916 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



transmitting terminal conditions a television signal for carrier transmis- 

 sion over the system simultaneously with a group of 600 telephone mes- 

 sages. The receiving terminal reconverts the carrier signal at each re- 

 ceiving point along the cable route. Primarily, the transmitting terminal 

 IS a modulator which translates the composite video picture spectrum of 

 frequencies up to the carrier band of frequencies and the receiving 

 terminal is a detector which retranslates the carrier spectrum back to its 

 original band of frequencies. 



Particular characteristics of the transmitted television signal, which 

 are intended to aid in achieving optimum transmission quality, have 

 necessitated the departures from past techniques in modulation and 

 demodulation processes that are described in the following. Described 

 also are the methods employed to achieve transmitted picture quality 

 adequate for tandem operation of as many as six pairs of transmitting 

 and receiving terminal equipments in a 4,000-mile television transmission 

 circuit. Operation with several pairs of terminals in tandem occurs when 

 L3 coaxial systems are interconnected with LI coaxial systems or micro- 

 wave radio systems. 



L3 television terminal development has been in progress since early 

 in 1948. Two transmitting and two receiving terminals have been built 

 on a preproduction basis and currently are being tested under field con- 

 ditions as part of the L3 system field trial. Development effort is con- 

 tinuing on the terminals with emphasis on equipment reliability, includ- 

 ing means for maintaining and improving transmission quality. 



Frequency Allocations 



The L3 coaxial system was designed to have as broad a transmission 

 band as the economics of repeater spacing together with presently rea- 

 lizable feedback ampUfier performance permit.^ The band extends from 

 300 kc to 8.5 mc. In comparison with this band a broadcast television 

 signal occupies the frequency spectrum from zero frequency up to 4.5 

 mc. 



From the foregoing it is evident that the television spectrum will not 

 occupy fully the available system transmission band. It is feasible and 

 attractive to allocate part of the transmission band for television trans- 

 mission and the remainder for transmission of message channels. De- 

 tailed allocations then result from a compromise among transmission 

 performance, cost and the number of message channels made available. 



From these considerations vestigial sideband transmission of the tele- 

 vision signal rather than double sideband transmission is called for. 

 The smaller the vestigial band of transmitted frequencies is made the 



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