THE L3 SYSTEM DESIGN 799 



are shifted with respect to one another in the band, a process known as 

 frogging. 



In the L3 system, signal-to-noise performance is substantially im- 

 proved by frogging the supergroups at intervals of about 800 miles. In 

 the 1,860 channel all-message application, the busy hour signal-to-noise 

 performance of 4,000-mile circuits is alike to within two db with all 

 channels meeting the objective of +29 dba at the — 9 db level. In con- 

 trast, if frogging were not specified, a substantial number of circuits 

 (10 to 20 per cent would fail to meet the objectives while the perfor- 

 mance of other channels would be better than required by six db or more. 



When the system is used for combined message-television signals, 

 the message circuits are frogged in supergroup blocks at approximate 

 800-mile intervals except for supergroups Nos. 113 and 114 which must 

 be frogged at 400-mile intervals. This procedure is necessary to prevent 

 second order sum and difference products of message and television 

 signal components from cumulating excessively, expecially those pro- 

 ducts which involve television signal components close to the television 

 carrier. Frogging these supergroups more frequently than others results 

 in a 3 db improvement in television signal-to-noise performance. 



2.18 Special Services Transmission 



During the early design stages, requirements based on the trans- 

 mission of message and television signals were used to set repeater spac- 

 ing, to determine the bandwidth and frequency allocations and to fix 

 important design parameters of the amplifiers. Concurrently, the ob- 

 jectives for the transmission of telegraph, program, and telephotograph 

 signals were studied and before the system design crystallized, analyses 

 were made to assure that these special services objectives would be met. 



In a few instances it was found that the special services objectives 

 tended to dominate and the system requirements and design were ad- 

 justed accordingly. For the most part, however, channels which meet 

 message circuit objectives are satisfactory for special services trans- 

 mission. In L3, telegraph and telephotograph signals may be transmitted 

 without restriction provided the proportion of these signals does not 

 materially exceed the proportion now installed in the plant. Program 

 signals may be transmitted in the 1,860 all-message arrangement with- 

 out restriction but when television transmission is provided, program 

 circuits are restricted to supergroups Nos. 113 and 114. This restriction 

 is due to the fact that program circuits are usually more than 4 kc wide; 

 interferences of high magnitude which normally fall between 3,300 and 



