by differential equations. Discrete- time (Digital) systems are described by difference equations. Then it is possible to convert the difference equation to a transfer function H(z) by using z-transform. Digital systems are sometimes described by Delay-Adder-Gain block diagrams. Example: y[n] = 0.5 x[n] + 0.4 x[n-1] - 0.2 x[n-2] - 0.5y[n-1] + 2y[n-2] This is 2"d order difference equation. Three basic unit are required for realization structure 1. Delay element (x[n-1], y[n-1], x[n-2], etc...) ZA-1 2, Coefficient Multiplier (numerator and denominator) 3. Summer or Adder