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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



the band is largely due to the failure of the fixed equalizer to match the 

 very sharp cut-off of the line between 8.35 and 8.50 mc. Since telephone 

 channels are not transmitted in this region and since television require- 

 ments are less severe at such high video frequencies (4.2 mc) this effect 

 does not limit the transmission quality. Also note that long television 

 circuits, over 400 miles, will have a further level of equaUzation, D. 

 There is no visible impairment of ordinary television pictures due to 

 insertion of the 200-mile L3 line in their path. With critical types of 

 test patterns there is a slight effect. While there are problems yet to be 

 solved before 4,000-mile transmission can be obtained, the 200-mile 

 performance is most encouraging. 



In the past the adjustment of manual equalizers has often been a 

 difficult and time consuming task. The cosine adjusting set described 



0123456789 

 FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 17 — Final gain characteristic of a 100 mile L3 line after equalication with 

 15 cosine shapes. 



previously appears to have made sizable inroads on this problem. The 

 adjustment of the 25 controls used for television is a three minute job. 

 The fact that the equalization operator is working toward a unique solu- 

 tion and therefore knows when he is done appears to be of material 

 value. 



Regulation 



The I^ regulation system is in many respects, similar to the LI sys- 

 tem. However, the design of a stable regulation system for over five 

 hundred regulators in tandem introduces unusual problems. Also the 

 accuracy and stability requirements have led to the use of novel regu- 

 lator circuits including, for example, an analog computer as an element 

 of the system. 



Six pilots are used, 308, 556, 2,064, 3,096, 7,266 and 8,320 kc. These 

 frecjuencies were selected to l)est measure the anticipated system changes 

 as restricted by where signal allocations would permit their insertion. 



