852 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



MANUAL GAIN EQUALIZERS 



One of the more difficult equalization design choices is the selection 

 of shapes for the manual equalizers. In the LI system some of the shapes 

 used were so-called "bump" shapes. This type of shape reduces the 

 amount of shape overlap and thereby tends to reduce the adjustment 

 problem when conventional adjustment methods are used. For the L3 

 system many types of shapes and circuits were carefully studied. The 

 computational concepts of equalizer adjustment resulted in the con- 

 sideration of shapes that would otherwise be impractical. 



The shapes finally chosen for L3 are "cosine" shapes. Any continuous 

 function may be matched over a 180-degree interval by a Fourier series 

 of cosines only. By making the 180-degree interval the frequency range 

 from to 8.5 mc the cosines are cosines of frequency and can match 

 any gain characteristic if enough terms are used. These cosine equalizers 

 have the following advantages. 



1. The range required for any term is less than the total shape to be 

 matched, usually less than half. 



2. The residues are always higher harmonics and therefore easily 

 matched if necessary. 



0.114 



r<3^AMn 



18.561 \2466 



nm^--^ 



nm^—c 



• — ^AA/ — ' 



30 < Rt < 753 



FLAT LOSS = 7.2 DB 



OHMS, >UH, /U/LIF 



Fig. 7 — Circuit of the regulating network for tube aging. 



