784 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



4. Sufficient bandwidth should be provided so that a black and white 

 television signal of at least four-megacycle quality could be transmitted 

 simultaneously with 600-message signals, the message capacity of the 

 LI system. Alternatively, as many message channels as possible should 

 be transmitted when there was no need for television service. 



5. The channels should meet Bell System high quality signal-to-noise 

 and equalization objectives after 4,000 miles of transmission. 



Section 2 of the paper is devoted to a discussion of the principal 

 system design problems and descriptions of methods used in solving 

 these problems. Section 3 contains descriptions of the components of 

 the system, their locations and their functions. 



2.0 Transmission Design 



With a given cable loss, the line repeaters determine in large measure 

 the bandwidth and quality of transmission and the economics of the 

 system. The basic system plan therefore evolves from a consideration of 

 the signal-to-noise and equalization performance — i.e., the transmission 

 stability — that can be designed into the repeaters. This leads to the 

 development of broad signal-to-noise and equalization analyses which 

 guide and coordinate the system design. 



2.1 SIGNAL-TO-NOISE DESIGN 



Simply stated, the signal-to-noise problem is to adjust the repeater 

 spacing and bandwidth of the system so that channel objectives can be 

 met with the repeater noise, linearity, and gain performance that the 

 electron tube and circuit art permit. In detail this means the following: 

 (1) to translate the broad transmission objectives on message and tele- 

 vision channels into detailed requirements on noise, specific modulation 

 products and compression; (2) analyzing the amount of these inter- 

 ferences that result from various repeater design choices; (3) determining 

 the effect of signal wave form and frequency allocations on both the 

 channel requirements and the repeater performance ; and (4) integrating 

 these studies into a specific system design plan that meets the objectives. 



2.11 Telephone Channel Interference Objectives 



The amount of noise, tone interferences or crosstalk that is con- 

 sidered tolerable in telephone channels is generally determined by judge- 

 ments involving the subjective reactions of representative observers to 

 specific interferences on typical transmitted signals and by the cost of 

 providing a given grade of service. The broad objectives for message 



