A NEW KEY WEST— HAVANA CARRIER TELEPHONE CABLE 211 



tion in these circuit elements must be kept extremely small. Some or- 

 dinary resistances, mica condensers, etc., have been found to have 

 enough modulation to be serious. Ordinary iron core transformers are 

 likely to be very bad. Special design was required to reduce the modu- 

 lation in the impedance matching transformers to tolerable limits. 



Noise Prevention 



As noted previously, at the receiving end the important problem is 

 naturally to keep the low level receiving circuit free from interference. 

 In taking the necessary precautions along these lines, a large number 

 of sources of noise were investigated. These included crosstalk from 

 other carrier telephone or telegraph systems, high-frequency oscil- 

 lations set up by d-c. telegraph apparatus in the vicinity, radio stations, 

 power wires, submarine telegraph cables, and many minor sources. 



Space requirements prevent a complete discussion of this work but 

 a tabulation of a few of the remedial measures may be interesting. 



Special filters in all telephone office power supply sources. 



Special shielding arrangements in the construction of the re- 

 ceiving amplifiers. 



Series "choke" coils in all the d-c. telegraph circuits which 

 enter the underground cable. 



D-c. telegraph apparatus in the office with special "spark- 

 killer" and high-frequency suppression units. 



Frequency limiting apparatus in the Commercial Cable Com- 

 pany's submarine telegraph circuit. 



Frequency limiting filters in the carrier telegraph circuits of 

 the 1921 cables. 



Special shielding to reduce induction from the other carrier 

 telephone equipment in the terminal offices. 



Improvement in the balance (to ground) of some of the appa- 

 ratus previously installed. 



Various special grounds in the apparatus and cable circuits 

 both at the telephone office and in the cable hut. 



Of particular interest was the case where the Commercial Cable 

 Company's submarine telegraph circuits from Havana to New York, 

 a communication facility of low inherent frequency range, interfered 

 with the carrier channels, having frequencies up to 28 kc, by induc- 

 tion in the underground cable through which both circuits passed. 

 In this case, by the generous cooperation of the Commercial Cable 

 Company, frequency limiting equipment was added to their cable 

 transmitter. 



