It is easy to show that the TDL of Fig.2.19 is equivalent to a parallel combination of three feedforward comb filters, each as in Fig.2.23. To see this, we simply add the three comb-filter transfer functions of Eq. (2.3) and equate coefficients:
which implies
We see that parallel comb filters require more delay memory ( elements) than the corresponding TDL, which only requires elements.