196 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



The loading units having the digit 4 in their code designations are "full- 

 weight" units for H31-18 voice-frequency entrance cable described in Part V 

 of this review. The loading units having the digit 2 in their designations 

 are used in H44-25 four-wire repeatered circuits. The other loading units 

 are used in two-wire repeatered circuits, and also in circuits not long enough 

 to require repeaters. 



(13) Improved Loading eor (Long-Distance) Cable 

 Program Transmission Circuits 



13.1 General 



In the early days of radio chain-broadcasting (during the early 1920*s), 

 the links which connected the broadcasting stations with the studios where 

 the programs originated usually were open-wire voice-frequency telephone 

 circuits modified to meet the special requirements of this new type of 

 service. In some instances, toll cable circuits were used for links not more 

 than a few hundred miles long. 



Where available, side circuits of H44-25 facilities, previously described, 

 were preferred for the cable program-transmission service because of their 

 high cut-off frequency. By using suitable equalizing and regulating net- 

 works in conjunction with modified one-way amplifiers, a fairly satisfactory 

 transmission-medium could be obtained, providing a frequency-band ranging 

 from about 100 cycles up to about 4000 cycles. While these circuits were 

 adequate for speech broadcasting they were not equally satisfactory for 

 classical music programs by symphony orchestras. 



As early as 1924, some preliminary studies were started on entirely new 

 types of loaded cable facilities primarily for use in transmitting the highest 

 grade of radio-broadcast program material. These studies showed 16 ga. 

 cable pairs to be desirable for long program circuits. At the time, however, 

 there was considerable question as to what the ultimate performance- 

 requirements should be, and some doubt as to whether the commercial 

 demand would be sufficiently large to warrant the high cost of developing 

 and providing a suitable new type of facility. 



During the following years, there was a large increase in the number of 

 broadcasting stations and in the need for inter-connecting links. Also the 

 toll cable network expansion had commenced to accelerate rapidly. Be- 

 ginning about 1926, the new toll cables that were installed usually included 

 a small complement of 16-gauge non-quadded pairs (generally 6) in anticipa- 

 tion of their ultimate use for improved program facilities, if the then ex- 

 p>ected demand should eventually materialize. Studies of improved loading, 

 and of the very important equalization and temperature regulation problems 

 were renewed. This work progressed sufficiently so that early in 1927 a trial 



