730 



BELL SYSTEM TECHNICAL JOURNAL 



111 terms of B{x~) directly and a desired transmission ao at the angular cut- 

 off frequency coo , equation (10) becomes 



^(.^2) = /ao^- 



-2fc/(i) 



(11) 



where A' = ^~^"°. Equation (11) now represents the characteristic that is 

 to be approximated over the useful frequency band while Fig. 15 shows a 

 plot of this function. 



Fig. 15 — Specification for B{x'') over useful frequencj* band. 



/f(x2)+f2V^2(x) 



f2Vn2(x) 



lp-2kf (X) 



Fig. 16— Combined approximating function for 5(.v2) over entire frequency band. 

 Now, if B{x-) is broken up into two parts and represented as 



B{x'-) = j{x') + eWl{x) 



(12) 



16 It is important to note that eq. (12) now represents the api)roxiniating function over 

 the entire frequency range as compared to eq. (11) which represents the function to be 

 approximated only over the useful range. 



