THE L3 SYSTEM DESIGN 821 



The telephone terminals consist of modulators (and related trans- 

 mission equipment) and carrier and pilot generating equipment. The 

 transmission components of a terminal for an all message system are 

 shown on Fig. 19. The channel, group and supergroup equipment are 

 designs previously used in the LI system. The designs of the submaster- 

 group and mastergroup units employ circuit arrangements similar to 

 those used in the supergroup equipment. The greater bandwidths, 

 higher frequencies and more severe stability requirements required new 

 components and improved circuit and layout techniques. 



Fig. 20 shows the modulation steps and location in the frequency 

 spectrum of the supergroups, submastergroups and mastergroups when 

 the L3 system is used for telephone and television or all telephone. Mas- 

 tergroup one comprises the first ten sixty-channel supergroups. This 

 mastergroup is placed directly on the Hne in the 564 to 3,084-kc frequency 

 band for both the telephone-television and all-telephone cases. When the 

 system is used entirely for telephone, two additional mastergroups are 

 formed by modulating mastergroup one up into the desired frequency 

 bands. 



Mastergroup No. 1 is subdivided into two submastergroups. The 

 lower six supergroups, comprising submastergroup one are modulated 

 directly up from the basic supergroup located in the 312 to 552-kc band. 

 The modulation and carrier supply equipment for these supergroups 

 are the same units that are employed in the LI system. The upper four 

 supergroups comprising submastergroup two are obtained by modulating 

 four supergroups located in the same frequency range as the top four 

 supergroups in submastergroup one into the top part of mastergroup 

 one. 



The supergroup numbering system used for L3 has been adopted for 

 easy identification of supergroups in their high-frequency positions. 

 Each supergroup is given a three digit number. The first digit identifies 

 the mastergroup, the second digit identifies the submastergroup, and 

 the third digit identifies the LI supergroup from which it was originally 

 derived. 



In the 1860 channel all-telephone allocation, supergroup No. 112, 

 which corresponds to the basic LI supergroup No. 2, may be used for 

 high quahty, long haul message circuits. When the system is used for 

 telephone and television, supergroup No. 112 is restricted to circuits 

 under 200 miles in length because of intolerable second order cross 

 modulation between these signals and the television signal. 



With these groupings of channels new modulating and carrier supply 

 equipment is required for submastergroup No. 2 and mastergroups Nos. 



