One possible signal flow graph (or system diagram) for Eq. (5.1) is given in Fig.5.1a for the case of and . Hopefully, it is easy to see how this diagram represents the difference equation (a box labeled ``'' denotes a one-sample delay in time). The diagram remains true if it is converted to the frequency domain by replacing all time-domain signals by their respective z transforms (or Fourier transforms); that is, we may replace by and by . transforms and their usage will be discussed in Chapter 6.