132 BELL SYSTEM TECHNICAL JOURNAL 



end, after detection, are proportional to the product of the carrier 

 wave and the side-band. If the carrier as well as the side-band is trans- 

 mitted through the medium, then a given variation in the transmission 

 efficiency of the medium will affect both components and will change 

 the received speech in proportion to the square of the variation, as 

 compared to the first power if only the side-band is transmitted and 

 the carrier is supplied locally. Thus it will be seen that the omission 

 of the carrier from the sending end and the resupplying of it from 

 the constant source at the receiving end gives greater stability of 

 transmission. 



Without discussing the system in further detail the advantages of 

 it may be summarized as follows: 



1. It conserves the frequency (wave length) band required for radio 

 telephony, which is particularly important at long wave lengths. 



2. It conserves power, in that all of the power transmitted is useful 

 signal-producing power. This is particularly important also in 

 long distance transmission which requires the use of large powers. 



3. The fact that only a single-band of frequencies is transmitted 

 simplifies the antenna problem at long wave lengths, where the 

 resonance band becomes too narrow to transmit both side-bands. 



4. As compared with a system which eliminates the carrier but 

 transmits both side-bands the simple side-band system has the 

 important advantage of not requiring an extreme accuracy of 

 frequency in the carrier which is resupplied at the receiver. Were 

 both side-bands transmitted very perfect synchronism would be 

 required for good quality. 



5. It improves the transmission stability of the radio circuit since 

 variations in the ether attenuation affect only one (the side-band) 

 of the two components effective in carrying out the detecting 

 action in the receiver. 



6. The receiving part of the overall system has two advantages: 



a. It need accept only half of the frequency band which would be 

 required in double side-band transmission, thereby accepting 

 only half of the "static" interfering energy. 



b. By stepping down the frequency of the received currents and 

 filtering and amplifying at the low-frequency stage a very sharp 

 cutoff is obtained for frequencies outside of the desired band and 

 a very stable and easily maintained amplifying system is obtained. 



