138 BELL SYSTEM TECHNICAL JOURNAL 



pressed as shown at 63. A means for doing this is the Carson balanced 

 tube modulator circuit illustrated below in the figure. 



A reduction of the total band can be effected by filtering out one 

 of them as shown at ^4- The remaining single band is the simplest 

 component of the modulation process with which intelligence can 

 be transmitted to the distant end. Upon arriving at the receiving 

 end at b^, this side band is fed into the detector along with a carrier 

 of the same frequency as that employed at the sending end; these 

 two components demodulate one another, with the result that the 

 side band is shifted down to its original audio-frequency position in 

 the scale, as indicated at be,. 



Actually this general method of transmission, involving both 

 carrier suppression and side band elimination, is being employed in 

 wire carrier systems in the Bell Telephone Plant. ^ It is briefly ex- 

 plained here because it represents a valuable improvement in wire 

 transmission which should have important application in radio. 



From the standpoint of transmission levels its application is in 

 showing that the real intelligence-carrying component of a radio wave 

 is the side-band and not the carrier itself. In considering transmission 

 levels accurately care should be taken, therefore, to deal in terms of 

 the level or wave-intensity of the side-band component and not the 

 carrier. It is because of this that as nearly complete modulation 

 as possible is desired at the transmitting station. 



It follows that the power resident in the carrier is a pure waste in 

 so far as overcoming interference is concerned. An important power 

 saving can be effected in the transmitting station by providing some 

 such means as is illustrated whereby the carrier power is held back 

 in the circuit. The two side-bands together can never be greater in 

 current and voltage value than the carrier, and each side-band alone 

 cannot be greater than half the carrier. The power of the carrier is 

 therefore always at least four times the power of one side-band or 

 twice that of both together. Thus by " holding-back " the carrier 

 at the transmitter we can transmit with but one-third the power 

 ordinarily required. Actually the power saving is much greater 

 than this because of the necessity of normally working with larger 

 ratios between carrier and side-band in order to accommodate the 

 peaks of the telephone waves and thereby preserve the quality of 

 transmission. The power saving is, of course, especially important 

 in long distance work. 



The suppression of one of the two side-bands halves the frequency 



•"Carrier Current Telephony and Telegraphy," by Colpitis and Blackwell, 

 Journal of the American Institute of Electrical Engineers, February 16-18, 1921. 



