SHORT-HAUL CARRIER TELEGRAPH 667 



designed for application in large groujjs at telegraph central offices and 

 for trunk-ser\'icc operation o\cr toll telephone circuits emplojdng stand- 

 ard levels. It has proved very economical in this field. However, the 

 very features which make for economy in large installations (such as 

 amplitude modulation, common carrier supply and testing equipment, 

 and standardized operating conditions) cause this equipment to be 

 costly when it is applied a few channels at a time in outlying offices; 

 these may not be ecjuipped with either telegraph battery supplies or 

 telegraph boards. Moreover, the -40C1 equipment, being a carrier-on- 

 for-mark and carrier-off-for-space system, does not lend itself to the 

 pro\'ision of TWX toll subscriber line supervision identical to that of 

 local stations without the addition of rather complex and expensive 

 superNisory applicjue circuits. Where TWX supervision is involv^ed these 

 super^^sory circuits are required to generate and recognize supervisory 

 signal patterns capable of being distinguished from transmission space 

 signals and communication breaks, which are long space signals. 



Consequently, it was decided to develop a new carrier telegraph 

 system especially aimed at the needs of fringe areas. One of the problems 

 to which much thought was given concerned the choice between ampli- 

 tude-modulation and frequency-shift operation. A frequency-shift sys- 

 tem provides some reduction in the effect of noise and other interference 

 on transmission and it is also less affected by rapid level changes. Al- 

 though these advantages were attractive, it was not clear that they were 

 sufficient to justify the added complexity and cost entailed by the 

 adoption of this type of transmission, in view of the quiet and stable cir- 

 cuits encountered in the Bell System plant. What finally s^vlnlg the bal- 

 ance to a frequency-shift system was its advantage in handling TWX 

 supervisory signals. With transmission accomplished by shifting the car- 

 rier frequency, supervisory signals could be sent by turning the carrier 

 on and off. A cheap and simple circuit might then be used to distinguish 

 between transmission and supervision. 



From the foregoing discussion it will be evident that during the twelve 

 years since the 40C1 system was developed the needs of the Bell System 

 have changed. Fortunately, the designer's art has concurrently made 

 great strides in making available new miniature apparatus and elec- 

 tronic techniques such as have been exploited so successfully in the 

 143A type electronic telegraph regenerative repeater, the V3 telephone 

 repeater and the N-1 carrier telephone system. As a result, the 

 channel terminal of the new 43A1 carrier telegraph system, being small, 

 inexpensive, self-contained and all-electronic with no electro-mechanical 



