426 BELL SYSTEM TECHNICAL JOURNAL 



considered with particular reference to loaded cables in the paper by 

 Mr. A. A. Clokey published in this issue of the Bell System Technical 

 Journal. 



The Necessity of Cable Signal Amplifiers and Their 

 Requirements 



Telegraph signals passing over a long submarine cable are distorted 

 so severely that only a small fraction of the ultimate speed would be 

 possible if extraordinary means were not taken to compensate for this 

 distortion. It may be found that a certain cable attenuates very low 

 frequencies to only one half of their original voltage while the higher 

 frequencies may be received at less than one ten-thousandth of their 

 initial strength. The transmission of an ideally perfect signal requires 

 that its components of all frequencies be received at amplitudes 

 proportional to those transmitted, consequently the reshaping of a 

 signal received from a cable involves equalizing the strength of all its 

 component frequencies by reducing the amplitude of the lower fre- 

 quencies and amplifying the higher frequencies. 



As the voltage which may be impressed upon a cable is limited by 

 considerations of the safety of its insulation, the sensitivity of the 

 receiver to currents of the highest frequency necessary in a properly 

 defined signal is one of the limitations of the speed at which a cable 

 may be operated. Unfortunately this is not the only limitation or it 

 would be possible to increase the speed indefinitely by simply in- 

 creasing the sensitivity of the receiver. All cables are exposed to 

 extraneous interfering currents, some natural in origin and some the 

 result of human activities, and while a great deal may be accomplished 

 by proper design of the cable and the associated apparatus the speed 

 is ultimately limited by interference. A large proportion of this 

 interference is similar in nature to "static" and the bane of radio 

 communication is also, but to a lesser degree, the bane of cable 

 communication . 



Experience has shown that continuous communication of the high 

 standard of accuracy required in the transmission of code and cypher 

 messages cannot be maintained unless the voltage received at the 

 nominal signaling ^ frequency is between two and five millivolts. A 

 receiver must therefore be capable of responding to voltages of this 

 order at the signaling frequency in order to utilize the cable efficiently. 

 On an average cable the power available at this voltage is of the order 

 of 2 X 10~^ watts and there is at present no signal recording device 



2 This is defined in the case of the Morse cable code as the fundamental frequency 

 in a series of alternate dots and dashes. 



