842 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



are used, 19 and 28 repeaters, and the residue is corrected by the manual 

 equaUzers. This residue may be positive or negative depending on 

 whether the fixed ecfuaUzer over or under compensates. Thus there is 

 only a small tendency for these residues to accumulate in long systems. 

 However the inaccuracies of match between the fixed equalizer and the 

 gain of the average repeater section tend to accumulate systematically 

 and must therefore be kept small. This brings out the importance of 

 statistical quality control of amplifier manufacture since any systematic 

 shift in the amplifier gain characteristic will accumulate and may con- 

 sume excessive manual equalizer range or may lead to excessive equaliza- 

 tion errors. For example, a shift of only 0.05 db in the gain of the average 

 amplifier w^ould represent 1.5 db in 30 repeaters, 50.0 db in 1000 am- 

 plifiers and thereby becomes an extremely serious matter. Thus quality 

 control of the amplifier is a vital part of the solving of the equalization 

 problem. 



The various effects that consume the range of the manual equalizers 

 produce for some shapes an unsymmetrical consumption of range. If 

 uncorrected this would produce larger range requirements in the manual 

 equalizers as well as introduce new shapes to be equalized. The manual 

 equalizer shapes are symmetrical and their errors cancel if equal amounts 

 of positive and negative range occur in the system. Any systematic 

 offset of a particular shape tends to introduce new shapes due to the 

 manual equalizer networks themselves. By appropriate modification 

 of the shape of the fixed equalizer it is possible to recenter the manual 

 equalizers so that on the average the manual shapes are in the center 

 of their range. 



DYNAMIC EQUALIZERS 



Any long transmission system suffers from relatively rapid gain 

 changes and in the L3 coaxial system, as in many previous systems, 

 the necessary corrections are performed automatically by pilot con- 

 trolled regulators. Pilot tones are transmitted over the line at a reference 

 level and, at appropriate points, regulators pick the pilots off the line, 

 observe the deviation in pilot levels from the reference values and re- 

 store the pilots to or very nearly to the reference values by the use of 

 regulating networks. 



There an; fundamentally two causes of fast gain changes, time and 

 temperature. Time produces vacuum tube aging and in spite of their 

 feedback the liru; amplifiers change gain. In one week a 4,000-mile sys- 

 tem is expected to change by as much as 5 db due to the aging of the 

 6000 tulxjs or so in the transmission path. Because of different thermal 



