796 BELL SYSTEM TECHNICAL JOURNAL 



tremely complicated and expensive that the reduction in the ampUtude 

 of the reflected waves by improvement of the reflection coefficient at 

 these junctions is of considerable economic importance. The imped- 

 ances of the terminals and repeaters at the junctions at which reflec- 

 tions occur are chiefly determined by their filters, which are the ap- 

 paratus immediately facing the line. A detailed study of the relation- 

 ship between the actual input impedance of a filter and mismatches of 

 characteristic impedance which may occur at further junction points 

 in the circuit shows that by far the simplest method of obtaining a 

 low reflection coefficient at the line terminals is to produce a match of 

 characteristic impedances at all junction points of the filter system. 

 Fortunately speech currents beyond the transmitted band of the filters 

 carry so little energy that the reflection coefficient of the structure in 

 these ranges is of no importance. The technical problem therefore 

 reduces to the construction of a new type of filter section for use at 

 terminations, the new filter section having an image impedance within 

 the transmitted band which at one end matches that of the standard 

 sections forming the main body of the structure and at the other ap- 

 proximates a constant resistance, matching the terminating im- 

 pedances. Of course the new filter sections must also be so chosen that 

 they will not impair the transmission properties of the system. 



This immediate problem has been solved. It still leaves unsettled, 

 however, the question as to whether we can devise a type of network 

 capable of correcting for reflection effects not only at these particular 

 junctions but also at any other impedance irregularity in the circuit. 

 Such a structure would transform one arbitrary impedance character- 

 istic into another preassigned characteristic without decreasing the 

 transmission efficiency of the circuit, much as the familiar attenuation 

 equalizer changes the attenuation characteristic of a circuit by a pre- 

 assigned amount without changing its impedance and without greatly 

 affecting its phase characteristic. The mathematical analysis under- 

 lying the sections which have been developed for filter impedance cor- 

 rection is easily extended to a much broader class of terminating im- 

 pedances. Judged from a purely formal standpoint, therefore, the 

 networks appear to be a long step forward in the development of such a 

 general impedance equalizing device. Unfortunately, it seems certain 

 from other considerations that much of the promise thus inherent in 

 the formal mathematical analysis may not be realized in practical 

 applications, but since the network has been thoroughly studied only 

 in its application to filters, its precise limitations are still uncertain. 

 In the discussion which follows the general method of impedance cor- 

 rection is first sketched briefly, and is succeeded by a detailed treat- 



