130 BELL SYSTEM TECHNICAL JOURNAL 



atlantic telegraphy at present pretty well preempts frequencies below 

 30,000 cycles (10,000 meters). Therefore, for the present at least 

 transatlantic telephony is limited to a range of some 30,000 cycles. 

 Now transmission of speech requires as a minimum for good quality 

 a single-side-band 3000 cycles wide. Allowing for variations and 

 clearances between channels it is doubtful if the channels could be 

 made to average closer than one every 4000 cycles for single-side- 

 band transmission and one every 7000 cycles for the ordinary double- 

 side-band transmission. This means that even were the whole range 

 from 30,000 to 60,000 cycles devoted to telephony to the exclusion of 

 telegraphy, only about four channels could be obtained by the older 

 methods and some seven by the present one. 



It is a rather interesting commentary to note that a somewhat 

 similar situation as to limitation in frequency range exists in the 

 case of carrier-current transmission over wires. The transmission 

 efficiency falls off with increase in frequency and limits the range 

 of frequencies which can be economically used, in much the same 

 way as it is limited in long distance radio transmission. It is because 

 of this limitation in the case of wires and the value which attaches 

 to conserving the frequency range consumed per message that single- 

 side-band transmission was first developed for wire carrier current 

 systems. Its development in wire transmission has been of con- 

 siderable aid in adapting the method to the present purpose of trans- 

 atlantic operation. 



The second of the outstanding characteristics of the present system 

 resides in the large power economy which it permits. Transatlantic 

 telephony requires hundreds of kilowatts of high-frequency power. 

 Since it is difficult and expensive to produce this power it is important 

 that every effort be made to increase its efficiency or effectiveness in 

 transmitting the voice. To illustrate how the present system effects 

 economies in power, consider the case of a carrier wave completely 

 modulated by a single frequency tone. In such a completely modu- 

 lated wave, only 1/3 of the total power contains the message, the 

 remaining 2/3 conveying only the carrier frequency which can as 

 well be supplied from an oscillator of small power at the receiving 

 station. It is obvious, therefore, that by eliminating the carrier 

 only 1/3 as much power need be used as would be required were all 

 the elements of the completely modulated wave transmitted. To 

 realize the maximum advantage of this mode of operation, the system 

 eliminates the carrier at low power and, thereby, the high-power 

 apparatus is devoted exclusively to the amplification of the essential 

 part of the signal. 



