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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



of this term is ±0.5 db as is that of all higher terms (to 24). Terms 1, 2, 

 3 have full range, 4, 5, 6 have 1.5 db and 7, 8, 9 have 1 db. The O term, 

 flat gain, is also 1 db because additional flat shape is obtainable from 

 the flat amplifiers used to make up for the equalizer loss. 



The shapes provided by the first three terms are shown on Fig. 11. 

 Note that the shapes are cosines of a warped frequency variable and 

 that on this warped scale the shapes are orthogonal. In all, 24 such 

 harmonics plus flat gain are used in the high frequency line for combined 

 systems. For all telephone use only 14 harmonics plus flat gain are 

 required. Fig. 12 shows the construction of one of the cosine networks. 

 These are mounted in groups of five as indicated in Fig. 13. The fixed 

 equaUzer and regulating networks are mounted to the rear of the cosine 

 assembly. 



Harmonic Adjusting Set 



To make a mental harmonic analysis of a complicated gain char- 

 acteristic is difficult if not impossible. Therefore a special cosine-equalizer 

 adjusting set has been developed which eliminates trial and error from 

 the adjustment process and which leads to a unique optimum adjust- 

 ment. Broadly the method consists of using sweep frequency methods 

 to convert the gain-frequency characteristic into a repetitive voltage- 

 time function. Gain cosines on the warped frequency scale are converted 

 to voltage cosines of time and the audio harmonic-spectrum components 



IN 



Fig. 10 — Cosine equalizer circuit (tenth hMrnioiiic) showinjj; dissipation and 

 range corrections. 



