THE L3 SYSTEM — EQUALIZATION AND REGULATION 863 



For example, the wide gap from 3,096 to 7,266 is largely due to the 

 difficulty of inserting and removing pilots in the lower video frequencies 

 of television signals. The problem of finding a satisfactory set of pilot 

 levels and frequencies which will at the same time, be compatible with 

 the desired signals is an important part of the system design problem. ^ 

 The change of four-mile cable loss with temperature is so large (±1.2 

 db) that regulation is required at each repeater. It takes three months 

 or more for the cable loss to change 2 db but the normal line maintenance 

 interval is of this order. A gain error of this magnitude could not be 

 allowed to accumulate over very many repeaters before the signal to 

 noise performance of the system would collapse. Other effects such as 

 vacuum tube aging and repeater temperature changes can be allowed to 

 accumulate over as many as 30 repeaters before regulation. These facts 

 dictate the location of regulators in the system. At each repeater there 

 is a ''line" regulator controlling a square-root-of -frequency-shape regu- 

 lating network. Then at equalizing points and dropping points "office" 

 regulators correct for the remaining effects. 



CHAIN ACTION 



Pilot controlled dynamic regulators derive much of their advantage 

 from the fact that they prevent gain changes from accumulating from 

 repeater to repeater. This advantage is one manifestation of what might 

 be called the ''chain action" of a series of regulators. There is however a 

 corresponding disadvantage, disturbances of the pilot cause the accumu- 

 lation of unwanted gain fluctuations. In previous systems this disad- 

 vantage has been aggravated by positive envelope feedback (l-MiS less 

 than one), at some frequencies, an effect known as "gain enhancement". 

 In the L3 system the "gain enhancement" is nearly negligible but the 

 television requirements still require careful control of certain types of 

 gain fluctuations. 



The advantage noted above can easily be demonstrated by a simple 

 example. Consider a chain of regulators each having 20 db envelope 

 feedback so that pilot level changes are reduced by 10 to 1. Now con- 

 sider what happens if each cable section changes loss by one db. Table 

 I illustrates the action. 



The first regulator inserts a gain change of 0.9 db in response to the 

 1.0 db input change. The 0.1 db error increases the input change to the 

 second regulator to 1.1 db and it therefore inserts a 0.99 db correction. 



The total resultant error of 0.11 db adds to the change at the third 

 regulator input, etc. Simply stated: The error of the first regulator rides 

 through the system forcing the other regulators to make an accurate 



