Exercise 12 (b).  Construct a filter using the zeros   .   What signals are "zeroed out" ?

Solution 12 (b).

See text and/or instructor's solution manual.

Answer.   Compute the product  

                    .  

The desired filter is  

                    .  

Solution.   Use the zeros in part (a) and the additional conjugate pair of zeros   ,  

and the "zero out factors"   ,       and   .  

Then calculate  

                       

        Now use the General Filter Equation (9-29) and the corresponding Transfer Function (9-34) in the case  .     

Get the new fact that the transfer function  

                    ,  

corresponds to the filter  

                    .  

        For this exercise, we use   ,    and      in these equations to get the desired recursive formula  

                    .  

Therefore,  the desired filter for (b) is

                    .  

for "zeroing out" the signals   .

We are done.   

Aside.  We can let Mathematica double check our work.

Aside.  The Maple commands are similar  

  

                                                              


  

                                                              

We are really done.   

Aside.  We can graph the amplitude response for the filter   .  

                    Amplitude response      and zero-pole plot of   ,  

                    for the filter   .

                    We can see that the mid-range frequencies are attenuated, and      for   .

Aside.  In Exercise 18 (a) we will investigate the filter   .  

We are really really done.   

Aside.  Let us investigate how well the filter works to eliminate signals   which are close to the "zero-out" frequencies.

        For illustration purposes we will explore the casual input signal   .

The signal component    will be amplified by the factor  .  

The signal component    will be attenuated by the factor  ,  and

The signal component    will be attenuated by the factor  ,  and

The signal component    will be attenuated by the factor  .  

                    The causal input sequence      and the corresponding causal output sequence.

                    We can see that the filter practically eliminates the signals   which are close to the "zero-out" frequencies.

This solution is complements of the authors.

(c) 20098 John H. Mathews, Russell W. Howell