118 BELL SYSTEM TECHNICAL JOURNAL 



method, the narrowest possible band of wave lengths in the ether is 

 used, and all of the energy radiated has maximum effectiveness in 

 transmitting the message. 



As had been pointed out in other papers 3 , when a carrier is modulated 

 by telephone waves, the power given out is distributed over a fre- 

 quency range, and may be conveniently considered in three parts: 

 (1) energy at the carrier frequency itself, (2) energy distributed in a 

 frequency band extending from the carrier upward, and having a 

 width equal to the frequencies appearing in the telephone waves, 

 and (3) energy in a band extending from the carrier downward, and 

 having a similar width. The power at the carrier frequency itself 

 makes up somewhat more than two-thirds of the total power, even 

 when modulation is as complete as possible. Furthermore, this 

 energy can, in itself, convey no message, as is self evident. In the 

 present method, therefore, the carrier-frequency component is elim- 

 inated, by methods explained in detail below with the result that a 

 large saving in power is effected. Each of the remaining frequency 

 ranges, generally known as the upper and the lower side-band re- 

 spectively, transmits power representing -the complete message. It is 

 therefore unnecessary to transmit both of these side-bands, so that 

 in the present method one of them is eliminated. In this way the 

 transmission of the message uses only half the frequency band re- 

 quired in the usual method of operation. Similarly the frequency- 

 band accepted by the receiving set is narrowed to conform to a single 

 side-band as compared with the usual double side-band reception, 

 and as a result the ratio of signal to interference is improved. Certain 

 other advantages of this method will be brought out in the further 

 discussion. 



While these advantages of the single side-band eliminated carrier 

 method hold good for radio telephone transmission generally, they 

 become of the utmost importance in transoceanic work, because of 

 the necessity of conserving power in a system where the transmitting 

 powers are large, and also because the very limited frequency range 

 available for long distance transmission makes it imperative that 

 each part of the range shall be utilized with the greatest of care. 

 Before discussing the method further, the circuits and apparatus 

 which are actually used in the tests will be described. 



3 "Carrier Current Telephony and Telegraphy" by Colpitts and Blackwell. 

 Journal A. I. E. E., April, 1921. 



"Application to Radio of Wire Transmission Engineering" by Lloyd Espenschied. 

 Proc. Inst. Radio Engrs., Oct. 1922. 



"Relations of Carrier and Side-bands in Radio Transmission" by R. V. L. Hartley. 

 Proc. Inst. Radio Engrs., Feb. 1923. 



