THE L3 SYSTEM DESIGN 801 



ning was the balance of factors such that there was less than three db 

 clear margin. 



Margin handled in this way becomes a carefully husbanded asset of 

 the whole system. In designing or analyzing a part of the system a 

 major effort must be made to achieve the performance introduced into 

 the initial determination of repeater spacing and bandwidth. The design 

 of each individual part of the system cannot be allowed a margin which 

 can be used up as the individual designer chooses. 



2.20 Equalization Design 



The term "equalization" is used to describe the process of obtaining 

 flat gain and delay characteristics for the system transmission. The 

 system and equipment designs to accomplish this function represent two 

 of the major engineering features of the L3 system. In an overall sense, 

 equalization includes the following: (1) determining deviation objectives 

 for the gain and delay characteristics of the system and its component 

 parts; (2) designing the auxiliary repeater so that the most economical 

 over-all system equalization is obtained; and (3) specifying the location, 

 form and control methods for the mop-up equalizers that are used at 

 intervals along the system. Equalization and its related process, regula- 

 tion, are the subject of a companion paper ;^^ therefore, in this paper only 

 those aspects of equalization will be covered which are necessary for an 

 appreciation of the over-all system design. 



2.21 Transmission Objectives 



The requirements on the gain characteristic of a band used for multi- 

 channel telephony depend on two message channel objectives. One of 

 these is that the gain of a message channel must not vary by more than 

 two db over the 4-kc band. To meet this requirement, broad changes in 

 the transmission characteristic of the message band are held to less than 

 0.5 db for 150-mile links. The second objective stems primarily from the 

 need to transmit telephotograph signals. Since these signals are rela- 

 tively intolerant of level changes, the transmission characteristics of 

 working lines and protection lines are made alike to within d=0.25 db. 



The requirements on the gain and delay characteristics of the tele- 

 vision band are based on the subjective determination that an echo de- 

 layed by about two microseconds or more in a representative picture is 

 considered tolerable by 95 per cent of the viewers when the peak-to- 

 peak voltage of the echo signal is 39 db below the peak-to-peak signal 

 voltage." The translation of this echo objective to allowable variations 



