A CARRIER TELEPHONE SYSTEM FOR TOLL CABLES 91 



keep high-frequency noise from entering the cable where open-wire 

 pairs tap into it and frequently also where branch cables are con- 

 nected. For this purpose simple noise suppression filters are em- 

 ployed. With these and other measures the noise on the carrier pairs 

 can, at the highest frequencies where the amplification is greatest, be 

 brought within a few db of the basic noise due to thermal agitation of 

 electricity in the conductors themselves. 



Velocity of Transmission 



Voice waves travel through loaded cable circuits at from 10,000 to 

 20,000 miles per second, the higher speed being used for the longer 

 circuits. On very long connections, even if echo suppressors are 

 employed this velocity results in transmission delays which introduce 

 difficulties in conversation.* The use of non-loaded conductors for 

 the type K systems results in an overall velocity of transmission of 

 about 100,000 miles per second, a speed so high that such difficulties 

 are greatly reduced and satisfactory telephone conversations are 

 possible over the longest distances for which connections may be 

 required. 



Repeaters 



Since the noise level in the cable circuits can be made quite low, the 

 carrier currents may be permitted to drop to levels below those used 

 on voice-frequency circuits or on open-wire carrier circuits, and the 

 repeaters may have higher gains. In the cable carrier system, the 

 noise has been so reduced that the level of the top channel at the 

 repeater input may on the average be dropped about 60 db below the 

 voice level at the transmitting switchboard. The amplifier gains at 

 the top frequency range from about 50 to 75 db, and the output level 

 of each of the 12 channels is about 10 db above that at the switchboard. 

 The average repeater spacing is about 17 miles. 



The tube which was developed for the gain stage of the amplifier 

 is a pentode with indirect heater. The heater requires a potential 

 of 10 volts and a current of 0.32 ampere and the plate 150 volts. The 

 tube in the power stage is similar in type but requires a heater current 

 of 0.64 ampere at 10 volts. With this power tube a feedback of about 

 40 db has been found to provide a satisfactory reduction of inter- 

 channel modulation.^ Both tubes were designed to have long life 

 with very reliable performance. 



Description of Amplifier 



Each repeater comprises two amplifiers of the type illustrated in 

 Fig. 8. A schematic diagram of the amplifier circuit is shown in 



