THE L3 SYSTEM — DESIGN 



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channels stem from early unpublished work on transmission standards. 

 The interference and load capacity requirements for transmission sys- 

 tems involving large numbers of message channels were developed by 

 Dixon, Holbrook and Bennett. ' In effect, they provide techniques for 

 translating channel objectives into linearity and power handling require- 

 ments on repeaters, taking into account the statistical properties of 

 individual and multi-channel speech. Based on the data and techniques 

 in these papers, the requirements on individual channels shown in 

 Table I can be derived. These requirements in themselves form an im- 

 portant basis for the signal-to-noise design of the system. However, in a 

 highly refined system design it is necessary to extend our notions of 

 requirements somewhat further. 



In the L3 signal-to-noise design the message channel requirements of 

 Table I were used as the initial basis for study. However, when specific 

 interferences of a complex nature were found to be limiting, the wave 

 forms and the probability of their occurrence w^ere examined in detail. 

 As a result of these studies, two distinctive types of interferences were 

 found to be important when the system is used to carry message and 

 television signals simultaneously. The first of these, due to both second 

 and third order modulation involving multifrequency key pulse signals 

 and components of the television signal, has the characteristics of inter- 

 mittent musical tones. The second, due to the second order difference 

 products generated by the television signal components, produces tones 

 in the message channels which vary in amplitude and frequency as the 

 television signal changes with picture content. Both types of interference 

 were generated in the laboratory and recorded on tapes. From these 

 tapes, records were cut and then used in a series of subjective tests 



Table I — Summary of Message Circuit Objectives 

 {Allowable Zero Level Interference in 3 kc band) 



* The translations from dba to dbm are effected by noting that a 3000 cycle 

 band of flat noise with one milliwatt of power equals +82 dba and that one milli- 

 watt of 1000 cycle single frequency is equal to +85 dba. 



t Interference assumed evenly distributed over 3000 cycle band. 



I Tones assumed to be at 1000 cycles. 



