An Improved Three-Channel Carrier Telephone System 



By J. T. O'LEARY, E. C. BLESSING and J. W. BEYER 



This paper describes an improved three-channel carrier telephone 

 system for use on open-wire lines. It employs recent advances in the 

 telephone art to bring about many economies and circuit simplifica- 

 tions as compared with previous models of the three-channel sys- 

 tem. A new type of automatic regulating equipment is included. 



Introduction 



THERE are now in service in the Bell System approximately 

 750,000 miles of telephone circuit which are furnished by carrier 

 systems. Of this total, almost 90 per cent is provided by some 600 

 Type C systems, ranging from about 75 miles to over 2000 miles in 

 length. Basically designed to add three carrier channels to the normal 

 voice channel on open-wire lines, the Type C system has also been 

 used in special cases to provide additional circuits over deep sea 

 cables of moderate lengths. 



The system was first described in this Journal in the July 1928 issue.' 

 Improved designs and the application of new circuit elements have 

 recently permitted a very extensive revision of the terminal and re- 

 peater equipment which results not only in striking reductions in 

 size and cost as compared with the older equipment, but also gives a 

 considerable improvement in transmission performance. A new type 

 of automatic regulating equipment has been provided for both the 

 terminal and repeater. 



The improved system employs heater type pentode tubes, copper- 

 oxide modulators and demodulators and makes use of the negative feed- 

 back type of amplifier at both terminal and repeater points. The 

 terminal band filters are newly designed to give improved transmission 

 frequency characteristics on all channels. Each channel is arranged to 

 terminate on a four-wire basis in the same manner as the Type K 

 system for cables.^ 



An outstanding feature of the modified design is the large saving in 

 space in comparison with the previous equipment. As shown on 

 Fig. 1, the complete terminal with its regulating equipment occupies 

 a single bay, whereas the older system without regulating equipment 

 required two and one-half bays. The repeater space savings, while 

 not so large, are nevertheless substantial. The number of vacuum 



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